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In this section we describe how to bring existing applications into the framework.
A Gridded Component is a self-contained piece of code which will be initialized, will be called once or many times to run, and then will be finalized. It will be expected to either take in data from other components/models, produce data, or both.
Generally a computational model like an ocean or atmosphere model will map either to a single component or to a set of multiple nested components.
A component provides data to other components using an ESMF State object. A component should fill the State object with a description of all possible values that it can export. Generally, a piece of code external to the component (the AppDriver, or a parent component) will be responsible for marking which of these items are actually going to be needed. Then the component can choose to either produce all possible data items (simpler but less efficient) or only produce the data items marked as being needed. The component should consult the CF data naming conventions when it is listing what data it can produce.
A component gets data from other components using an ESMF State object. The application developer must figure out how to get any required fields from other components in the application.
A component should communicate to other components only through the framework. All global data items should be private to Fortran modules, and ideally should be isolated to a single derived type which is allocated at run time.
A component needs to provide 3 routines which handle initialization, running, and finalization. (For codes which have multiple phases of initialize, run, and finalize it is possible to have multiple initialize, run, and finalize routines.)
The initialize routine needs to allocate space, initialize data items, boundary conditions, and do whatever else is necessary in order to prepare the component to run.
For a sequential application in which all components are on the same set of processors, the run phase will be called multiple times. Each time the model is expected to take in any new data from other models, do its computation, and produce data needed by other components. A concurrent model, in which different components are run on different processors, may execute the same way. Alternatively, it may have its run routine called only once and may use different parts of the framework to arrange data exchange with other models. This feature is not yet implemented in ESMF.
The finalize routine needs to release space, write out results, close open files, and generally close down the computation gracefully.
Components need to provide only a single externally visible entry point. It will be called at start time, and its job is to register with the framework which routines satisfy the initialize, run, and finalize requirements. If it has a single derived type that holds its private data, that can be registered too.
An ESMF State object is fundamentally an annotated list of other ESMF items, most often expected to be ESMF FieldBundles (groups of Fields on the same grid). Other things which can be placed in a State object are Fields, Arrays (raw data with no gridding/coordinate information) and other States (generally used by coupling code). Any data which is going to be received from other components or sent to other components needs to be represented as an ESMF object.
To create an ESMF Field the code must create an ESMF Array object to contain the data values, and usually an ESMF Grid object to describe the computational grid where the values are located. If this is an observational data stream the locations of the data values will be held in an ESMF Location Stream object instead of a Grid.
During the execution of the run routine, information about time is transferred between components through ESMF Clocks. The component needs to be able to at least query a Clock for the current time using framework methods.
The ESMF framework provides a rich set of time management functions, data management and query functions, and other utility routines which help to insulate the user's code from the differences in hardware architectures, system software, and runtime environments. It is up to the user to select which parts of these functions they choose to use.
Select from the set of ESMF components available.
Examine what data is produced by each component and what data is needed by each component. The role of Coupler Components in the ESMF is to set up any necessary regridding and data conversions to match output data from one component to input data in another.
Decide on a strategy for how to do the coupling. There can be a single coupler for the application or multiple couplers. Single couplers follow a "hub and spoke" model. Multiple couplers can couple between subsets of the components, and can be written to couple either only one-way (e.g. output of component A into input of component B), or two-way (both A to B and B to A).
The coupler must understand States, Fields, FieldBundles, Grids, and Arrays and ESMF execution/environment objects such as DELayouts.
The main program can be a copy of a driver found in any of the system_tests sub-directories. The customization needed is to use the correct Component module files, to gain access to the SetServices routines.
Although ESMF provides example source code for the main program, it is not considered part of the framework and can be changed by the user as needed.
The final thing the main program must do is call ESMF_Finalize(). This will close down the framework and release any associated resources.
The main program is responsible for creating a top-level Gridded Component, which in turn creates other Gridded and Coupler Components. We encourage this hierarchical design because it aids in extensibility - the top level Gridded Component can be nested in another larger application. The top-level component contains the main time loop and is responsible for calling the SetServices entry point for each child component it creates.
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