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 . > 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 . > > 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  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  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 . > 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 > > >  https://etherpad.openstack.org/p/integrating-task-into-neutron >  http://paste.openstack.org/show/81184/ >  https://etherpad.openstack.org/p/sillythings > > > > > _______________________________________________ > OpenStack-dev mailing list > OpenStackfirstname.lastname@example.org > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev > _______________________________________________ OpenStack-dev mailing list OpenStackemail@example.com http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev