The YAC interface usage

Table of Contents

• The Initialisation Phase
• The Definition Phase
  • Component Definition
  • Grid Definition
    • Ordering of edges
  • Point Definition
  • Decomposition information
  • Definition of Masks
  • Definition of Coupling Fields
  • Definition of Metadata
  • Definition of couples
  • Synchronisation of definitions (optional)
  • Additional Definitions
• End of definitions
• Data Exchange
  • Completition of asynchronous data exchanges
  • Advancing internal clock without data exchange operation
• The Finalisation Phase
  • Restarting YAC

The Initialisation Phase

There are multiple methods to initialise YAC (more details: Initialising YAC). The basic method, which is collective for all processes in MPI_COMM_WORLD is:
yac_cinit ( );	CALL yac_finit ( )	from yac import * yac = YAC()
Alternatively, it is possible to initialise YAC with a previously generated MPI communicator (using for example MPI handshake algorithm).
char const * yac_group_name = "yac"; MPI_Comm yac_comm; yac_cmpi_handshake( MPI_COMM_WORLD, 1, &yac_group_name, &yac_comm ); yac_cinit_comm ( yac_comm );	CHARACTER(len=YAC_MAX_CHARLEN) :: yac_group_name(1) INTEGER :: yac_comm(1) yac_group_name(1) = "yac" CALL yac_fmpi_handshake( & mpi_comm_world, yac_group_name, yac_comm) CALL yac_finit_comm ( yac_comm(1) )
After the initialisation we have the possibility to read in a coupling configuration file.
char const * yaml_filename = "coupling.yaml"; yac_cread_config_yaml(yaml_filename)	CHARACTER(LEN=YAC_MAX_CHARLEN) :: yaml_filename ... yaml_filename = "coupling.yaml" CALL yac_fread_config_yaml(yaml_filename)	yaml_filename = "coupling.yaml" yac.read_config_yaml(yaml_filename)
If not defined in a coupling configuration file, the calendar has to be set. It is required for YAC be able to interpret the start- and end date, and timesteps provided by the field definition.
yac_cdef_calendar(YAC_PROLEPTIC_GREGORIAN)	CALL yac_fdef_calendar(YAC_PROLEPTIC_GREGORIAN)	def_calendar(Calendar.PROLEPTIC_GREGORIAN)
The start- and end date needs to be defined by at least one process in the YAC instance. This information is synchronized across all processes in the YAC instance by the synchronisation of definitions. It can be defined within a coupling configuration file or set through the interface.
const char * start_datetime = "01-01-1850T00:00:00"; const char * end_datetime = "31-12-1850T00:00:00"; // Both arguments are optional (can be NULL) yac_cdef_datetime ( start_datetime, end_datetime );	character(len=YAC_MAX_CHARLEN) :: start_datetime character(len=YAC_MAX_CHARLEN) :: end_datetime start_datetime = '01-01-1850T00:00:00' end_datetime = '31-12-1851T00:00:00' ! Both arguments are optional CALL yac_fdef_datetime ( start_datetime = start_datetime, & end_datetime = end_datetime )	start_datetime = "01-01-1850T00:00:00" end_datetime = "31-12-1850T00:00:00" yac.def_datetime ( start_datetime, end_datetime )
A coupled run configuration may consist of multiple executables or programs, e.g. model_a.x and model_b.x. If the processes of a single excutable have to register multiple components, these processes may required their own individual communicator that contain only the processes of their respective executable in order to be able to determine the component associated to each process. Initialising YAC contains more information on how to handle more complex setups such as the one described above.

The Definition Phase

Component Definition

Each process can be part of zero, one, or more components. The components are identified by a unique component name. The definition of the component associated with each process is a collective call for all processes in the communicator passed to the initialisation routine (or MPI_COMM_WORLD if none was provided) and is called once. For each defined component an ID is returned, which is used in subsequent calls to identify the respective component.
int comp_id; char const * comp_name = "ocean"; yac_cdef_comp ( comp_name, &comp_id );	INTEGER :: comp_id CHARACTER(LEN=YAC_MAX_CHARLEN) :: comp_name comp_name = 'Ocean' CALL yac_fdef_comp ( comp_name, & ! [IN] comp_id ) ! [out]	component = yac.def_comp ( "ocean" )
int const nbr_comps = 2; char const * comp_names[nbr_comps] = { "ocean", "carbon" }; int comp_ids[nbr_comps]; yac_cdef_comps ( comp_names, nbr_comps, comp_ids );	INTEGER :: comp_ids(2) CHARACTER(LEN=YAC_MAX_CHARLEN) :: comp_names(2) comp_names(1) = 'ocean' comp_names(2) = 'carbon' CALL yac_fdef_comps ( 2, & ! [IN] comp_names, & ! [IN] comp_ids ) ! [out]
Once, all components have been defined, a component communicator can be retrieved for each component identified by its local comp ID. In case of python a mpi4py communicator can be obtained.
MPI_Comm comp_communicator; yac_cget_comp_comm ( comp_id, &comp_communicator );	INTEGER :: comp_comm CALL yac_fget_comp_comm ( comp_id, & ! [IN] comp_comm ) ! [out]	comp_communicator = component.comp_comm
In addition, YAC can provide a communicator, which encompasses all processes of a provided list of components. Not all components in this list have to be locally defined. The generation of this communicator is collective for all processes that defined at least one component in this list. The list of component names has to be consistent across all involved processes.
MPI_Comm oce_atm_comm; char const * comp_names[2] = { "ocean", "atmosphere" }; yac_cget_comps_comm ( comp_names, 2, &oce_atm_comm );	INTEGER :: oce_atm_comm CHARACTER(LEN=YAC_MAX_CHARLEN) :: comp_names(2) comp_names(1) = 'ocean' comp_names(2) = 'atmosphere' CALL yac_fget_comps_comm ( comp_names, & ! [IN] 2, & ! [IN] oce_atm_comm ) ! [out]	oce_atm_comm = yac.get_comps_comm(["ocean", "atmosphere"])
In some use-cases it might be difficult to provide all component names at once for the collective component definition. In this case it possible to "pre-define" components. The component ids for these "pre-defined" components are valid, however the component query routines can only be called after collective component definition is finished.
int comp_id; char const * comp_name = "I/O"; yac_cpredef_comp ( comp_name, &comp_id );	INTEGER :: comp_id CHARACTER(LEN=YAC_MAX_CHARLEN) :: comp_name comp_name = 'I/O' CALL yac_fpredef_comp ( comp_name, & ! [IN] comp_id ) ! [out]	component = yac.predef_comp ( "I/O" )

Grid Definition

YAC requires the user to provide basic informations about the grids used in the coupling. This information consists of the geographical locations of all vertices, connectivity information (i.e. which vertices have to be taken to form a particular cell) and information about which line on the sphere the edges follow (great circles or circles of longitude or latitude). Each process has to provide this information only for its local part of the grid. There are grid definition routines available for various grid types. For an unstructed grids geographical locations of the vertices and the connectivity has to be provided explicitly. For this grid type it is assumed that all grid edges follow great circles.
int grid_id; int m,n; int const nbr_vertices = 20; int const nbr_cells = 5; int nbr_vertices_per_cell[nbr_cells]; int cell_to_vertex[nbr_cells*nbr_vertices]; double x_vertices[nbr_vertices]; double y_vertices[nbr_vertices]; char * grid_name = "ocean_grid"; ... yac_cdef_grid_unstruct ( grid_name, nbr_vertices, nbr_cells, nbr_vertices_per_cell, x_vertices, y_vertices, cell_to_vertex, &grid_id );	CHARACTER(LEN=YAC_MAX_CHARLEN) :: grid_name INTEGER, PARAMETER :: nbr_vertices = 20 INTEGER, PARAMETER :: nbr_cells = 5 INTEGER :: grid_id INTEGER :: nbr_vertices_per_cell(nbr_cells) INTEGER :: cell_to_vertex(nbr_cells,nbr_vertices) DOUBLE PRECISION :: x_vertices(nbr_vertices) DOUBLE PRECISION :: y_vertices(nbr_vertices) grid_name = 'ocean_grid' ... CALL yac_fdef_grid ( grid_name, & ! [IN] nbr_vertices, & ! [IN] nbr_cells, & ! [IN] nbr_vertices_per_cell, & ! [IN] x_vertices, & ! [IN] y_vertices, & ! [IN] cell_to_vertex, & ! [IN] grid_id ) ! [out]	nbr_vertices_per_cell = [...] x_vertices = [...] y_vertices = [...] cell_to_vertex = [...] grid = UnstructuredGrid ( "ocean_grid", nbr_vertices_per_cell, x_vertices, y_vertices, cell_to_vertex)

Ordering of edges

Depending on the grid definition routine being used, the local ordering of cells and vertices is given implicitly or explicitly by the user. The ordering of the edges of the grid on the other hand is defined as follows:

Generate for each edge a tuple containing the local indices of the adjacent vertices (lowest index first). Sort this list by the vertex indices; this gives the the local order of edges.

Point Definition

If data do not represent values of the complete cell, it is possible to define sets of points. Here we specify points at some location (in radian) inside of a cell (location can be YAC_LOCATION_CELL, YAC_LOCATION_CORNER, or YAC_LOCATION_EDGE). As for the grid defintion, there are multiple routines for defining points. For an unstructured grid points at the center of each cell may be defined as follows:
int cell_point_id; int nbr_cells = 200; x_points = malloc ( nbr_cells * sizeof(*x_points) ); y_points = malloc ( nbr_cells * sizeof(*y_points) ); ... yac_cdef_points_unstruct ( grid_id, nbr_cells, YAC_LOCATION_CELL, x_points, y_points, &cell_point_id );	INTEGER, PARAMETER :: nbr_points = 5 INTEGER :: point_id DOUBLE PRECISION :: x_points(nbr_points) DOUBLE PRECISION :: y_points(nbr_points) ... CALL yac_fdef_points ( grid_id, & ! [IN] nbr_points, & ! [IN] yac_location_cell, & ! [IN] x_points, & ! [IN] y_points, & ! [IN] point_id ) ! [out]	nbr_cells = 200 x_points = [...] y_points = [...] ... cell_points = grid.def_points ( Location.CELL, x_points, y_points)

Decomposition information

The user can provide global ids for all cell, vertices, and/or edges, if they are available. These have to be consistent across all processes. Otherwise, YAC will generate them. However, the generation of global ids may take some time. Therefore, it is recommended to provide them. Additionally, this makes interpretation of weight files (if they are generated) easier.
int * global_cell_ids; ... yac_cset_global_index ( global_cell_ids, YAC_LOCATION_CELL, grid_id );	INTEGER :: global_cell_ids(nbr_cells) ... CALL yac_fset_global_index ( global_cell_ids, & yac_location_cell, & grid_id )	global_cell_ids = [...] ... grid.set_global_index ( global_cell_ids, Location.CELL)
The core mask allows the user to define cells, vertices, and/or edges that YAC is supposed to ignore, for example because they are halo points that do not contain valid data. These points will not be used as a source or destination in a put/get/exchange operation.
int * is_core_cell; ... yac_cset_core_mask ( is_core_cell, YAC_LOCATION_CELL, grid_id )	INTEGER :: is_core_cell(nbr_cells) ... CALL yac_fset_core_mask ( is_core_cell, & yac_location_cell, & grid_id ) The core mask can be specified as LOGICAL or INTEGER in the Fortran interface (see yac_fset_core_mask (*)).	is_core_cell = [...] ... grid.set_core_mask ( is_core_cell, Location.CELL)

Definition of Masks

While core masks are supposed to allow YAC to differentiate between the compute domain and halo/dummy points. YAC also allows to define additional masks that can set for each field individually (see here). These masks can be used to specify that YAC should for example only consider coast or coean cells. The is_valid array should have the appropriate size for the number of cells, vertices, or edges of the respective grid.
int coast_mask_id; int * is_coast_cell; ... yac_cdef_mask ( grid_id, nbr_cells, YAC_LOCATION_CELL, is_coast_cell, &coast_mask_id );	INTEGER :: coast_mask_id INTEGER :: is_coast_cell(nbr_cells) ... CALL yac_fdef_mask ( grid_id, & ! [IN] nbr_cells, & ! [IN] yac_location_cell, & ! [IN] is_coast_cell, & ! [IN] coast_mask_id ) ! [out]	is_coast_cell = np.array( ... ) coast_mask = grid.def_mask ( Location.CELL, is_coast_cell)
It is also possible to assign a default mask to a set of previously define points. The is_valid array should again have the approriate size.
int * is_ocean_cell; ... yac_cset_mask ( is_ocean_cell, cell_point_id );	LOGICAL :: is_ocean_cell (nbr_cells) ... CALL yac_fset_mask ( is_ocean_cell, & ! [IN] cell_point_id ) ! [in]	is_ocean_cell = [...] ... cell_points.set_mask ( is_ocean_cell )
A field usually gets assigned a mask when it is defined. However, masks for the source and target fields can also be defined for each couple either using the user interface or the through the configuration file. In these cases the masks are referenced through their names, which can be set when defining them.
int coast_mask_id; int * is_coast_cell; char const * mask_name = "ocean mask"; ... yac_cdef_mask_named ( grid_id, nbr_cells, YAC_LOCATION_CELL, is_coast_cell, mask_name, &coast_mask_id );	` INTEGER :: coast_mask_id INTEGER :: is_coast_cell(nbr_cells) CHARACTER(LEN=YAC_MAX_CHARLEN) :: mask_name ... mask_name = 'ocean mask' CALL yac_fdef_mask_name ( grid_id, & ! [IN] nbr_cells, & ! [IN] yac_location_cell, & ! [IN] is_coast_cell, & ! [IN] mask_name, & ! [IN] coast_mask_id ) ! [out]	is_coast_cell = np.array( ... ) coast_mask = grid.def_mask ( Location.CELL, is_coast_cell, "ocean mask")

Definition of Coupling Fields

A field consists of one or more point sets and is identified by it name. YAC only supports 2D-fields on the sphere. However if multiple 2D-fields have the same configuration or the associated the field has more than one level, these can be processed in a single step using the collection size. In case a field is configured to be coupled, the point sets have to match with the interpolation stack. Most interpolation methods only support a single point set and some interpolations are limited to certain point locations (for example Conservative interpolation only support points with location YAC_LOCATION_CELL).
int sst_field_id; char const * field_name = "sea_surface_temperature"; int const num_point_sets = 1; int point_ids[num_point_sets] = {cell_point_id}; int collection_size = 1; char const * timestep = "PT15M"; yac_cdef_field ( field_name, comp_id, point_ids, num_point_sets, collection_size, timestep, YAC_TIME_UNIT_ISO_FORMAT, &sst_field_id );	INTEGER :: field_id CHARACTER(LEN=YAC_MAX_CHARLEN) :: field_name INTEGER, PARAMETER :: num_point_sets = 1 INTEGER :: point_ids(num_point_sets) INTEGER, PARAMETER :: collection_size = 1 CHARACTER(LEN=YAC_MAX_CHARLEN) :: timestep field_name = 'sea_surface_temperature' timestep = 'PT15M' point_ids(1) = cell_point_id CALL yac_fdef_field ( field_name, & ! [IN] comp_id, & ! [IN] point_ids, & ! [IN] num_point_sets, & ! [IN] collection_size, & ! [IN] timestep, & ! [IN] yac_time_unit_iso_format, & ! [IN] field_id ) ! [out]	field_name = "sea_surface_temperature" num_point_sets = 1 points = [cell_points] collection_size = 1 timestep = "PT15M" sst_field = Field.create ( field_name, component, points, collection_size, timestep, TimeUnit.ISO_FORMAT)
If the user wants to specify a mask for a field (in addition to the core mask), he has to either set a default mask to the respective points or pass the mask id to the field definition. The location associated with the provided masks has to match with the ones of the points and they have to be based on the same grid. Before a field can be defined, a calendar has to be set.
int sst_field_id; char const * field_name = "sea_surface_temperature"; int const num_point_sets = 1; int point_ids[num_point_sets] = {cell_point_id}; int mask_ids[num_point_sets] = {ocean_mask_id}; int collection_size = 1; char const * timestep = "PT15M"; yac_cdef_field_mask ( field_name, comp_id, point_ids, mask_ids, num_point_sets, collection_size, timestep, YAC_TIME_UNIT_ISO_FORMAT, &sst_field_id );	INTEGER :: field_id CHARACTER(LEN=YAC_MAX_CHARLEN) :: field_name INTEGER, PARAMETER :: num_point_sets = 1 INTEGER :: point_ids(num_point_sets) INTEGER :: mask_ids(num_point_sets) INTEGER, PARAMETER :: collection_size = 1 CHARACTER(LEN=YAC_MAX_CHARLEN) :: timestep field_name = 'sea_surface_temperature' timestep = 'PT15M' point_ids(1) = cell_point_id mask_ids(1) = ocean_mask_id CALL yac_fdef_field_mask ( field_name, & ! [IN] comp_id, & ! [IN] point_ids, & ! [IN] mask_ids, & ! [IN] num_point_sets, & ! [IN] collection_size, & ! [IN] timestep, & ! [IN] yac_time_unit_iso_format, & ! [IN] field_id ) ! [out]
Once a field is defined it can be enabled for dynamic fractional masking (see Dynamic fractional masking). When enabling dynamic fractional masking a fallback value has to be provided. If in an exchange operation the mask for all source points used to interpolate a target point is zero, then this value will be assigned.
int sst_field_id; char const * comp_name = "ocean"; char const * grid_name = "ocean_grid"; char const * field_name = "sea_surface_temperature"; double frac_mask_fallback_value = 0.0; yac_cenable_field_frac_mask ( comp_name, grid_name, field_name, frac_mask_fallback_value );	CHARACTER(LEN=YAC_MAX_CHARLEN) :: comp_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: grid_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: field_name DOUBLE PRECISION :: frac_mask_fallback_value comp_name = 'ocean' grid_name = 'grid_name' field_name = 'sea_surface_temperature' frac_mask_fallback_value = 0.0; CALL yac_fenable_field_frac_mask ( & comp_name, & ! [IN] grid_name, & ! [IN] field_name, & ! [IN] frac_mask_fallback_value) ! [in]	frac_mask_fallback_value = 0.0 yac.enable_field_frac_mask( comp_name, grid_name, field_name, frac_mask_fallback_value)

Definition of Metadata

Once a component, grid, or field is defined, it is possible to attach metadata to it. In YAC metadata comes in the form of a string. It is sufficient if at least one process defines the metadata for a component, grid, or field. If multiple processes define metadata, it has to be consistent across all of these processes. The metadata will be synchronised and made available for all other processes by the the synchronisation of definitions and/or the end of the definition.
char const * comp_metadata = "some metadata"; yac_cdef_component_metadata(comp_name, comp_metadata);	CHARACTER (LEN=256) :: comp_metadata ... comp_metadata = "some metadata" CALL yac_fdef_component_metadata(comp_name, comp_metadata)	comp_metadata = "some metadata" yac.def_component_metadata(comp_name, comp_metadata.encode())
char const * grid_metadata = "some metadata"; yac_cdef_grid_metadata(grid_name, grid_metadata);	CHARACTER (LEN=256) :: grid_metadata ... grid_metadata = "some metadata" CALL yac_fdef_grid_metadata(grid_name, grid_metadata)	grid_metadata = "some metadata" yac.def_grid_metadata(grid_name, grid_metadata.encode())
char const * field_metadata = "some metadata"; yac_cdef_field_metadata( comp_name, grid_name, field_name, field_metadata);	CHARACTER (LEN=256) :: field_metadata ... field_metadata = "some metadata" CALL yac_fdef_field_metadata( & comp_name, grid_name, field_name, field_metadata)	field_metadata = "some metadata" yac.def_field_metadata( comp_name, grid_name, field_name, field_metadata.encode())

Definition of couples

The coupling between two fields can either be defined through a configuration file or through the interface. As for the definition of fields, the definition of couples also requires a previously defined calendar. To define a couple using the interface, the user has to define a interpolation stack.
int interp_stack_id; yac_cget_interp_stack_config( &interp_stack_id ) yac_cadd_interp_stack_config_average( interp_stack_id, YAC_AVG_ARITHMETIC, 1) yac_cadd_interp_stack_config_fixed( interp_stack_id, -1.0 );	INTEGER :: interp_stack_id CALL yac_fget_interp_stack_config( & interp_stack_id ) CALL yac_fadd_interp_stack_config_average( & interp_stack_id, yac_avg_arithmetic, 1) CALL yac_fadd_interp_stack_config_fixed( & interp_stack_id, -1)	interp_stack = InterpolationStack() interp_stack.add_average(AverageReductionType.YAC_AVG, 1) interp_stack.add_fixed( -1.0 )
If a coupling stack is not needed anymore, it can be freed.
yac_cfree_interp_stack_config(interp_stack_id);	CALL yac_ffree_interp_stack_config(interp_stack_id)	del interp_stack
Once the interpolation stack is defined, it can be used in any number of couple definitions. It is sufficient to define a couples on a single process. It will be synchronised to all other processes at synchronisation of definitions and/or the end of the definitions. However, if a couple (identified by the same source and target field) is defined on multiple processes and/or in a configuration file, the definition must be consistent.
char const * src_comp_name = "atmo" char const * src_grid_name = "icon_atmos_grid" char const * tgt_comp_name = "ocean"; char const * tgt_grid_name = "icon_ocean_grid"; char const * field_name = "sea_surface_temperature"; char const * coupling_timestep = "PT15M"; int time_unit = YAC_TIME_UNIT_ISO_FORMAT; int time_reduction = YAC_REDUCTION_TIME_NONE; int src_lag = 0; int tgt_lag = 0; yac_cdef_couple( src_comp_name, src_grid_name, field_name, tgt_comp_name, tgt_grid_name, field_name, coupling_timestep, time_unit, time_reduction, interp_stack_id, src_lag, tgt_lag);	CHARACTER(LEN=YAC_MAX_CHARLEN) :: src_comp_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: src_grid_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: tgt_comp_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: tgt_grid_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: field_name INTEGER :: time_unit CHARACTER(LEN=YAC_MAX_CHARLEN) :: timestep INTEGER :: time_reduction src_comp_name = 'atmo' src_grid_name = 'icon_atmos_grid' tgt_comp_name = 'ocean' tgt_grid_name = 'icon_ocean_grid' field_name = 'sea_surface_temperature' time_unit = yac_time_unit_iso_format timestep = 'PT15M' time_reduction = yac_reduction_time_none CALL yac_fdef_couple ( & src_comp_name, src_grid_name, field_name, & tgt_comp_name, tgt_grid_name, field_name, & time_unit, timestep, time_reduction, interp_stack_id)	src_comp_name = "atmo" src_grid_name = "icon_atmos_grid" tgt_comp_name = "ocean" tgt_grid_name = "icon_ocean_grid" field_name = "sea_surface_temperature" coupling_timestep = "PT15M" time_unit = TimeUnit.ISO_FORMAT time_reduction = Reduction.INSTANT yac.def_couple( src_comp_name, src_grid_name, field_name, tgt_comp_name, tgt_grid_name, field_name, coupling_timestep, time_unit, time_reduction, interp_stack)
YAC provides a set of additional parameters like weight file name and scaling factor, which can be set for each couple. While for Fortran and Python this is implemented as optional arguments, for C the extended coupling configuration has to be used:
char const * src_comp_name = "atmo" char const * src_grid_name = "icon_atmos_grid" char const * tgt_comp_name = "ocean"; char const * tgt_grid_name = "icon_ocean_grid"; char const * field_name = "sea_surface_temperature"; char const * coupling_timestep = "PT15M"; int time_unit = YAC_TIME_UNIT_ISO_FORMAT; int time_reduction = YAC_REDUCTION_TIME_NONE; int src_lag = 0; int tgt_lag = 0; // additional coupling parameters int ext_couple_config; char const * weight_file_name = "weights.nc" double celcius2kelvin = 273.15; int mapping_on_source = 1; char const * const src_mask_name = "ocean mask" yac_cget_ext_couple_config(&ext_couple_config); // activate writing of weight files and // set weight file name yac_cset_ext_couple_config_weight_file( ext_couple_config, weight_file_name); // activation conversion from degree Celcius to Kelvin yac_cset_ext_couple_config_scale_summand( ext_couple_config, celcius2kelvin); yac_cset_ext_couple_config_mapping_side( ext_couple_config, mapping_on_source); yac_cset_ext_couple_config_src_mask_names( ext_couple_config_id, 1, &src_mask_name) yac_cdef_couple_custom( src_comp_name, src_grid_name, field_name, tgt_comp_name, tgt_grid_name, field_name, coupling_timestep, time_unit, time_reduction, interp_stack_id, src_lag, tgt_lag, ext_couple_config); yac_cfree_ext_couple_config(ext_couple_config);	CHARACTER(LEN=YAC_MAX_CHARLEN) :: src_comp_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: src_grid_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: tgt_comp_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: tgt_grid_name CHARACTER(LEN=YAC_MAX_CHARLEN) :: field_name INTEGER :: time_unit CHARACTER(LEN=YAC_MAX_CHARLEN) :: timestep INTEGER :: time_reduction CHARACTER(LEN=YAC_MAX_CHARLEN) :: weight_file_name DOUBLE PRECISION :: celcius2kelvin INTEGER :: mapping_on_source CHARACTER(LEN=YAC_MAX_CHARLEN) :: weight_file_name TYPE(yac_string) :: src_mask_name(1) src_comp_name = 'atmo' src_grid_name = 'icon_atmos_grid' tgt_comp_name = 'ocean' tgt_grid_name = 'icon_ocean_grid' field_name = 'sea_surface_temperature' time_unit = yac_time_unit_iso_format timestep = 'PT15M' time_reduction = yac_reduction_time_none ! additional coupling parameters weight_file_name = 'weights.nc' celcius2kelvin = 273.15 mapping_on_source = 1 src_mask_name(1)%string = 'ocean mask' CALL yac_fdef_couple ( & src_comp_name, src_grid_name, field_name, & tgt_comp_name, tgt_grid_name, field_name, & time_unit, timestep, time_reduction, interp_stack_id, & weight_file = weight_file_name, & mapping_side = mapping_on_source, & scale_summand = celcius2kelvin, & src_mask_names = src_mask_name)	src_comp_name = "atmo" src_grid_name = "icon_atmos_grid" tgt_comp_name = "ocean" tgt_grid_name = "icon_ocean_grid" field_name = "sea_surface_temperature" coupling_timestep = "PT15M" time_unit = TimeUnit.ISO_FORMAT time_reduction = Reduction.INSTANT src_mask_name = ["ocean mask"] # additional coupling parameters weight_file_name = "weights.nc" celcius2kelvin = 273.15 mapping_on_source = 1 yac.def_couple( src_comp_name, src_grid_name, field_name, tgt_comp_name, tgt_grid_name, field_name, coupling_timestep, time_unit, time_reduction, interp_stack, weight_file = weight_file_name, mapping_side = mapping_on_source, scale_summand = celcius2kelvin, src_masks_names = src_mask_name)

Synchronisation of definitions (optional)

Once all components, grids and fields are defined, the configuration can be synchronized explicitly between all processes. This is optional and can be done by only a subset of all tasks, and is done implicitly by the end of definitions. Afterwards, the local configuration contains the all definitions from all other processes that were done until their synchronisation of definitions or end of definitions (in case they did not explicitly synchronised their definitions).
yac_csync_def();	CALL yac_fsync_def()	yac.sync_def()

Additional Definitions

After the synchronisation of definitions, processes have the option to query the definitons done by all processes. YAC provides a number of routines for this task.
int nbr_comps = yac_cget_nbr_comps(); char const ** comp_names = malloc((size_t)nbr_comps * sizeof(*comp_names)); yac_cget_comp_names(nbr_comps, comp_names); for (int comp_idx = 0; comp_idx < nbr_comps; ++comp_idx) { char const * comp_metadata = yac_cget_component_metadata(comp_names[comp_idx]); if (comp_metadata) printf("%s (%s)\n", comp_names[comp_idx], comp_metadata); int nbr_grids = yac_cget_comp_nbr_grids(comp_names[comp_idx]); char const ** grid_names = malloc((size_t)nbr_grids * sizeof(*grid_names)); yac_cget_comp_grid_names(comp_names[comp_idx], nbr_grids, grid_names); for (int grid_idx = 0; grid_idx < nbr_grids; ++grid_idx) { char const * grid_metadata = yac_cget_grid_metadata(grid_names[grid_idx]); if (grid_metadata) printf( "%s::%s (%s)\n", comp_names[comp_idx], grid_names[grid_idx], grid_metadata); int nbr_fields = yac_cget_nbr_fields(comp_names[comp_idx], grid_names[grid_idx]); char const ** field_names = malloc((size_t)nbr_fields * sizeof(*field_names)); yac_cget_field_names( comp_names[comp_idx], grid_names[grid_idx], nbr_fields, field_names); for (int field_idx = 0; field_idx < nbr_fields; ++field_idx) { char const * field_metadata = yac_cget_field_metadata( comp_names[comp_idx], grid_names[grid_idx], field_names[field_idx]); if (field_metadata) printf( "%s::%s::%s (%s)\n", comp_names[comp_idx], grid_names[grid_idx], field_names[field_idx], field_metadata); char const * field_timestep = yac_cget_field_timestep( comp_names[comp_idx], grid_names[grid_idx], field_names[field_idx]); int field_collection_size = yac_cget_field_collection_size( comp_names[comp_idx], grid_names[grid_idx], field_names[field_idx]); int field_role = yac_cget_field_role( comp_names[comp_idx], grid_names[grid_idx], field_names[field_idx]); printf( "%s::%s::%s timestep: %s " "collection_size: %d role: src(%c) tgt(%c) " "none(%c)\n", comp_names[comp_idx], grid_names[grid_idx], field_names[field_idx], field_timestep, field_collection_size, (field_role == YAC_EXCHANGE_TYPE_SOURCE)?'X':'O', (field_role == YAC_EXCHANGE_TYPE_TARGET)?'X':'O', (field_role == YAC_EXCHANGE_TYPE_NONE)?'X':'O'); } free(field_names); } free(grid_names); } free(comp_names);	INTEGER :: comp_idx, grid_idx, field_idx TYPE(yac_string), ALLOCATABLE :: comp_names(:) TYPE(yac_string), ALLOCATABLE :: grid_names(:) TYPE(yac_string), ALLOCATABLE :: field_names(:) CHARACTER(len=:), ALLOCATABLE :: field_timestep INTEGER :: field_collection_size INTEGER :: field_role comp_names = yac_fget_comp_names() DO comp_idx = 1, SIZE(comp_names) IF (yac_fcomponent_has_metadata( & comp_names(comp_idx)%string)) & WRITE (*, "(A,' (',A,')')") & comp_names(comp_idx)%string, & yac_fget_component_metadata( & comp_names(comp_idx)%string) grid_names = & yac_fget_comp_grid_names( & comp_names(comp_idx)%string) DO grid_idx = 1, SIZE(grid_names) IF (yac_fgrid_has_metadata( & grid_names(grid_idx)%string)) & WRITE (*, "(A,'::',A,' (',A,')')") & comp_names(comp_idx)%string, & grid_names(grid_idx)%string, & yac_fget_grid_metadata( & grid_names(grid_idx)%string) field_names = & yac_fget_field_names( & comp_names(comp_idx)%string, & grid_names(grid_idx)%string) DO field_idx = 1, SIZE(field_names) IF (yac_ffield_has_metadata( & comp_names(comp_idx)%string, & grid_names(grid_idx)%string, & field_names(field_idx)%string)) & WRITE (*, "(A,'::',A,'::',A,' (',A,')')") & comp_names(comp_idx)%string, & grid_names(grid_idx)%string, & field_names(field_idx)%string, & yac_fget_field_metadata( & comp_names(comp_idx)%string, & grid_names(grid_idx)%string, & field_names(field_idx)%string) field_timestep = & yac_fget_field_timestep( & comp_names(comp_idx)%string, & grid_names(grid_idx)%string, & field_names(field_idx)%string) field_collection_size = & yac_fget_field_collection_size( & comp_names(comp_idx)%string, & grid_names(grid_idx)%string, & field_names(field_idx)%string) field_role = & yac_fget_field_role( & comp_names(comp_idx)%string, & grid_names(grid_idx)%string, & field_names(field_idx)%string) WRITE(*,"(A,'::',A,'::',A,' timestep: ',A, & ' collection_size: ',I0, & ' role: src(',A,') tgt(',A, & ') none(',A,')')") & comp_names(comp_idx)%string, & grid_names(grid_idx)%string, & field_names(field_idx)%string, & field_timestep, field_collection_size, & merge('X', 'O',field_role .eq. & yac_exchange_type_source), & merge('X', 'O',field_role .eq. & yac_exchange_type_target), & merge('X', 'O',field_role .eq. & yac_exchange_type_none) DEALLOCATE(field_timestep) DEALLOCATE(field_names(field_idx)%string) END DO DEALLOCATE(field_names) DEALLOCATE(grid_names(grid_idx)%string) ENDDO DEALLOCATE(grid_names) DEALLOCATE(comp_names(comp_idx)%string) ENDDO DEALLOCATE(comp_names)	for comp_name in yac.component_names: comp_metadata = yac.get_component_metadata(comp_name) if comp_metadata is not None: print(f"{comp_name} ({comp_metadata})") for grid_name in yac.get_comp_grid_names(comp_name): grid_metadata = yac.get_grid_metadata(grid_name) if grid_metadata is not None: print(f"{comp_name}::{grid_name} ({grid_metadata})") for field_name in yac.get_field_names(comp_name, grid_name): field_metadata = \ yac.get_field_metadata(comp_name, grid_name, field_name) if field_metadata is not None: print(f"{comp_name}::{grid_name}::{field_name} " + f"({field_metadata})") field_timestep = \ yac.get_field_timestep(comp_name, grid_name, field_name) field_collection_size = \ yac.get_field_collection_size(comp_name, grid_name, field_name) print(f"{comp_name}::{grid_name}::{field_name} " + f"timestep: {field_timestep} " + f"collection_size: {field_collection_size}")
Even after synchronisation of definition the processes can define additional grids, fields, and couples. However, these definitions will not be synchronised with the other processes until the end of definitions. These additions definitions can be used for example by output components, that first want to query what the model components have defined. They can then use this information to decide which data should be sent to the output and define the respective couples accordingly.

End of definitions

Once all processes have finished their definitions they have to collectively end the definition phase. At first, the definitions are synchronised between the processes and checked for consistency. Afterwards all internal data required for the exchanges is generated. This includes the generation of communication matrices, the computation of interpolation weights, and the reading/writing of weight files (if applicable).
yac_cenddef ( );	CALL yac_fenddef ( )	yac.enddef ( )

Data Exchange

For sending of data YAC provides a put operation. If mapping_on_source is used, this call is collective for all source processes. The sending of data to the target processes is done using non-blocking MPI function calls. Thus, the routine returns even if the data have not been transfered to the receiver. The data are buffered internally and the user is free to reuse the send_field buffer for a next message. However, a put will always wait until the previous put for the same field has been completed. Depending on the coupling configuration, the data provided through the put operation may be received by one or more get operations. In case no couple is defined for the field, the put can even return without any data being sent.
int const nbr_hor_points = 512; int const nbr_pointsets = 1; int const collection_size = 4; int info; int ierror; double *** send_field = malloc(collection_size * sizeof(*send_field)); for (int i = 0; i < collection_size; i++) { send_field[i] = malloc((size_t)nbr_pointsets * sizeof(**send_field)); for (int j = 0; j < nbr_pointsets; j++) send_field[i][j] = malloc((size_t)nbr_hor_points * sizeof(***send_field)); } for (int i = 0; i < collection_size; i++) for (int j = 0; j < nbr_pointsets; j++) for (int k = 0; k < nbr_hor_points; k++) send_field[i][j][k] = ... ; ... yac_cput ( field_id, collection_size, send_field, &info, &ierror );	INTEGER, PARAMETER :: nbr_hor_points = 512 INTEGER, PARAMETER :: collection_size = 4 INTEGER, PARAMETER :: nbr_pointsets = 1 INTEGER :: info INTEGER :: ierror DOUBLE PRECISION :: send_field(nbr_hor_points, & nbr_pointsets, & collection_size) DO i = 1, collection_size DO j = 1, nbr_pointsets DO k = 1, nbr_hor_points send_field(k,j,i) = ... END DO END DO END DO ... CALL yac_fput ( field_id, & ! [IN] nbr_hor_points, & ! [IN] nbr_pointsets, & ! [IN] collection_size, & ! [IN] send_field, & ! [IN] info, & ! [OUT] ierror ) ! [out]	nbr_hor_points = 512 nbr_pointsets = 1 collection_size = 4 send_field = np.empty(shape=(collection_size, nbr_pointsets, nbr_hor_points)) # ... fill send_field somehow info = field.put (send_field)
The receiving of data is implemented as a get operation. YAC provides a sychronous and an asynchronous version. The sychronous one will return after all required data has been received. The asynchronous version will return immediately and will receive the data in the background. Only after the user has made sure that an asynchronous get operation has been completed, it is save to access the buffers provided to the respective get call (see Completition of asynchronous data exchanges). If no couple is defined for the field, the get will return without any changes to the receiving field data array.
int const nbr_hor_points = 1024; int const collection_size = 4; int info; int ierror; double ** recv_field = malloc((size_t)collection_size * sizeof(*recv_field)); for (int i = 0; i < collection_size; ++i) recv_field[i] = malloc((size_t)nbr_hor_points * sizeof(**recv_field)); yac_cget ( field_id, collection_size, recv_field, &info, &ierror );	INTEGER, PARAMETER :: nbr_hor_points = 1024 INTEGER, PARAMETER :: collection_size = 4 INTEGER :: info INTEGER :: ierror DOUBLE PRECISION recv_field(nbr_hor_points, collection_size) CALL yac_fget ( field_id, & ! [IN] nbr_hor_points, & ! [IN] collection_size, & ! [IN] recv_field, & ! [OUT] info, & ! [OUT] ierror ) ! [out]	nbr_hor_points = 1024 collection_size = 4 recv_field = np.empty(shape=(collection_size, nbr_hor_points)) recv_field, info = field.get ( recv_field ) Preallocating the buffer is optional. Also None (default argument) can be passed. In that case a buffer in the correct size is allocated. See also Coroutines for an example how to use the python coroutines interface.
In addition to the put and get operation, YAC also provides an exchange operation. This can be used in case of a bi-directional exchange. It executes a put and get operation at the same time and returns after both have been completed. By combining both operations, the internal buffer usage is more efficient.
int const send_nbr_hor_points = 512; int const send_nbr_pointsets = 1; int const recv_nbr_hor_points = 1024; int const collection_size = 4; int send_info; int recv_info; int ierror; double *** send_field = malloc((size_t)collection_size * sizeof(*send_field)); double ** recv_field = malloc((size_t)collection_size * sizeof(*recv_field)); for (int i = 0; i < collection_size; i++) { send_field[i] = malloc(send_nbr_pointsets * sizeof(**send_field)); recv_field[i] = malloc( (size_t)recv_nbr_hor_points * sizeof(**recv_field)); for (int j = 0; j < send_nbr_pointsets; j++) send_field[i][j] = malloc(send_nbr_hor_points * sizeof(***send_field)); } for (int i = 0; i < collection_size; i++) for (int j = 0; j < send_nbr_pointsets; j++) for (int k = 0; k < send_nbr_hor_points; k++) send_field[i][j][k] = ... ; ... yac_cexchange ( send_field_id, recv_field_id, collection_size, send_field, recv_field, &send_info, &recv_info, &ierror );	INTEGER, PARAMETER :: send_nbr_hor_points = 512 INTEGER, PARAMETER :: send_nbr_pointsets = 1 INTEGER, PARAMETER :: recv_nbr_hor_points = 1024 INTEGER, PARAMETER :: collection_size = 4 INTEGER, PARAMETER :: collection_size = 4 INTEGER :: send_info INTEGER :: recv_info INTEGER :: ierror DOUBLE PRECISION :: send_field(nbr_hor_points, & nbr_pointsets, & collection_size) DOUBLE PRECISION recv_field(nbr_hor_points, & collection_size) DO i = 1, collection_size DO j = 1, nbr_pointsets DO k = 1, nbr_hor_points send_field(k,j,i) = ... END DO END DO END DO ... CALL yac_fexchange ( send_field_id, & ! [IN] recv_field_id, & ! [IN] send_nbr_hor_points, & ! [IN] send_nbr_pointsets, & ! [IN] recv_nbr_hor_points, & ! [IN] collection_size, & ! [IN] send_field, & ! [IN] recv_field, & ! [OUT] send_info, & ! [OUT] recv_info, & ! [OUT] ierror ) ! [out]

Completition of asynchronous data exchanges

There are two utility routines that help working with asynchronous data exchanges.
The test operation checks whether for a provided field an asynchronous data exchange operation is still active.
int flag; yac_ctest(field_id, &flag); if (!flag) fputs("Last async operation is still active.", stdout);	LOGICAL :: flag ... CALL yac_ftest ( field_id, flag ) IF (.NOT. flag) print *, "Last async operation is still active."	if field.test() == 0: print("Last async operation is still active")
The second utility routine is the wait operation. It does not return until all active asynchronous data exchanges associated with the provided field have been completed. (This can easily result in a deadlock. The user has to make sure, that this does not occure.)
yac_cwait(field_id); fputs("Last async operation has been completed.", stdout);	CALL yac_fwait ( field_id ) print *, "Last async operation has been completed."	field.wait() print("Last async operation has been completed.") See also Coroutines for an example how to use the python coroutines interface.

Advancing internal clock without data exchange operation

YAC has for each field an internal event timer. With each put, get, or exchange operation call it is advanced according to the field time step provided in the field definition. However, sometime it may be desirable to advance the internal clock without calling a data exchange operation, if it is save to do so. For each field it is possible to check the action that will occure in the next call of a data exchange operation. If appropriate, the update operation can be executed instead of a data exchange operation.
int action; yac_cget_action(field_id, action); if (action == YAC_ACTION_NONE) { yac_cupdate(field_id); } else { // do data exchange ... }	INTEGER :: action CALL yac_fget_action(field_id, action) IF (action == yac_action_none) THEN CALL yac_fupdate(field_id) ELSE ! do data exchange ... END IF	if field.action == yac.Action.NONE: field.update() else: # do data exchange ...

The Finalisation Phase

Once all exchanges have been completed, YAC has to be finalised, which frees all memory allocated by YAC.
yac_cfinalize ( );	CALL yac_ffinalize ( )	del yac
In case MPI_Init was initialised by YAC, MPI_Finalize will be called in this finalisation phase. If the user has called MPI_Init himself (before the YAC initialisation), he also has to call MPI_Finalize once the finalisation phase has been completed.

Restarting YAC

It is possible to restart YAC. To do that the user has to clean up YAC in the finalisation phase instead of finalising it.
yac_ccleanup ( );	CALL yac_fcleanup ( )
After the clean up, the user can restart YAC by going through the Initialisation, Definition, and Data Exchange Phase as before. The restarted YAC can have a different configuration than the previous one. In case the user initialised yaxt himself, he has to finalise it after the clean up in order to be able initialise YAC again.