External fields

The LFRic infrastructure provides field objects which hold field data in a form useful to the infrastucture. Other models, libraries and tooling will hold field data in a different form, useful to their needs. Sometimes it is necessary to transfer data between these different representations when linking models together.

The mechanism on offer to handle these occasions is the “External field.”

Design

The LFRic field section described how the infrastructure goes to some lengths to protect the raw field data from interference. This provides a lot of value in the form of reliability and maintainability.

Transforming field data for an external user must, by definition, compromise that protection. The concept of external fields exists to minimise the scope of that vulnerability.

Each external field is, in fact, a mapping between an LFRic field and a non-LFRic field. It contains any encapsulation breakage within itself, thus preventing the internal data of fields from being exposed.

Fig. 2 Class diagram showing an external field linking an LFRic field with another type of field.

Although the class diagram above shows a mapping between two field classes there is no reason why the external field needs be an actual object. In the case of coupling through OASIS the external “field” is, in fact, a call to the transmit or receive functions of OASIS.

Implementing an External Field

Developing a new external field should not be too hard. The most complicated part is working out how the external field data is presented.

Start by deriving a new class from the abstract external field class.

Due to the way Fortran handles object construction plain initialiser routines are used instead but they can be treated like constructors.

The initialiser of the new class should take an LFRic field pointer and some unique reference to the external target. This may be the target’s field representation (class or primitive array) or some address or ID string. Whatever is appropriate.

The initialiser should call the abstract initialiser with the LFRic field pointer. The external reference should be stored for future use.

The two deferred methods (copy_from_lfric and copy_to_lfric) then need to be implemented. This is where the work happens.

They both use the field proxy object to gain access to the field’s contents and either populate the external field with that content or update the content from the external field.