On Wed, Nov 30, 2011 at 11:32:55PM -0500, Itamar Heim wrote:
-----Original Message----- From: Adam Litke [mailto:agl@us.ibm.com] Sent: Thursday, December 01, 2011 0:41 AM To: vdsm-devel@lists.fedorahosted.org; engine-devel@ovirt.org Cc: Daniel P. Berrange; Chris Wright; Dan Kenigsberg; Itamar Heim Subject: Proposed next-generation vdsm API
Recently we've had some very productive discussions concerning the VDSM API. I want to attempt to refocus the discussion around an emerging proposal and see if we can agree on a sensible path forward.
Based on the discussion, I have identified the following requirements that a new API for vdsm should have:
1.) Single API that can be consumed by ovirt-engine and ISVs - We don't want to maintain multiple parallel APIs - To develop a vendor ecosystem, we must have a robust external API to vdsm
2.) Full vdsm capabilities are exposed without requiring ovirt-engine - ovirt components should be modular and independently useful - Some deployments might want to manage nodes without ovirt-engine
3.) Standardized protocol with low overhead - Required for widespread adoption
4.) Support for asynchronous tasks and events - Needed by ovirt-engine and other consumers
Based on these requirements, the following proposal has started to emerge:
Create a REST API that will provide all of the functionality that is currently available via the xmlrpc interface (with the goal of deprecating xmlrpc once it becomes mature enough). To support advanced clustering features that ovirt-engine is planning, we'll write an QMF broker that can proxy the REST API onto a message bus. ovirt-engine will interact with vdsm exclusively over this bus but the REST API will be the principle API and the entry point for ISV apps. A REST API provides a light-weight and standard way to access all of the vdsm functionality.
The REST API will handle events by exposing a new 'events' collection at the api root. REST users will use some sort of polling to collect these events. The details of this interface are being worked on. Several ways for minimizing the impact of polling have been discussed. The QMF broker can expose a publish/subscribe model for events as appropriate.
Is this model an acceptable way to improve the vdsm API? I would like to hear the opinions of ovirt-engine developers, vdsm developers, and other stakeholders. Thanks for providing feedback on this proposal!
Why things non native to REST and wrap it in QMF, rather than do the reverse? Or just to them in parallel, since it sounds like both are going to be first class citizens?
Parallel APIs mean dual maintenance. There will be inherent incompatibilities as each API would naturally have small differences. The reason for beginning with REST because of its low overhead and simplicity. Users of the REST API would not need to concern themselves with QMF at all but if that extra set of features is desired it can be easily added. -- Adam Litke <agl@us.ibm.com> IBM Linux Technology Center