On 06/18/2012 10:02 AM, Saggi Mizrahi wrote:
I would like to put on to the table for descussion the growing need for a way
to more easily reuse of the functionality of VDSM in order to service projects
other than Ovirt-Engine.

Originally VDSM was created as a proprietary agent for the sole purpose of
serving the then proprietary version of what is known as ovirt-engine. Red Hat,
after acquiring the technology, pressed on with it's commitment to open source
ideals and released the code. But just releasing code into the wild doesn't
build a community or makes a project successful. Further more when building
open source software you should aspire to build reusable components instead of
monolithic stacks.

We would like to expose a stable, documented, well supported API. This gives
us a chance to rethink the VDSM API from the ground up. There is already work
in progress of making the internal logic of VDSM separate enough from the API
layer so we could continue feature development and bug fixing while designing
the API of the future.

In order to achieve this though we need to do several things:
    1. Declare API supportability guidelines

Adding danpb and DV as I think they can provide good advice here.

Practically speaking, I think the most important thing to do is clearly declare what's supported and not supported in more detail than you probably want to. Realistically, you have to just support whatever you have. I don't know that designing a "supportable" interface can be really successful unless you start with that tomorrow.

So basically, unless you plan on removing the XML-RPC interface in the next release, you should plan on supporting it forever...

    2. Decide on an API transport (e.g. REST, ZMQ, AMQP)

We spent so much time trying to find the best transport in QEMU with the resulting being something I'm ultimately unhappy with.

The best decision we've made recently on this front is to move to a schema-based RPC mechanism where the transport code is all autogenerated. Python has an advantage in that it supports introspection although a disadvantage in that it's easy to end up with an ad-hoc interface by relying on passing around dictionaries.

    3. Make the API easily consumable (e.g. proper docs, example code, extending
       the API, etc)

Documentation is by far the most important thing IMHO. I actually think that simply taking the existing XML-RPC interface and adding documentation ought to be the first step even..

    4. Implement the API itself

I think the biggest risk in an effort like this is letting perfect become the enemy of good. If the goal is to open VDSM up to other applications, you can start today but just documenting what you have with plans to deprecate and improve later.

Honestly, worrying about XML-RPC vs. REST vs. AMQP is likely going to result in a lot of bike shedding and grand plans.


Anthony Liguori

All of these are dependent on one another and the permutations are endless.
This is why I think we should try and work on each one separately. All
discussions will be done openly on the mailing list and until the final version
comes out nothing is set in stone.

If you think you have anything to contribute to this process, please do so
either by commenting on the discussions or by sending code/docs/whatever
patches. Once the API solidifies it will be quite difficult to change
fundamental things, so speak now or forever hold your peace. Note that this is
just an introductory email. There will be a quick follow up email to kick start
the discussions.
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