Task #988

Definition of "public API"

Added by Teemu Murtola over 8 years ago. Updated over 1 year ago.

core library
Target version:


One of the goals stated on the wiki for 5.0 is for libgromacs to behave more like a real library. To this end, we should think about what we want to expose as the "public API" of libgromacs and how fixed are we ready to keep those parts between releases.

There are at least the following aspects to consider:
  • Which symbols (classes/functions/types) we want to export from our shared library/libraries? (see #701)
  • Which symbols we document as being part of our public API? (see Doxygen documentation instructions in the Doxygen-generated documentation for the current approach)
  • Which symbols are declared in installed headers?
    • Should symbols that are not part of a public API, but are declared in installed headers, be within a separate namespace such as internal or detail?
  • Which symbols are used in our executables?
  • What is the division between the library and the executables?
  • Which symbols are used in our unit tests?

This issue is mostly about discussing the above aspects. The outcome should be a clear definition of how the above-mentioned aspects should relate to each other in our code.

A few points to support the discussion:
  • Symbols used in installed binaries must be exported.
  • Symbols used in unit tests should be exported to avoid a lot of headaches and build system complications.
  • If we have our library internally divided into modules, it may also make a lot of sense to unit test also some of the interfaces between modules, even if they are not public.
  • Currently, with the exception of mdrun and gmx view, essentially all the code for the programs is within libgromacs, and the executable is only a shell. If we move more content into the executables, can we support things like Python bindings to the analysis tools?

Related issues

Related to GROMACS - Task #701: Add symbol visibility macrosNew
Related to GROMACS - Bug #999: add MYLIB_EXPORT for public APIClosed09/03/2012
Related to GROMACS - Task #1013: Library division for tools and generic Gromacs codeClosed09/30/2012
Related to GROMACS - Task #2045: API design and language bindingsNew
Related to GROMACS - Task #2698: gmxapi documentation integrationIn Progress
Related to GROMACS - Task #2912: C++ extension module for Python bindingsClosed
Related to GROMACS - Feature #3034: Python gmxapi exception hierarchyClosed

Associated revisions

Revision 91ea2482 (diff)
Added by Paul Bauer over 1 year ago

Removes checker for installed files

As the plan is to remove the current handling of installed header files,
this removes the checker parts from the doxygen check-source script for
this attribute.

Refs #2382, #2139, #988

Change-Id: I76b519f39a5c793c9f1ea8c1eb5eebb39b4a9352

Revision 52e3d670 (diff)
Added by Paul Bauer over 1 year ago

Remove installed headers from CMake

Removed the gmx_install_header sections from CMake files, as well as the
CMake code used to add and check them.

Refs #2382, #2139, #988

Change-Id: I4525ea14d2967f83d940300daeb2ade08717ed5d

Revision 393f214e (diff)
Added by Paul Bauer over 1 year ago

Remove section checking api level

Removes the check for api levels in check-source to allow modernization
without having to account for the legacy api layout.

Refs #988

Change-Id: I12cd3f6765bc57801ff4dd81583b7836dc4f18fb

Revision feba5ffb (diff)
Added by Christoph Junghans over 1 year ago

cmake: add option to install legacy headers

This is needed for package like VOTCA to use the C-API until
a real GMX API got established.

Refs #988

Change-Id: I87b6f81641cb5bcac5ceb6ff6fb3dbd9e6650ea5


#1 Updated by Teemu Murtola over 8 years ago

One slightly related issue for Doxygen documentation is that we should also consider how can one find the documentation for the public API. Currently the Doxygen setup builds a sepatate "public API" documentation that only includes a subset of functions and classes. There is also a custom \inpublicapi tag that groups all public API functions and classes under one page. For details, see

The main issue with this setup is that \inpublicapi conflicts with tags like \ingroup module_utility for most other items except classes and structs. So a function can only be specified either as being part of a public API, or as belonging to a module (i.e., a subdirectory under src/gromacs/). I think that in many cases, it may be more useful to be able to see an overview of everything that is part of a module than the whole public API. It could be enough that we have that separate "public API" documentation: if a function appears there and does not have an explicit comment that it does not belong to the public API, then it is in public API.

So for this part, I think there are at least these three alternatives:
  • Follow the current guideline, which suggests using \inpublicapi everywhere, and leave the \ingroup away if it is not allowed. It also suggests to document things such that everything that appears in installed headers also appears in the public API documentation. The original rationale for this approach was that if someone is browsing through the installed headers, it could be nice to explain everything that is there. But I now think that it may be better to not include this in the public API documentation; the Doxygen comments are still there in the headers if someone wants to read them.
  • Prefer \ingroup over \inpublicapi. Still make everything from installed headers appear in the public API docs, with explicit comments for those members that are in installed headers only for implementation purposes.
  • Prefer \ingroup over \inpublicapi. Only make those members of installed headers that are actually in the public API to appear in the public API documentation.

For the last two, we may want to remove \inpublicapi completely as it may not be that useful. The main benefit that it adds in these cases (for classes only) is that there is a nice "Public API" tag just under the class name. This may be useful when viewing more complete documentation, where also other functions than just public API is included.

Whatever is decided here for \inpublicapi, a similar \inlibraryapi construct should probably follow suit.

#2 Updated by Teemu Murtola over 8 years ago

  • Description updated (diff)

Updated the description with one more point to consider.

#3 Updated by Teemu Murtola almost 7 years ago

  • Tracker changed from Task to Feature
  • Project changed from Source code reorganization to GROMACS
  • Description updated (diff)
  • Category set to core library
  • Target version changed from 5.0 to future

The discussion did not happen for 5.0, but the points in the description are still valid. Added one more issue to consider into the list.

#4 Updated by Peter Kasson over 6 years ago

Not sure if this is the best place to bump the discussion, but...

What features do we want for a public API?
Would it make sense to differentiate into a high-level API and a low-level API?
If so, it might be natural to have different stability standards for these two e.g. the high-level API is more stable.

If we want just a single-level public API, any thoughts on which pieces we should prioritize?
The idea of porting analysis tools to use the API is attractive.
Another possibility that would be useful but maybe involve more low-level functionality would be external programs that interface with Gromacs.

#5 Updated by Mark Abraham over 6 years ago

IMO, an external API is what people call when they want to re-use existing code. That could take the form of things like
  • something like mdrun that they write that calls our (parallel) neighbour-search, force, update or constraint routines (and likely paying some memory-copy costs if they want human-convenient data structures)
  • something like an analysis tool that wants access to things like reading trajectories, making selections (but IMO we should target having a Python-extension API sitting on our C++ analysis-framework API)
  • installing some plug-in for mdrun to call back, which might either react to or modify a trajectory in progress (e.g. like NAMD's scripting thing, which people ask for occasionally)

The first point has the most internal benefit, because we can use our own API when testing modules, so one or other implementation can develop from the other. Interest from others is probably divided mostly over the first two points.

For example, the PLUMED implementation of (at least) REST would have been straightforward to implement within mdrun if there was a cleaner separation of responsibilities so that mdrun could maintain n distinct Hamiltonians to apply to the coordinates to suit the algorithm in use. (As it is, they do things like gather the coordinates to send them to another replica where the Hamiltonian exists, re-do domain decomposition, then get the new energy/force.) Conversely, if they'd been able to call API functions to load a .tpr and set up a Hamiltonian, then they'd have been able to organize doing that n times on each replica (or just applying the REST scaling operation to one .tpr loaded), and then just call do_force however they'd like to do it, and then send whatever forces they want into the update stage. Ideally, we'd make available a handle-style implementation so that the client doesn't have to pay anything for data that they don't actually extract (e.g. a REST client needs to get potentials, but would be quite happy for the forces that might be used later for an update to live out on the compute resources where the update might happen).

Such Hamiltonian flexibility probably has other applications too, e.g. less ad-hoc implementation of whatever free-energy methods use foreign-lambda energy evaluations now.

So, to answer Peter's questions I'd say one high-level API that stays highly stable, but I consider the feature sets of mdrun and analysis tools so distinct that I would treat their APIs as separate entities.

And none of this is likely to happen in the near term unless we get people with dedicated support to do it! :-)

#6 Updated by Mark Abraham about 4 years ago

  • Tracker changed from Feature to Task

#7 Updated by Eric Irrgang almost 4 years ago

  • Related to Task #2045: API design and language bindings added

#8 Updated by Eric Irrgang almost 2 years ago

  • Related to Task #2698: gmxapi documentation integration added

#9 Updated by Eric Irrgang almost 2 years ago

Update: The installed headers are deprecated in GROMACS 2019, and an attempt at a well-defined public API has been postponed to allow these questions to be answered unconstrained by technical debt.

Note: This task seems to be waiting for some decisions to be made. What should be the process for approaching and declaring a consensus? One possibility would be to iterate on a document in a git branch and to use the code review machinery.

More detailed discussion of what is in the API, targeted use cases, etc, seem to be in #2045.

#10 Updated by Eric Irrgang almost 2 years ago

  • Related to Task #2912: C++ extension module for Python bindings added

#11 Updated by Eric Irrgang over 1 year ago

  • Related to Feature #3034: Python gmxapi exception hierarchy added

#12 Updated by Mark Abraham over 1 year ago

Side discussion of the possible value in leveraging modern CMake for exporting headers from modules, rather than Doxygen annotation, occured at

#13 Updated by Eric Irrgang over 1 year ago

Mark Abraham wrote:

Side discussion of the possible value in leveraging modern CMake for exporting headers from modules, rather than Doxygen annotation, occured at

Responding to the external discussion:

Yes, and we definitely need more than location. For example, one cannot differentiate between e.g. domdec.h and domdec_internal.h purely on location because they're both in src/gromacs/domdec. Somewhere there has to be an explicit annotation that directs that domdec.h is installed - CMake can do that.

The only common way I know to robustly distinguish between header files in the same directory is with ClangModules, but I don't think we need to introduce that additional machinery. We could annotate headers with CMake properties, but that isn't nearly clear enough and means a whole new pile of arcane infrastructure.

The only check employed by most projects is to build client test code against the installation in the CI testing. Of course, we can and should guide development structurally before edits begin. The easiest solution (and the solution used in other KitWare projects, I believe) is to put public headers in directories that only contain public headers. One option would be an include directory parallel to the src directory, rather than inside it. If we prefer to assert that all public headers belong to a specific module and should be in a subdirectory of the module directory, then we should not include src in any include path, and segregate module public and private headers in a manner like src/api/cpp/include. Then, every directory or subdirectory has a clear API level that can (a) be easily distinguished with CMake interface levels, (b) easily installed from, and (c) easily segregated when building internal vs. external documentation.

Likewise, there must be an annotation that provides impedance to devs and reviewers if someone would try to include domdec_internal.h from another module - that's what the absence of \inlibraryapi and/or absence of \libinternal indicated. However, I don't think we can do that with CMake unless we make every module an object library and make use of PUBLIC/INTERFACE/PRIVATE machinery.

OBJECT library targets can't be linked against, so they can't confer PUBLIC/INTERFACE/PRIVATE machinery in the usual way. It would be worth revisiting the real-world cost of making modules into STATIC targets. We would want to move to default hidden symbols. After that, I don't know how many platforms of interest still lack good link-time-optimization. Actually, moving to default hidden symbols might already save us more than we currently save by not using STATIC targets per module. As above, though, I think the best annotation is file location.

if we can avoid the custom lump of python to analyze header dependencies because someone abusing domdec_internal.h can't compile without changing the CMake annotation, then that's quite an advantage.)

Yes, please! :-)

Do you have something specific in mind? Is the separation of interfaces in the gmxapi target a reasonable model?

If it is deemed necessary to continue a three-level access model for header files, then module-public non-library-public headers can be in a third (relative) location. Modules can then each have an INTERFACE target that exposes just that location. The gmxapi-detail target sort of does this, but it does not leave a third location unexposed because it wasn't particularly relevant to do so.

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