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On 23/05/14 01:34, Salvatore Orlando wrote:
> 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.

fwiw, I was previously involved with the DMTF CIMI spec - the 'Job'
resource defined there seems match the notion of 'task' as below [1].
The spec has a notion of nesting Jobs but any scheduling (as discussed in
the etherpad you link below) was under discussion/tbd.


> 
> At the summit a broad range of items were discussed during the session, and
> most of them have been reported in the etherpad [1].

fwiw, there is mention in that etherpad about deferring a task/api call
(so once you have a notion of 'Job' you can then schedule its
execution). IMO this is an aside/added/unnecessary complication to what
is potentially already a huge change.
> 
> 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.

(as an aside, in the example, another issue is what to return for GET
/actual_resource whilst the task is still pending, you kind of cover
that with the expanded status semantics discussed later):

> 
> 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.

+1 - I don't like this; a client will always get back a 'task' with a
reference to the actual resource being acted upon. The client would then
need to make a further API call to retrieve the (e.g.) created resource.
BUT, how can we enable a task API without this model? Could we use an
extension header to communicate the task URI? So API would remain
unchanged, returning all the usual status but with the task URI included
where necessary

> 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.

you mean e.g. for create, client gets back 200 OK with the created
resource inline, OR 202 Accepted for async tasks (so in the latter case
resource state will be 'pending' until its completed, like server create
in nova for example?).

> 
> --> On workflow-oriented operations

please excuse my ignorance, but I am not entirely sure what the
distinction between 'task' and 'workflow' is (I wasn't at
summit/discussion if that is some excuse). Is a workflow a directed
graph of tasks?

> 
> 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


+1 to expanding current status, an additional status_attribute is
confusing imo


> 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).



which subgroup do you see this being discussed in?

thanks! marios

[1]
http://dmtf.org/sites/default/files/standards/documents/DSP0263_1.1.0.pdf#page=184&zoom=auto,69,345



> 
> 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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