Re: [Qemu-devel] QOM vs QAPI for QMP APIs

[Stefan Hajnoczi]

On Fri, Feb 21, 2014 at 10:16 AM, Stefan Hajnoczi <address@hidden> wrote:
> Maybe I just need some convincing but it seems that QAPI is the simplest
> and cleanest way to define external APIs.
>
> Disagree?  Tell me why :).

I'm replying to myself because we had an interesting discussion on
IRC.  Thanks Paolo, Markus, and Peter!

I'm biased, but here are the points collected from the discussion:

Why QOM *should* be QEMU's external API:
 * QOM eliminates hand-written QMP API code.  We get query-foo APIs
for free using qom-list, qom-get, qom-set, and friends

Why QOM should *not* be QEMU's external API:
 * Internal objects will diverge from the external QOM objects over
time, we'll have to maintain a backwards compatible QOM object layer
on top of QEMU's internal objects - that defeats the whole point of
getting APIs for free from QOM
 * We lose the easy-to-review qapi-schema.json.  Would need new
documentation and review tools for QOM model changes.
 * query-foo APIs are simple to implement, not a huge win to get them via QOM
 * QAPI is mature and well-understood, QMP is incomplete
 * qom-list, qom-get, qom-set are too low-level, need more powerful
APIs to save clients from elaborate back-and-forth conversations

Stefan