Clarity on dependencies in host metadata

[grantfirl]

With the new "dependencies" value of the [ccpp-table-properties] tag, what exactly needs to be listed there? The documentation is not super clear.

For example, for host metadata for the IAP model, host modules with metadata have many USE statements. I am listing those in the metadata. But, what if those dependency modules have further dependencies? Do the list of dependencies need to be recursive? Do we need to list dependencies for all subroutines in a host module even though they may not be used any CCPP physics schemes?

Any clarity would be helpful. I'm assuming that these dependencies are used to construct makefile snippets for the build system, so does what gets listed as "dependencies" also depend on the existing host build system (i.e. order of what gets compiled first, etc.)?

[gold2718]

I do not really see the point of using the dependencies keyword in host model metadata.

My understanding of the need for this feature (requested by NOAA) is to tell the build system what modules (files) need to be compiled when a particular scheme is used in a suite. Therefore, for a scheme, I think the complete list needs to be present, however, I would expect that a host model already knows what modules to compile for internal (non-physics-suite) use.

[climbfuji]

This has to do with the fact that the host model - in the ccpp_prebuild world - needs to allocate the memory for internal physics variables. This includes DDTs, and the host model needs to know about the DDTs. When the Fortran code is compiled, the compiler sees references to a certain DDT and will complain if it can't find it. Even if that particular scheme is not used in the selected suites (and the variable not allocated to save memory).

To answer Grant's question, yes, dependencies are recursive. If a compiler sees a use statement then it will try to compile the module referenced there, which may also contain use statements. The host model build system uses this information to compile a list of files that need to be compiled. Modern build systems like cmake+make are able to figure out the order automatically (in most cases at least).

[gold2718]

So I guess the answer (host to use the dependencies keyword in host-model metadata) is completely host-model specific and is not really a CCPP Framework issue.

[grantfirl]

@gold2718 Agreed. I was asking here under the "host model integration" tag because I thought the answer might be of future use to others.