source: XIOS/trunk/doc/XIOS_user_guide.lyx @ 1171

#LyX 2.2 created this file. For more info see http://www.lyx.org/
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\begin_body

\begin_layout Title
XIOS User Guide
\end_layout

\begin_layout Standard
\begin_inset CommandInset toc
LatexCommand tableofcontents

\end_inset


\end_layout

\begin_layout Chapter
Calendar
\end_layout

\begin_layout Section
How to define a calendar
\end_layout

\begin_layout Standard
XIOS has an embedded calendar module which needs to be configured before
 you can run your simulation.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Only the calendar type and the time step used by your simulation are mandatory
 to have a well defined calendar.
 For example, a minimal calendar definition could be:
\end_layout

\begin_layout Itemize
from the XML configuration file:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1.5h" />
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset


\end_layout

\begin_layout Itemize
from the Fortran interface:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

TYPE(xios_context) :: ctx_hdl
\end_layout

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

! Context initialization ommited, see the corresponding section of this
 user manual and of the reference manual
\end_layout

\begin_layout Plain Layout

CALL xios_get_handle("test",ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_current_context(ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_define_calendar(type="Gregorian", timestep=1.5*xios_hour)
\end_layout

\end_inset


\end_layout

\begin_layout Standard
The calendar type definition is done once and for all, either from the XML
 configuration file or the Fortran interface, and cannot be modified.
 However there is no such restriction regarding the time step which can
 be defined at a different time than the calendar type and even redefined
 multiple times.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

For example, it is possible to the achieve the same minimal configuration
 as above by using both the XML configuration file:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" />
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

and the Fortran interface:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

TYPE(xios_context) :: ctx_hdl
\end_layout

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

! Context initialization ommited, see the corresponding section of this
 user manual and of the reference manual
\end_layout

\begin_layout Plain Layout

CALL xios_get_handle("test",ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_current_context(ctx_hdl)
\end_layout

\begin_layout Plain Layout

! xios_define_calendar cannot be used here because the type was already
 defined in the configuration file.
\end_layout

\begin_layout Plain Layout

! Ommiting the following line would lead to an error because the timestep
 would be undefined.
\end_layout

\begin_layout Plain Layout

CALL xios_set_timestep(timestep=1.5*xios_hour)
\end_layout

\end_inset

The calendar also has two optional date parameters:
\end_layout

\begin_layout Itemize
the start date which corresponds to the beginning of the simulation
\end_layout

\begin_layout Itemize
the time origin which corresponds to the origin of the time axis.
\end_layout

\begin_layout Standard
If they are undefined, those parameters are set by default to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
0000-01-01 00:00:00
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
 If you are not interested in specific dates, you can ignore those parameters
 completely.
 However if you wish to set them, please note that they must not be set
 before the calendar is defined.
 Thus the following XML configuration file would be for example invalid:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <!-- Invalid because the calendar type cannot be known at that point -->
\end_layout

\begin_layout Plain Layout

                <calendar start_date="2011-11-11 13:37:42" />
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

while the following configuration file would be valid:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <!-- The order of the arguments does not matter so this is valid -->
\end_layout

\begin_layout Plain Layout

                <calendar time_origin="2011-11-11 13:37:42" type="Gregorian" />
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

Of course, it is always possible to define or redefine those parameters
 from the Fortran interface, directly when defining the calendar:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

TYPE(xios_context) :: ctx_hdl
\end_layout

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

! Context initialization ommited, see the corresponding section of this
 user manual and of the reference manual
\end_layout

\begin_layout Plain Layout

CALL xios_get_handle("test",ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_current_context(ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_define_calendar(type="Gregorian", time_origin=xios_date(1977,
 10, 19, 00, 00, 00), start_date=xios_date(2011, 11, 11, 13, 37, 42))
\end_layout

\end_inset

or at a later time:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

TYPE(xios_context) :: ctx_hdl
\end_layout

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

! Context initialization ommited, see the corresponding section of this
 user manual and of the reference manual
\end_layout

\begin_layout Plain Layout

CALL xios_get_handle("test",ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_current_context(ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_define_calendar(type="Gregorian")
\end_layout

\begin_layout Plain Layout

CALL xios_set_time_origin(time_origin=xios_date(1977, 10, 19, 00, 00, 00))
\end_layout

\begin_layout Plain Layout

CALL xios_set_start_date(start_date=xios_date(2011, 11, 11, 13, 37, 42))
\end_layout

\end_inset

To simplify the use of dates in the XML configuration files, it is possible
 to partially define a date as long as the omitted parts are the rightmost.
 In this case the remainder of the date is initialized as in the default
 date.
 For example, it would be valid to write: 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
start_date="1977-10-19"
\end_layout

\end_inset

 instead of 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
start_date="1977-10-19 00:00:00"
\end_layout

\end_inset

 or even 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
time_origin="1789"
\end_layout

\end_inset

 instead of 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
time_origin="1789-01-01 00:00:00"
\end_layout

\end_inset

.
 Similarly, it is possible to express a date with an optional duration offset
 in the configuration file by using the 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
date + duration
\end_layout

\end_inset

 notation, with 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
date
\end_layout

\end_inset

 potentially partially defined or even completely omitted.
 Consequently the following examples are all valid in the XML configuration
 file:
\end_layout

\begin_layout Itemize
\begin_inset Flex Code
status open

\begin_layout Plain Layout
time_origin="2011-11-11 13:37:00 + 42s"
\end_layout

\end_inset


\end_layout

\begin_layout Itemize
\begin_inset Flex Code
status open

\begin_layout Plain Layout
time_origin="2014 + 1y 2d"
\end_layout

\end_inset


\end_layout

\begin_layout Itemize
\begin_inset Flex Code
status open

\begin_layout Plain Layout
start_date="+ 36h"
\end_layout

\end_inset

.
\end_layout

\begin_layout Section
How to define a user defined calendar
\end_layout

\begin_layout Standard
Predefined calendars might not be enough for your needs if you simulate
 phenomenons on another planet than the Earth.
 For this reason, XIOS can let you configure a completely user defined calendar
 by setting the 
\series bold
type
\series default
 attribute to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
user_defined
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
 In that case, the calendar type alone is not sufficient to define the calendar
 and other parameters should be provided since the duration of a day or
 a year are not known in advance.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Two approaches are possible depending on whether you want that your custom
 calendar to have months or not: either use the 
\series bold
month_lengths
\series default
 attribute to define the duration of each months in days or use the 
\series bold
year_length
\series default
 attribute to define the duration of the year in seconds.
 In both cases, you have to define 
\series bold
day_length
\series default
, the duration of a day in seconds.
 Those attributes have to be defined at the same time than the calendar
 type, either from the XML configuration file or the Fortran interface,
 for example:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="user_defined" day_length="86400" month_lengths="(1, 12)
 [31 28 31 30 31 30 31 31 30 31 30 31]" />
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

or
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

TYPE(xios_context) :: ctx_hdl
\end_layout

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

! Context initialization ommited, see the corresponding section of this
 user manual and of the reference manual
\end_layout

\begin_layout Plain Layout

CALL xios_get_handle("test",ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_current_context(ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_define_calendar(type="Gregorian", day_length=86400, year_length=315576
00)
\end_layout

\end_inset

Note that if no months are defined, the format of the dates is modified
 in the XML configuration file since the month must be omitted.
 For example, 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
"2015-71 13:37:42"
\end_layout

\end_inset

 would be the correct way to refer to the 71st day of the year 2015 at 13:37:42.
 If you use the Fortran interface, the month cannot be omitted but you have
 to make sure to always set it to 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
1
\end_layout

\end_inset

 in that case.
 For example, use 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_date(2015, 01, 71, 13, 37, 42)
\end_layout

\end_inset

for 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
"2015-71 13:37:42"
\end_layout

\end_inset

.
 Moreover, it is possible that the duration of the day is greater than the
 duration of the year on some planets.
 In this case, it obviously not possible to define months so you have to
 use the 
\series bold
year_length
\series default
 attribute.
 Additionally the day must also be omitted from the dates in the configuration
 file (for example 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
"2015 13:37:42"
\end_layout

\end_inset

) and must always be set to 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
1
\end_layout

\end_inset

 when using the Fortran interface (for example 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_date(2015, 01, 01, 13, 37, 42)
\end_layout

\end_inset

).
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

If months have been defined, you might want to have leap years to correct
 the drift between the calendar year and the astronomical year.
 This can be achieved by using the 
\series bold
leap_year_drift
\series default
 and 
\series bold
leap_year_month
\series default
 attributes and optionally the 
\series bold
leap_year_drift_offset
\series default
 attribute.
 The idea is to define 
\series bold
leap_year_drift
\series default
, the yearly drift between the calendar year and the astronomical year as
 a fraction of a day.
 This yearly drift is summed each year to know the current drift and each
 time the current drift is greater or equal to one day, the year is considered
 a leap year.
 In that case, an extra day is added to the month defined by 
\series bold
leap_year_month
\series default
 and one day is subtracted to the current drift.
 The initial drift is null by default but it can be fixed by the 
\series bold
leap_year_drift_offset
\series default
 attribute.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The following configuration file defines a simplified Gregorian calendar
 using the user calendar feature:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="user_defined"
\end_layout

\begin_layout Plain Layout

                                        day_length="86400"
\end_layout

\begin_layout Plain Layout

                                        month_lengths="(1, 12) [31 28 31 30 31 30 31 31 30 31 30 31]"
\end_layout

\begin_layout Plain Layout

                                        leap_year_month="2"
\end_layout

\begin_layout Plain Layout

                                        leap_year_drift="0.25"
\end_layout

\begin_layout Plain Layout

                                        leap_year_drift_offset="0.75"
\end_layout

\begin_layout Plain Layout

                                        time_origin="2012-02-29 15:00:00"
\end_layout

\begin_layout Plain Layout

                                        start_date="2012-03-01 15:00:00" />
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

As you know, the astronomical year on Earth is approximately a quarter of
 day longer than the Gregorian calendar year so we have to define the yearly
 drift as 0.25 day.
 In case of a leap year, an extra day is added at the end of February which
 is the second month of the year so 
\series bold
leap_year_month
\series default
 should be set to 2.
 We start our time axis in 2012 which was a leap year in the Gregorian calendar.
 This means there was previously three non-leap years in a row so the current
 drift was (approximately) 
\begin_inset Formula $3\times0.25$
\end_inset

 days, hence 
\series bold
leap_year_drift_offset
\series default
 should be set to 0.75.
 At the beginning of 2013, the drift would have been 
\begin_inset Formula $0.75+0.25=1$
\end_inset

 day so 2012 will be a leap year as expected.
\end_layout

\begin_layout Section
How to use the calendar
\end_layout

\begin_layout Standard
The calendar is created immediately after the calendar type has been defined
 and thus can be used even before the context definition has been closed.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Once the calendar is created, you have to keep it updated so that it is
 in sync with your simulation.
 To do that, you have to call the 
\series bold
xios_update_calendar
\series default
 subroutine for each iteration of your code:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

INTEGER :: ts
\end_layout

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

DO ts=1,end
\end_layout

\begin_layout Plain Layout

        CALL xios_update_calendar(ts)
\end_layout

\begin_layout Plain Layout

        ! Do useful stuff
\end_layout

\begin_layout Plain Layout

ENDDO
\end_layout

\end_inset

The current date is updated to 
\begin_inset Formula $start\_date+ts\times timestep$
\end_inset

 after each call.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Many other calendar operations are available, including:
\end_layout

\begin_layout Itemize
accessing various calendar related information like the time step, the time
 origin, the start date, the duration of a day or a year, the current date,
 etc.
 
\end_layout

\begin_layout Itemize
doing arithmetic and comparison operations on date:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

TYPE(xios_date) :: date1, date2
\end_layout

\begin_layout Plain Layout

TYPE(xios_duration) :: duration
\end_layout

\begin_layout Plain Layout

LOGICAL :: res
\end_layout

\begin_layout Plain Layout

! we suppose a calendar is defined
\end_layout

\begin_layout Plain Layout

CALL xios_get_current_date(date1)
\end_layout

\begin_layout Plain Layout

duration = xios_duration(0, 0, 1, 0, 0, 0, 0, 0) + 12 * xios_hour
\end_layout

\begin_layout Plain Layout

date2 = date1 + duration + 0.5 * xios_hour
\end_layout

\begin_layout Plain Layout

res = date2 > date1
\end_layout

\begin_layout Plain Layout

duration = date2 - date1
\end_layout

\end_inset


\end_layout

\begin_layout Itemize
converting dates to
\begin_inset Separator latexpar
\end_inset


\end_layout

\begin_deeper
\begin_layout Itemize
the number of seconds since the time origin, the beginning of the year or
 the beginning of the day,
\end_layout

\begin_layout Itemize
the number of days since the beginning of the year,
\end_layout

\begin_layout Itemize
the fraction of the day or the year.
\end_layout

\end_deeper
\begin_layout Standard
For more detailed about the calendar attributes and operations, see the
 XIOS reference guide.
\end_layout

\begin_layout Chapter
Files
\end_layout

\begin_layout Standard
Since files are central to an I/O server, the configuration of XIOS is built
 around file objects.
 Those objects correspond directly to files on the computer file system
 which are either to be written or to be read.
 Although, XIOS currently only supports the NetCDF format, XIOS files are
 a generic abstraction.
 Each file can contain one or more fields (each field being defined on a
 grid) and optionally variables.
 In the NetCDF nomenclature, fields defined in XIOS correspond to NetCDF
 variables and XIOS variables are NetCDF attributes.
 As fields, variables and grids are complex objects, they have their own
 chapters and we will focus only on files in this section.
\end_layout

\begin_layout Section
How to define your first file
\end_layout

\begin_layout Standard
If you wish to input or to output data using XIOS, you will need to define
 at least one file.
 This can be done from both the XML configuration file and the Fortran interface.
 Files are usually defined in the configuration file, although their definitions
 are sometimes amended using the API.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

File objects are defined with the 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
<file>
\end_layout

\end_inset

 tag and should always be inside the 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
<file_definition>
\end_layout

\end_inset

 section.
 Only the output frequency is mandatory to have a well defined file but
 it is generally a good idea to give it a name.
 The following example shows a minimal configuration file which defines
 one file.
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1ts">
\end_layout

\begin_layout Plain Layout

                        <!-- Content of the file ommited for now -->
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

Note that the file extension could depend of the format so it is automatically
 added to the chosen name by XIOS.
 Since XIOS only support NetCDF formats for now, the extension is always
 
\begin_inset Quotes eld
\end_inset

.nc
\begin_inset Quotes erd
\end_inset

.
 If the name is not set, XIOS will use the id of the file object instead.
 This id is generated automatically by XIOS if it was not set by the user.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The output frequency is particularly important since it defines the interval
 of time between two consecutive outputs, that is in NetCDF nomenclature
 the interval between two records.
 In the example, the data would be written for every timestep (independently
 of the timestep duration).
 It is possible to use any duration as the output frequency but be careful
 if you are not using a duration which is a multiple of the timestep duration
 since XIOS might not be doing what you want.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The same configuration could be obtained from the Fortran interface as well:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

TYPE(xios_context)   :: ctx_hdl
\end_layout

\begin_layout Plain Layout

TYPE(xios_file)          :: file_hdl
\end_layout

\begin_layout Plain Layout

TYPE(xios_filegroup) :: filegroup_hdl
\end_layout

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

! Context and calendar initializations ommited, see the corresponding section
 of this user manual and of the reference manual
\end_layout

\begin_layout Plain Layout

CALL xios_get_handle("test", ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_current_context(ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_get_filegroup_handle("file_definition", filegroup_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_add_file(filegroup_hdl, file_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_attr(file_hdl, name="output", output_freq=xios_timestep)
\end_layout

\end_inset

Another important parameter for file is the 
\series bold
mode
\series default
 attribute which is set by default to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
write
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
 You need to set it to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
read
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 if you want to use XIOS to handle inputs.
 Note that in this case the 
\series bold
output_freq
\series default
 attribute must correspond to the output frequency used to create the input
 file.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

When using the 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
write
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 mode, it is possible to append the data to an existing file instead of
 overwriting it by setting the 
\series bold
append
\series default
 attribute to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
true
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
 In this case, you must be careful not to modify the structure of the file,
 in particular no fields should be added, modified nor removed, or XIOS
 will throw an error.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

If you wish to disable a file without having to remove its definition from
 the configuration file, you can set the 
\series bold
enabled
\series default
 attribute to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
false
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
\end_layout

\begin_layout Section
How to use parallel I/O
\end_layout

\begin_layout Standard
By default XIOS will create one file by server, each file being suffixed
 with the rank of the server.
 For example, if the sample configuration used in the previous section was
 used with two servers, two files named 
\begin_inset Quotes eld
\end_inset

output_0.nc
\begin_inset Quotes erd
\end_inset

 and 
\begin_inset Quotes eld
\end_inset

output_1.nc
\begin_inset Quotes erd
\end_inset

 would be created.
 Each file would contain only the portion of the fields affected to the
 corresponding server.
 This default mode can also be explicitly configured by setting the 
\series bold
type
\series default
 attribute to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
multiple_file
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Using the 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
multiple_file
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 mode is often a reliable way to achieve good performances, particularly
 if you only have a few servers.
 However having multiple files also increases the complexity of the post-process
ing chains and it is often much easier to always get one file regardless
 of how many servers are used.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

It is possible to achieve such behavior in XIOS by setting the 
\series bold
type
\series default
 attribute to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
one_file
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
 This feature depends directly on the NetCDF library capabilities so you
 need to make sure that XIOS was properly linked with a parallel version
 of NetCDF.
 If the library was not compiled with parallel input/output support, XIOS
 will issue a warning and revert to the 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
multiple_file
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 mode.
\end_layout

\begin_layout Section
Supported NetCDF formats
\end_layout

\begin_layout Standard
XIOS supports only the version 4 or later of NetCDF library.
 It uses by default the new NetCDF-4 format which relies on HDF5 format
 as a back-end.
 This format can also be selected explicitly by setting the 
\series bold
format
\series default
 attribute to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
netcdf4
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Alternatively, it also possible to force NetCDF-4 to use the classic NetCDF-3
 binary format by setting the 
\series bold
format
\series default
 attribute to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
netcdf4_classic
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
 When using this older format, some features might be unavailable but current
 version of XIOS should not be affected much.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Depending on the format, there are some specific requirements on how the
 NetCDF library should have been compiled:
\end_layout

\begin_layout Itemize
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
netcdf4
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 format requires that HDF5 support has been enabled in NetCDF using the
 configuration option 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
-\SpecialChar nobreakdash
-enable-netcdf4
\end_layout

\end_inset

 and that the HDF5 library has been properly linked.
\end_layout

\begin_layout Itemize
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
netcdf4
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 format used in 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
one_file
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 mode requires that the HDF5 library has been compiled with parallel support
 using the configuration option 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
-\SpecialChar nobreakdash
-enable-parallel
\end_layout

\end_inset

.
\end_layout

\begin_layout Itemize
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
netcdf4_classic
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 format used in 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
one_file
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 mode requires that Parallel NetCDF support has been enabled in NetCDF using
 the configuration option 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
-\SpecialChar nobreakdash
-enable-pnetcdf
\end_layout

\end_inset

 and that the Parallel NetCDF library has been properly linked.
\end_layout

\begin_layout Section
How to use file splitting
\end_layout

\begin_layout Standard
Output files can often be quite huge, particularly if the 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
one_file
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 mode is used.
 In this case, it can be interesting to periodically split the file in order
 to have a few smaller files containing contiguous temporal portions of
 the output data.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

This behavior can be achieved in XIOS by setting the 
\series bold
split_freq
\series default
 attribute to the duration you want, as illustrated in the following example:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1d" split_freq="1y">
\end_layout

\begin_layout Plain Layout

                        <!-- Content of the file ommited for now -->
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

With this configuration, some data will be outputted every day and a new
 file will be created every year.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Note that the split frequency is the duration after which a new file will
 be created, it does not mean that a new file will be created at the beginning
 of each period.
 For example, if you start your simulation the first of June 2014 and run
 it for two years with a split frequency of one year:
\end_layout

\begin_layout Itemize
you will get two files containing respectively the period from June 1st,
 2014 to May 31th, 2015 and from June 1st, 2015 to May 31th, 2016.
\end_layout

\begin_layout Itemize
you will NOT get three files containing respectively the last six months
 of 2014, the full year of 2015 and the first six months of 2016.
\end_layout

\begin_layout Standard
XIOS automatically suffixes the file names with the start and end dates
 when using file splitting.
 By default, it will try to use the shortest date that still enables to
 distinguish the files.
 Thus in the above example, the files would be named 
\begin_inset Quotes eld
\end_inset

output_2014-2015.nc
\begin_inset Quotes erd
\end_inset

 and 
\begin_inset Quotes eld
\end_inset

output_2015-2016.nc
\begin_inset Quotes erd
\end_inset

.
 If you wish to force the date format used to prefix the files, you can
 define the 
\series bold
split_freq_format
\series default
 attribute to override the default behavior.
\end_layout

\begin_layout Section
A word about file synchronization
\end_layout

\begin_layout Standard
File synchronization is usually not something you should worry about.
 However, it is important to understand that data written by XIOS might
 not be immediately written on the disk in practice.
 Input/output libraries like NetCDF and HDF5 and parallel file systems generally
 use complex caching policies for performance reasons.
 This means that your data might still be stored in memory after it was
 supposedly written.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

It might become critical to control this behavior for two main reasons:
\end_layout

\begin_layout Itemize
if you want to mitigate the impact of a crash, as all buffered data would
 be lost ;
\end_layout

\begin_layout Itemize
if you want to be able to access the data from the output file immediately
 after writing it.
\end_layout

\begin_layout Standard
By default, XIOS will never force file synchronization but you can require
 it to do so by setting the 
\series bold
sync_freq
\series default
 attribute to the wanted duration.
 In this case, XIOS will regularly instruct NetCDF to synchronize the file
 on disk by flushing its internal buffers.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Note file synchronization must be used sparingly as it can have a disastrous
 impact on performance.
 Make sure to use a reasonably high synchronization frequency to avoid any
 issue.
\end_layout

\begin_layout Chapter
Fields and variables
\end_layout

\begin_layout Standard
XIOS outsources the input/output definitions in its XML configuration file.
 In the last chapter we presented some general points about file objects.
 This chapter focuses on how to use fields and variables (that is variables
 and attributes in NetCDF nomenclature) to populate files.
\begin_inset Newline newline
\end_inset


\end_layout

\begin_layout Section
How to define your first field
\end_layout

\begin_layout Standard
If you wish to input or to output data using XIOS, you will need to define
 at least one file with one field.
 This can be done from both the XML configuration file and the Fortran interface.
 Fields are often defined in the configuration file, although their definitions
 are sometimes amended using the API.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Field objects are defined with the 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
<field>
\end_layout

\end_inset

 tag and should always be inside a 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
<field_definition>
\end_layout

\end_inset

 or a 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
<file>
\end_layout

\end_inset

 section.
 Only the grid and the operation attached to the field are mandatory to
 have a well defined field but it is generally a good idea to give it an
 identifier.
 The following example shows a minimal configuration file which defines
 one file with one field.
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" type="one_file" output_freq="1ts">
\end_layout

\begin_layout Plain Layout

                        <field id="field_A" grid_ref="grid_A" operation="instant" />
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

It defines one file named 
\begin_inset Quotes eld
\end_inset


\emph on
output
\emph default

\begin_inset Quotes erd
\end_inset

 which contains one field 
\begin_inset Quotes eld
\end_inset


\emph on
field_A
\emph default

\begin_inset Quotes erd
\end_inset

 defined on a grid 
\begin_inset Quotes eld
\end_inset


\emph on
grid_A
\emph default

\begin_inset Quotes erd
\end_inset

.
 The file and the field are configured so that the data is written in the
 file at every timestep (using the 
\series bold
output_freq
\series default
 file attribute) without any transformation (using the 
\series bold
operation
\series default
 field attribute set to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
instant
\series default
\emph default

\begin_inset Quotes erd
\end_inset

).
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The corresponding Fortran simulation loop could be:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

DO ts=1,numberOfTimestep
\end_layout

\begin_layout Plain Layout

        ! Inform XIOS of the current timestep
\end_layout

\begin_layout Plain Layout

        CALL xios_update_calendar(ts)
\end_layout

\begin_layout Plain Layout

        ! Compute field_A for current timestep
\end_layout

\begin_layout Plain Layout

        ! ...
\end_layout

\begin_layout Plain Layout

        ! Output the data
\end_layout

\begin_layout Plain Layout

        CALL xios_send_field("field_A", field_A)
\end_layout

\begin_layout Plain Layout

ENDDO
\end_layout

\end_inset

As you can see, the 
\series bold
id
\series default
 of the field is used in the model to select the field for which data is
 being provided which makes this attribute extremely important.
 Note that it must be unique for all fields even if they are defined in
 different files.
 By default, the 
\series bold
id
\series default
 of a field is also used as the name of the corresponding NetCDF variable.
 It is however possible to override this default name using the field attribute
 
\series bold
name
\series default
.
 Two fields can share the same 
\series bold
name
\series default
 as long as they are not used in the same file.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The second argument of the 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_send_field
\end_layout

\end_inset

 function is an array containing the data.
 Its shape and content are not described here as they depend directly on
 the grid.
 For more information on the data layout, refer to the chapters focusing
 on grids, domains and axis.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The same configuration could also be obtained using the Fortran interface:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

TYPE(xios_context)   :: ctx_hdl
\end_layout

\begin_layout Plain Layout

TYPE(xios_file)          :: file_hdl
\end_layout

\begin_layout Plain Layout

TYPE(xios_filegroup) :: filegroup_hdl
\end_layout

\begin_layout Plain Layout

TYPE(xios_field)        :: field_hdl
\end_layout

\begin_layout Plain Layout

! ...
\end_layout

\begin_layout Plain Layout

! Context, calendar and grid initializations ommited, see the corresponding
 section of this user manual and of the reference manual
\end_layout

\begin_layout Plain Layout

CALL xios_get_handle("test", ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_current_context(ctx_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_get_filegroup_handle("file_definition", filegroup_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_add_file(filegroup_hdl, file_hdl)
\end_layout

\begin_layout Plain Layout

CALL xios_set_attr(file_hdl, name="output", output_freq=xios_timestep)
\end_layout

\begin_layout Plain Layout

CALL xios_add_field(file_hdl, field_hdl, "field_A")
\end_layout

\begin_layout Plain Layout

CALL xios_set_attr(field_hdl, grid_ref="grid_A", operation="instant")
\end_layout

\end_inset

Note that if you want to define a field on a grid with only one domain and/or
 one axis, it is possible to use the 
\series bold
domain_ref
\series default
 and 
\series bold
axis_ref
\series default
 attributes instead of the 
\series bold
grid_ref
\series default
 attribute.
 A temporary grid will be created based on the domain and/or axis defined
 this way.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

If you are using a grid with some masked points (see the relevant sections
 of this manual), you must set the 
\series bold
default_value
\series default
 attribute to define the default value that will replace the missing values
 in the output file.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

If you wish to disable a field without having to remove its definition from
 the configuration file, you can set the 
\series bold
enabled
\series default
 attribute to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
false
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
\end_layout

\begin_layout Section
How to use temporal operations
\end_layout

\begin_layout Standard
The last section showed a very basic example where the data was outputted
 at every timestep using the 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
instant
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 
\series bold
operation
\series default
.
 However in many use cases, it might be more interesting to output only
 the mean value on a certain period of time for example.
 This section describes the use of temporal operations available in XIOS.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The field attribute 
\series bold
operation
\series default
 currently supports six modes:
\end_layout

\begin_layout Itemize

\series bold
\emph on
instant
\series default
\emph default
: no temporal operation is applied which means the new data always overrides
 the previous one even if it was not outputted,
\end_layout

\begin_layout Itemize

\series bold
\emph on
average
\series default
\emph default
: compute and output the mean value,
\end_layout

\begin_layout Itemize

\series bold
\emph on
accumulate
\series default
\emph default
: compute and output the sum,
\end_layout

\begin_layout Itemize

\series bold
\emph on
minimum
\series default
\emph default
: compute and output the minimum value,
\end_layout

\begin_layout Itemize

\series bold
\emph on
maximum
\series default
\emph default
: compute and output the maximum value,
\end_layout

\begin_layout Itemize

\series bold
\emph on
once
\series default
\emph default
 : the data is written to the file only the first time it is received from
 the model, any subsequent data is ignored.
 The corresponding NetCDF variable does not have a time dimension.
\end_layout

\begin_layout Standard
The output frequency of the file defined by the 
\series bold
output_freq
\series default
 attribute is used as the temporal operation period (except for the 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
once
\series default
\emph default

\begin_inset Quotes erd
\end_inset

 
\series bold
operation
\series default
 for which there is no period).
 This means it is for example not possible to output a daily average and
 a weekly average in the same file.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

This updated example shows how to output the daily average instead of the
 instant data for all timesteps:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" type="one_file" output_freq="1d">
\end_layout

\begin_layout Plain Layout

                        <field id="field_A" grid_ref="grid_A" operation="average" />
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

Compared to the previous example, only the file attribute 
\series bold
output_freq
\series default
 and the field attribute 
\series bold
operation
\series default
 have been modified.
 Computing the weekly minimum instead of the daily average would be as simple
 as using 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
output_freq="7d"
\end_layout

\end_inset

and 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
operation="minimum"
\end_layout

\end_inset

.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Note that if you use a temporal operation and have 
\series bold
default_value
\series default
 defined, it might be useful to set the attribute
\series bold
 detect_missing_value
\series default
 to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
true
\series default
\emph default

\begin_inset Quotes erd
\end_inset

.
 This way temporal operations will not be applied when a default value is
 detected.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

For example, we consider the values of a 2x2 domain for three timesteps:
\begin_inset Formula 
\[
\begin{bmatrix}3 & -1\\
7 & 1
\end{bmatrix},\qquad\begin{bmatrix}5 & 6\\
-1 & 2
\end{bmatrix},\qquad\begin{bmatrix}-1 & 8\\
3 & 4
\end{bmatrix}.
\]

\end_inset

If we suppose that the field is configured to compute the average on three
 timesteps, the resulting field would be: 
\begin_inset Formula 
\[
\begin{bmatrix}\nicefrac{7}{3} & \nicefrac{13}{3}\\
3 & \nicefrac{7}{3}
\end{bmatrix}.
\]

\end_inset

If 
\series bold
default_value
\series default
 is set to 
\emph on

\begin_inset Quotes eld
\end_inset

-1
\begin_inset Quotes erd
\end_inset


\emph default
 and 
\series bold
detect_missing_value
\series default
 is set to 
\begin_inset Quotes eld
\end_inset


\series bold
\emph on
true
\series default
\emph default

\begin_inset Quotes erd
\end_inset

, the resulting field would be: 
\begin_inset Formula 
\[
\begin{bmatrix}4 & 7\\
5 & \nicefrac{7}{3}
\end{bmatrix}.
\]

\end_inset


\end_layout

\begin_layout Section
How to use a specific data sampling
\end_layout

\begin_layout Standard
It is sometimes useful to have more control on the data sampling.
 By default, the input data is used at every timestep but sometimes it is
 not what you want.
 The following examples illustrate such cases:
\end_layout

\begin_layout Enumerate
the model is not computing updated values at the same frequency for all
 fields (for example, a field is updated every two timesteps).
\end_layout

\begin_layout Enumerate
you want to output a specific instant value in the interval between two
 outputs.
\end_layout

\begin_layout Enumerate
you want to compute an average without taking into account all instant values
 in the interval between two outputs.
\end_layout

\begin_layout Standard
Data sampling can be controlled in XIOS using the 
\series bold
freq_op
\series default
 (one timestep by default) and 
\series bold
freq_offset
\series default
 (null by default) attributes.
 Those attributes define respectively how often data from the model must
 be used and the amount of time before starting to use it.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

For following excerpts of configuration files show you to use those attributes
 to handle the motivating examples.
\end_layout

\begin_layout Enumerate
In this example, we suppose that we get two fields from the model: 
\begin_inset Quotes eld
\end_inset

field_A
\begin_inset Quotes erd
\end_inset

 which is computed for each timestep and 
\begin_inset Quotes eld
\end_inset

field_B
\begin_inset Quotes erd
\end_inset

 which is only computed every two timesteps.
 For both fields, we show how to compute and output the sum of all values
 received during 6 timesteps:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<file_definition>
\end_layout

\begin_layout Plain Layout

        <file name="output" output_freq="6ts">
\end_layout

\begin_layout Plain Layout

                <field id="field_A" grid_ref="grid_A" operation="accumulate" />
\end_layout

\begin_layout Plain Layout

                <field id="field_B" grid_ref="grid_B" operation="accumulate" freq_op="2ts"
 />
\end_layout

\begin_layout Plain Layout

        </file>
\end_layout

\begin_layout Plain Layout

</file_definition>
\end_layout

\end_inset


\end_layout

\begin_layout Enumerate
In this example, we show how to output the 11th instant value every 12 timesteps
:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<file_definition>
\end_layout

\begin_layout Plain Layout

        <file name="output" output_freq="12ts">
\end_layout

\begin_layout Plain Layout

                <field id="field_A" grid_ref="grid_A" operation="instant" freq_op="11ts"
 freq_offset="12ts" />
\end_layout

\begin_layout Plain Layout

        </file>
\end_layout

\begin_layout Plain Layout

</file_definition>
\end_layout

\end_inset


\end_layout

\begin_layout Enumerate
In this example, we suppose that the timestep is equal to one hour and that
 the simulation starts at midnight.
 We show how to compute the weekly average of the field value at midday:
\begin_inset Newline newline
\end_inset


\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<file_definition>
\end_layout

\begin_layout Plain Layout

        <file name="output" output_freq="1w">
\end_layout

\begin_layout Plain Layout

                <field id="field_A" grid_ref="grid_A" operation="average" freq_op="1d"
 freq_offset="12h" />
\end_layout

\begin_layout Plain Layout

        </file>
\end_layout

\begin_layout Plain Layout

</file_definition>
\end_layout

\end_inset


\end_layout

\begin_layout Section
How to use field references
\end_layout

\begin_layout Standard
It is quite common that different temporal operations must be applied to
 the same instant data provided by the model.
 In theory, the only solution to handle this scenario would be to define
 a field for each operation, give them different 
\series bold
id
\series default
 and and call 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_send_field
\end_layout

\end_inset

 with the same array of data for each of those fields.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The following example illustrates this solution for a field for which we
 want to compute the average, minimal and maximal values:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1d">
\end_layout

\begin_layout Plain Layout

                        <field id="field_A_avg" grid_ref="grid_A" operation="average"
 />
\end_layout

\begin_layout Plain Layout

                        <field id="field_A_min" grid_ref="grid_A" operation="min" />
\end_layout

\begin_layout Plain Layout

                        <field id="field_A_max" grid_ref="grid_A" operation="max" />
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

To simplify the handling of such scenarios, XIOS has a 
\begin_inset Quotes eld
\end_inset

reference
\begin_inset Quotes erd
\end_inset

 feature which allows one field to inherit the attributes (except the 
\series bold
id
\series default
) and the instant data of another field.
 The above example can then be rewritten:
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1d">
\end_layout

\begin_layout Plain Layout

                        <field id="field_A" name="field_A_avg" grid_ref="grid_A" operation="av
erage" />
\end_layout

\begin_layout Plain Layout

                        <field field_ref="field_A" name="field_A_min" operation="min"
 />
\end_layout

\begin_layout Plain Layout

                        <field field_ref="field_A" name="field_A_max" operation="max"
 />
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

With this configuration, only one call to 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_send_field(
\begin_inset Quotes eld
\end_inset

field_A
\begin_inset Quotes erd
\end_inset

, field_A)
\end_layout

\end_inset

 is needed.
 Note how inherited attributes (like 
\series bold
name
\series default
 or 
\series bold
operation
\series default
 for example) are overwritten to obtain the desired configuration.
 Additionally, be aware that it is the instant values which are inherited,
 not the result of the operation on the field.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Similarly, it is sometimes useful to output the result of a temporal operation
 on a field for different periods.
 In this case, it does not really make sense to define the field that will
 be then inherited in one file rather than another.
 A solution is to make use of the 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
field_definition
\end_layout

\end_inset

 section so that it is clear that the field can be reused in any file.
 This is illustrated in the following sample configuration file:
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <field_definition>
\end_layout

\begin_layout Plain Layout

                <field id="field_A" name="field_A" grid_ref="grid_A" operation="average
" />
\end_layout

\begin_layout Plain Layout

                </field_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output_1d" output_freq="1d">
\end_layout

\begin_layout Plain Layout

                        <field field_ref="field_A" />
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                        <file name="output_1mo" output_freq="1mo">
\end_layout

\begin_layout Plain Layout

                        <field field_ref="field_A" />
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset


\end_layout

\begin_layout Section
How to use arithmetic operations
\end_layout

\begin_layout Standard
Since XIOS aims to reduce as much as possible the need for post-processing,
 it can apply some arithmetic operations on the data it handles (regardless
 of its provenance).
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

All usual operators (+, -, *, /, ^, that is addition, subtraction, multiplicatio
n, division and exponentiation) and some common functions (like cos, sin,
 tan, exp, log, log10, sqrt) are supported.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The following example shows how to use arithmetic operations:
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <field_definition>
\end_layout

\begin_layout Plain Layout

                <field id="field_A" grid_ref="grid_A" operation="average" />
\end_layout

\begin_layout Plain Layout

                </field_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1d">
\end_layout

\begin_layout Plain Layout

                        <field id="field_B" field_ref="field_A">field_A + 273.15</field>
\end_layout

\begin_layout Plain Layout

                        <field id="field_C" field_ref="field_A">log10(field_B)</field>
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

With this configuration, only one call to 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_send_field(
\begin_inset Quotes eld
\end_inset

field_A
\begin_inset Quotes erd
\end_inset

, field_A)
\end_layout

\end_inset

 is needed.
 In this example 
\series bold
field_ref
\series default
 is used only to inherit the attributes from 
\begin_inset Quotes eld
\end_inset

field_A
\begin_inset Quotes erd
\end_inset

, the instant values are not inherited since an expression has been given
 for 
\begin_inset Quotes eld
\end_inset

field_B
\begin_inset Quotes erd
\end_inset

 and 
\begin_inset Quotes eld
\end_inset

field_C
\begin_inset Quotes erd
\end_inset

.
 Note that it is possible to use fields obtained from an expression in another
 expression, thus the expression of 
\begin_inset Quotes eld
\end_inset

field_C
\begin_inset Quotes erd
\end_inset

 is equivalent to 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
log10(field_A + 273.15)
\end_layout

\end_inset

.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The special keyword 
\series bold
this
\series default
 can be used in an expression to refer to the instant data received from
 the model by the current field.
 For example, the previous configuration file could be rewritten as follow:
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1d">
\end_layout

\begin_layout Plain Layout

                        <field id="field_B" grid_ref="grid_A" operation="average">this
 + 273.15</field>
\end_layout

\begin_layout Plain Layout

                        <field id="field_C" field_ref="field_B">log10(field_B)</field>
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

and the Fortran call would be replaced by 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_send_field(
\begin_inset Quotes eld
\end_inset

field_B
\begin_inset Quotes erd
\end_inset

, field_A)
\end_layout

\end_inset

.
\end_layout

\begin_layout Section
How to chain multiple temporal operations
\end_layout

\begin_layout Standard
By default, all field names appearing in an expression refer to the instant
 data of those fields.
 To refer to the result of a temporal operation, the field name must be
 prefixed with 
\begin_inset Quotes eld
\end_inset

@
\begin_inset Quotes erd
\end_inset

.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

This feature allows to chain multiple temporal operations as illustrated
 bellow:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <field_definition>
\end_layout

\begin_layout Plain Layout

                <field id="field_A" grid_ref="grid_A" operation="average" />
\end_layout

\begin_layout Plain Layout

                </field_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="7d">
\end_layout

\begin_layout Plain Layout

                        <field name="field_A_min_of_average" grid_ref="grid_A" operation="min"
 freq_op="1d">@field_A</field>
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

This example shows how to compute the minimum on 7 days of the daily average
 of 
\begin_inset Quotes eld
\end_inset

field_A
\begin_inset Quotes erd
\end_inset

.
 In this context, the 
\series bold
freq_op
\series default
 attribute defines the period of the temporal operation for all fields pointed
 with the 
\begin_inset Quotes eld
\end_inset

@
\begin_inset Quotes erd
\end_inset

 operator in the expression.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Another use of this feature is to do arithmetic operations on the result
 of temporal operations.
  The following configuration file for example shows how to output the standard
 deviation for a field on a one day period: 
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <field_definition>
\end_layout

\begin_layout Plain Layout

                <field id="field_A" grid_ref="grid_A" operation="average" />
\end_layout

\begin_layout Plain Layout

                <field id="field_A_square" field_ref="grid_A">field_A * field_A</field>
\end_layout

\begin_layout Plain Layout

                </field_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1d">
\end_layout

\begin_layout Plain Layout

                        <field name="field_A_std_dev" grid_ref="grid_A" operation="instant"
 freq_op="1d">sqrt(@field_A_square - @field_A^2)</field>
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

Note that since an 
\series bold
\emph on

\begin_inset Quotes eld
\end_inset

instant
\begin_inset Quotes erd
\end_inset


\series default
 
\emph default
operation is used, 
\series bold
freq_op
\series default
 and 
\series bold
output_freq
\series default
 are identical in this scenario.
\end_layout

\begin_layout Section
How to access the data of a field
\end_layout

\begin_layout Standard
In order not to waste memory, the instant data of a field can be read from
 the model only if:
\end_layout

\begin_layout Itemize
it is part of a file whose attribute 
\series bold
mode
\series default
 is 
\series bold
\emph on

\begin_inset Quotes eld
\end_inset

read
\begin_inset Quotes erd
\end_inset


\end_layout

\begin_layout Itemize
or its attribute 
\series bold
read_access
\series default
 is set to 
\series bold
\emph on

\begin_inset Quotes eld
\end_inset

true
\begin_inset Quotes erd
\end_inset


\series default
\emph default
.
\end_layout

\begin_layout Standard
In any other case, trying to access the field data would cause an error.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The following configuration file:
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="input" output_freq="1ts">
\end_layout

\begin_layout Plain Layout

                        <field id="field_A" grid_ref="grid_A" operation="instant" />
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

can be used with this Fortran code:
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "language=Fortran,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

DO ts=1,numberOfTimestep
\end_layout

\begin_layout Plain Layout

        ! Get field_A for current timestep
\end_layout

\begin_layout Plain Layout

        CALL xios_recv_field("field_A", field_A) ! field_A must be an allocated
 array with the right size
\end_layout

\begin_layout Plain Layout

        ! Do useful things...
\end_layout

\begin_layout Plain Layout

        ! Inform XIOS of the current timestep
\end_layout

\begin_layout Plain Layout

        CALL xios_update_calendar(ts)
\end_layout

\begin_layout Plain Layout

ENDDO
\end_layout

\end_inset

The call to 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_recv_field
\end_layout

\end_inset

 might block for a while if the data was not yet received from the server(s)
 but it should not happen too often thanks to the prefetching done by XIOS.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

Since the 
\series bold
read_access
\series default
 attribute allows to the access fields which depend directly on data from
 the model, you must be very careful with the order of the 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_send_field
\end_layout

\end_inset

 and 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_recv_field
\end_layout

\end_inset

 calls.
 For example, consider the following configuration file (just a simple example
 as in practice it does not make much sense to use it):
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <field_definition>
\end_layout

\begin_layout Plain Layout

                <field id="field_A" grid_ref="grid_A" operation="instant" />
\end_layout

\begin_layout Plain Layout

                </field_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1ts">
\end_layout

\begin_layout Plain Layout

                        <field id="field_B" grid_ref="grid_A" operation="instant" read_access=
"true">field_A / 42</field>
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

If you call 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_recv_field(
\begin_inset Quotes eld
\end_inset

field_B
\begin_inset Quotes erd
\end_inset

, field_B)
\end_layout

\end_inset

 before 
\begin_inset Flex Code
status open

\begin_layout Plain Layout
xios_send_field(
\begin_inset Quotes eld
\end_inset

field_A
\begin_inset Quotes erd
\end_inset

, field_A)
\end_layout

\end_inset

, the requested data will never be available and a deadlock could occur.
 In practice, XIOS will detect the problem and throw an error.
\end_layout

\begin_layout Section
How to reduce the size of an output file
\end_layout

\begin_layout Standard
The size of the output files can sometimes become a problem.
 XIOS provides some features which may help to reduce the size of the output
 files losslessly.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The first solution is to use the compression feature provided by HDF5 which
 allows a field to be compressed using gzip.
 Since it depends directly on HDF5, this feature works only when the NetCDF-4
 format is used.
 Unfortunately, HDF5 does not support compression (yet) for parallel output
 so you have to use only one server or to engage the 
\series bold
\emph on

\begin_inset Quotes eld
\end_inset

multiple_file
\begin_inset Quotes erd
\end_inset


\series default
\emph default
 mode.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

To enable the gzip compression of a field, you need to set the 
\series bold
compression_level
\series default
 attribute to any integer between 1 and 9 (by default this attribute is
 set to 0 which means that compression is disabled).
 Using an higher compression level should improve the compression ratio
 at the cost of using more processing power.
 Generally using a compression level of 2 should be a good trade-off.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The following example illustrates the use of the gzip compression:
\begin_inset listings
lstparams "language=XML,breaklines=true,tabsize=2,frame=tb,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}}"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <context id="test">
\end_layout

\begin_layout Plain Layout

                <calendar type="Gregorian" timestep="1h" />
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <grid_definition>
\end_layout

\begin_layout Plain Layout

                        <grid id="grid_A"><!-- Definition ommited --></grid>
\end_layout

\begin_layout Plain Layout

                </grid_definition>
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

                <file_definition>
\end_layout

\begin_layout Plain Layout

                        <file name="output" output_freq="1ts" compression_level="2">
\end_layout

\begin_layout Plain Layout

                        <field id="field_A" grid_ref="grid_A" operation="average" compression_
level="4" />
\end_layout

\begin_layout Plain Layout

                        <field id="field_B" grid_ref="grid_A" operation="average" compression_
level="0" />
\end_layout

\begin_layout Plain Layout

                        <field id="field_C" grid_ref="grid_A" operation="average" />
\end_layout

\begin_layout Plain Layout

                        </file>
\end_layout

\begin_layout Plain Layout

                </file_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset

Note that the 
\series bold
compression_level
\series default
 attribute can also be set at a file level, in this case it is inherited
 by all fields of the file unless they explicitly override the attribute.
\begin_inset Newline newline
\end_inset


\begin_inset Newline newline
\end_inset

The second solution is available only if you are using a grid with masked
 values.
 In this case, you can choose to output the indexed grid instead of the
 full grid by setting the 
\series bold
indexed_output
\series default
 attribute to 
\series bold
\emph on

\begin_inset Quotes eld
\end_inset

true
\begin_inset Quotes erd
\end_inset


\series default
\emph default
.
 Missing values are then omitted and extra arrays are outputted so that
 the translation from the 
\begin_inset Quotes eld
\end_inset

compressed
\begin_inset Quotes erd
\end_inset

 indexes to the true indexes can be done.
 Due to those arrays of indexes, indexed output should be considered only
 if there is enough masked values.
 For more details about this feature, please refer to section 8.2 
\begin_inset Quotes eld
\end_inset

Compression by Gathering
\begin_inset Quotes erd
\end_inset

 of the Climate and Forecast (CF) Convention.
\end_layout

\begin_layout Standard
\begin_inset CommandInset include
LatexCommand include
filename "inputs/user/Grid.lyx"

\end_inset


\end_layout

\begin_layout Standard
\begin_inset CommandInset include
LatexCommand include
filename "inputs/user/Domain.lyx"

\end_inset


\end_layout

\begin_layout Standard
\begin_inset CommandInset include
LatexCommand include
filename "inputs/user/Axis.lyx"

\end_inset


\end_layout

\begin_layout Standard
\begin_inset CommandInset include
LatexCommand include
filename "inputs/user/Scalar.lyx"

\end_inset


\end_layout

\begin_layout Standard
\begin_inset CommandInset include
LatexCommand include
filename "inputs/user/Transformations.lyx"

\end_inset


\end_layout

\begin_layout Chapter
XIOS options
\end_layout

\begin_layout Standard
Some of XIOS behaviors can be configured using options.
 Those options must be expressed as variables in a specific context whose
 
\series bold
id
\series default
 must be 
\series bold
\emph on

\begin_inset Quotes eld
\end_inset

xios
\begin_inset Quotes erd
\end_inset


\series default
\emph default
 as shown below.
\end_layout

\begin_layout Standard
\begin_inset listings
lstparams "breaklines=true,frame=tb,language=XML,postbreak={\raisebox{0ex}[0ex][0ex]{\ensuremath{\rcurvearrowse\space}}},tabsize=2"
inline false
status open

\begin_layout Plain Layout

<?xml version="1.0"?>
\end_layout

\begin_layout Plain Layout

<simulation>
\end_layout

\begin_layout Plain Layout

        <!-- Actual context(s) used by the simulation ommited -->
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

        <context id="xios">
\end_layout

\begin_layout Plain Layout

                <variable_definition>
\end_layout

\begin_layout Plain Layout

                        <variable id="option_name" type="option_type">option_value</variable>
\end_layout

\begin_layout Plain Layout

                </variable_definition>
\end_layout

\begin_layout Plain Layout

        </context>
\end_layout

\begin_layout Plain Layout

</simulation>
\end_layout

\end_inset


\end_layout

\begin_layout Section
Buffer related options
\end_layout

\begin_layout Standard
By default, XIOS tries to guess the required buffers sizes to ensure efficient
 client-server communications.
 However it might sometimes be useful to tweak the buffers sizes so XIOS
 provides the following options:
\end_layout

\begin_layout Itemize

\series bold
optimal_buffer_size
\series default
 (type: 
\series bold
string
\series default
) can be either 
\series bold
\emph on

\begin_inset Quotes eld
\end_inset

memory
\begin_inset Quotes erd
\end_inset


\series default
\emph default
 or 
\series bold
\emph on

\begin_inset Quotes erd
\end_inset

performance
\begin_inset Quotes erd
\end_inset


\series default
\emph default
.
 When using the 
\series bold
\emph on

\begin_inset Quotes eld
\end_inset

memory
\begin_inset Quotes erd
\end_inset


\series default
\emph default
 mode, XIOS will try to use buffers as small as possible while still ensuring
 that the bigger message will fit.
 When using the 
\series bold
\emph on

\begin_inset Quotes erd
\end_inset

performance
\begin_inset Quotes erd
\end_inset


\series default
\emph default
 mode, XIOS will ensure that all active fields can be buffered without having
 to flush the buffers.
 This mode is used by default since it allows more asynchronous and thus
 better performance at the cost of being quite memory hungry.
\end_layout

\begin_layout Itemize

\series bold
minimum_buffer_size
\series default
 (type: 
\series bold
int
\series default
) defines the minimum buffer size in bytes (8192 by default).
 This value will be used by XIOS only for buffers whose detected size is
 smaller than the user defined minimum size.
\end_layout

\begin_layout Itemize

\series bold
buffer_size_factor 
\series default
(type: 
\series bold
int
\series default
) allows to modify the buffers sizes by multiplying the detected sizes by
 an user defined factor (
\begin_inset Formula $1.0$
\end_inset

 by default).
 For each allocated buffers, the used size is defined as 
\begin_inset Formula 
\[
{\scriptstyle used\_size\;=\;\min\left(minimum\_buffer\_size,\;detected\_size\;\times\;buffer\_size\_factor\right)}
\]

\end_inset


\end_layout

\end_body
\end_document