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Nachi,

I will be glad if the solution was as easy as sticking a task_state
attribute to a resource! I'm afraid however that would be only the tip of
the iceberg, or the icing of the cake, if you want.
However, I agree with you that consistency across Openstack APIs is very
important; whether this is a cross project discussion is instead debatable;
my feeling here is that taskflow is the cross-project piece of the
architecture, and every project then might have a different strategy for
integrating it - as long as it does not result in inconsistent APIs exposed
to customers!

It is something that obviously will be considered when designing how to
represent whether a DB resource is in sync with its actual configuration on
the backend.
I think this is something which might happen regardless of whether it will
be also agreed to let API consumers access task execution information using
the API.

Salvatore




On 23 May 2014 01:16, Nachi Ueno <nachi at ntti3.com> wrote:

> 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 <sorlando at 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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> >
>
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