Hi Salvatore

Thank you for your posting this.

IMO, this topic shouldn't be limited for Neutron only.
Users wants consistent API between OpenStack project, right?

In Nova, a server has task_state, so Neutron should do same way.

2014-05-22 15:34 GMT-07:00 Salvatore Orlando <sorla...@nicira.com>:
> As most of you probably know already, this is one of the topics discussed
> during the Juno summit [1].
> I would like to kick off the discussion in order to move towards a concrete
> design.
> Preamble: Considering the meat that's already on the plate for Juno, I'm not
> advocating that whatever comes out of this discussion should be put on the
> Juno roadmap. However, preparation (or yak shaving) activities that should
> be identified as pre-requisite might happen during the Juno time frame
> assuming that they won't interfere with other critical or high priority
> activities.
> This is also a very long post; the TL;DR summary is that I would like to
> explore task-oriented communication with the backend and how it should be
> reflected in the API - gauging how the community feels about this, and
> collecting feedback regarding design, constructs, and related
> tools/techniques/technologies.
> At the summit a broad range of items were discussed during the session, and
> most of them have been reported in the etherpad [1].
> First, I think it would be good to clarify whether we're advocating a
> task-based API, a workflow-oriented operation processing, or both.
> --> About a task-based API
> In a task-based API, most PUT/POST API operations would return tasks rather
> than neutron resources, and users of the API will interact directly with
> tasks.
> I put an example in [2] to avoid cluttering this post with too much text.
> As the API operation simply launches a task - the database state won't be
> updated until the task is completed.
> Needless to say, this would be a radical change to Neutron's API; it should
> be carefully evaluated and not considered for the v2 API.
> Even if it is easily recognisable that this approach has a few benefits, I
> don't think this will improve usability of the API at all. Indeed this will
> limit the ability of operating on a resource will a task is in execution on
> it, and will also require neutron API users to change the paradigm the use
> to interact with the API; for not mentioning the fact that it would look
> weird if neutron is the only API endpoint in Openstack operating in this
> way.
> For the Neutron API, I think that its operations should still be
> manipulating the database state, and possibly return immediately after that
> (*) - a task, or to better say a workflow will then be started, executed
> asynchronously, and update the resource status on completion.
> --> On workflow-oriented operations
> The benefits of it when it comes to easily controlling operations and
> ensuring consistency in case of failures are obvious. For what is worth, I
> have been experimenting introducing this kind of capability in the NSX
> plugin in the past few months. I've been using celery as a task queue, and
> writing the task management code from scratch - only to realize that the
> same features I was implementing are already supported by taskflow.
> I think that all parts of Neutron API can greatly benefit from introducing a
> flow-based approach.
> Some examples:
> - pre/post commit operations in the ML2 plugin can be orchestrated a lot
> better as a workflow, articulating operations on the various drivers in a
> graph
> - operation spanning multiple plugins (eg: add router interface) could be
> simplified using clearly defined tasks for the L2 and L3 parts
> - it would be finally possible to properly manage resources' "operational
> status", as well as knowing whether the actual configuration of the backend
> matches the database configuration
> - synchronous plugins might be converted into asynchronous thus improving
> their API throughput
> Now, the caveats:
> - during the sessions it was correctly pointed out that special care is
> required with multiple producers (ie: api servers) as workflows should be
> always executed in the correct order
> - it is probably be advisable to serialize workflows operating on the same
> resource; this might lead to unexpected situations (potentially to
> deadlocks) with workflows operating on multiple resources
> - if the API is asynchronous, and multiple workflows might be queued or in
> execution at a given time, rolling back the DB operation on failures is
> probably not advisable (it would not be advisable anyway in any asynchronous
> framework). If the API instead stays synchronous the revert action for a
> failed task might also restore the db state for a resource; but I think that
> keeping the API synchronous missed a bit the point of this whole work - feel
> free to show your disagreement here!
> - some neutron workflows are actually initiated by agents; this is the case,
> for instance, of the workflow for doing initial L2 and security group
> configuration for a port.
> - it's going to be a lot of work, and we need to devise a strategy to either
> roll this changes in the existing plugins or just decide that future v3
> plugins will use it.
> From the implementation side, I've done a bit of research and task queue
> like celery only implement half of what is needed; conversely I have not
> been able to find a workflow manager, at least in the python world, as
> complete and suitable as taskflow.
> So my preference will be obviously to use it, and contribute to it should we
> realize Neutron needs some changes to suit its needs. Growing something
> neutron-specific in tree is something I'd rule out.
> (*) This is a bit different from what many plugins do, as they execute
> requests synchronously and return only once the backend request is
> completed.
> --> Tasks and the API
> The etherpad [1] contains a lot of interesting notes on this topic.
> One important item it to understand how tasks affect the resource's status
> to indicate their completion or failure. So far Neutron resource status
> pretty much expresses its "fabric" status. For instance a port is "UP" if
> it's been wired by the OVS agent; it often does not tell us whether the
> actual resource configuration is exactly the desired one in the database.
> For instance, if the ovs agent fails to apply security groups to a port, the
> port stays "ACTIVE" and the user might never know there was an error and the
> actual state diverged from the desired one.
> It is therefore important to allow users to know whether the backend state
> is in sync with the db; tools like taskflow will be really helpful to this
> aim.
> However, how should this be represented? The main options are to either have
> a new attribute describing the resource sync state, or to extend the
> semantics of the current status attribute to include also resource sync
> state. I've put some rumblings on the subjects in the etherpad [3].
> Still, it has been correctly pointed out that it might not be enough to know
> that a resource is out of sync, but it is good to know which operation
> exactly failed; this is where exposing somehow tasks through the API might
> come handy.
> For instance one could do something like:
> GET /tasks?resource_id=<res_id>&task_state=FAILED
> to get failure details for a given resource.
> --> How to proceed
> This is where I really don't know... and I will therefore be brief.
> We'll probably need some more brainstorming to flush out all the details.
> Once that is done, it might the case of evaluating what needs to be done and
> whether it is better to target this work onto existing plugins, or moving it
> out to v3 plugins (and hence do the actual work once the "core refactoring"
> activities are complete).
> Regards,
> Salvatore
> [1] https://etherpad.openstack.org/p/integrating-task-into-neutron
> [2] http://paste.openstack.org/show/81184/
> [3] https://etherpad.openstack.org/p/sillythings
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