For what is worth we have considered this aspect from the perspective of
the Neutron plugin my team maintains (NVP) during the past release cycle.
The synchronous model that most plugins with a controller on the backend
currently implement is simple and convenient, but has some flaws:
- reliability: the current approach where the plugin orchestrates the
backend is not really optimal when it comes to ensuring your running
configuration (backend/control plane) is in sync with your desired
configuration (neutron/mgmt plane); moreover in some case, due to neutron
internals, API calls to the backend are wrapped in a transaction too,
leading to very long SQL transactions, which are quite dangerous indeed. It
is not easy to recover from a failure due to an eventlet thread deadlocking
with a mysql transaction, where by 'recover' I mean ensuring neutron and
backend state are in sync.
- maintainability: since handling rollback in case of failures on the
backend and/or the db is cumbersome, this often leads to spaghetti code
which is very hard to maintain regardless of the effort (ok, I agree here
that this also depends on how good the devs are - most of the guys in my
team are very good, but unfortunately they have me too...).
- performance & scalability:
- roundtrips to the backend take a non-negligible toll on the duration
of an API call, whereas most Neutron API calls should probably just
terminate at the DB just like a nova boot call does not wait for the VM to
be ACTIVE to return.
- we need to keep some operation serialized in order to avoid the
mentioned race issues
For this reason we're progressively moving toward a change in the NVP
plugin with a series of patches under this umbrella-blueprint [1].
For answering the issues mentioned by Isaku, we've been looking at a task
management library with an efficient and reliable set of abstractions for
ensuring operations are properly ordered thus avoiding those races (I agree
on the observation on the pre/post commit solution).
We are currently looking at using celery [2] rather than taskflow; mostly
because we've already have expertise on how to use it into our
applications, and has very easy abstractions for workflow design, as well
as for handling task failures.
Said that, I think we're still open to switch to taskflow should we become
aware of some very good reason for using it.
Regards,
Salvatore
[1]
https://blueprints.launchpad.net/neutron/+spec/nvp-async-backend-communication
[2] http://docs.celeryproject.org/en/master/index.html
On 19 November 2013 19:42, Joshua Harlow <harlo...@yahoo-inc.com> wrote:
> And also of course, nearly forgot a similar situation/review in heat.
>
> https://review.openstack.org/#/c/49440/
>
> Except theres was/is dealing with stack locking (a heat concept).
>
> On 11/19/13 10:33 AM, "Joshua Harlow" <harlo...@yahoo-inc.com> wrote:
>
> >If you start adding these states you might really want to consider the
> >following work that is going on in other projects.
> >
> >It surely appears that everyone is starting to hit the same problem (and
> >joining efforts would produce a more beneficial result).
> >
> >Relevant icehouse etherpads:
> >- https://etherpad.openstack.org/p/CinderTaskFlowFSM
> >- https://etherpad.openstack.org/p/icehouse-oslo-service-synchronization
> >
> >And of course my obvious plug for taskflow (which is designed to be a
> >useful library to help in all these usages).
> >
> >- https://wiki.openstack.org/wiki/TaskFlow
> >
> >The states u just mentioned start to line-up with
> >https://wiki.openstack.org/wiki/TaskFlow/States_of_Task_and_Flow
> >
> >If this sounds like a useful way to go (joining efforts) then lets see how
> >we can make it possible.
> >
> >IRC: #openstack-state-management is where I am usually at.
> >
> >On 11/19/13 3:57 AM, "Isaku Yamahata" <isaku.yamah...@gmail.com> wrote:
> >
> >>On Mon, Nov 18, 2013 at 03:55:49PM -0500,
> >>Robert Kukura <rkuk...@redhat.com> wrote:
> >>
> >>> On 11/18/2013 03:25 PM, Edgar Magana wrote:
> >>> > Developers,
> >>> >
> >>> > This topic has been discussed before but I do not remember if we have
> >>>a
> >>> > good solution or not.
> >>>
> >>> The ML2 plugin addresses this by calling each MechanismDriver twice.
> >>>The
> >>> create_network_precommit() method is called as part of the DB
> >>> transaction, and the create_network_postcommit() method is called after
> >>> the transaction has been committed. Interactions with devices or
> >>> controllers are done in the postcommit methods. If the postcommit
> >>>method
> >>> raises an exception, the plugin deletes that partially-created resource
> >>> and returns the exception to the client. You might consider a similar
> >>> approach in your plugin.
> >>
> >>Splitting works into two phase, pre/post, is good approach.
> >>But there still remains race window.
> >>Once the transaction is committed, the result is visible to outside.
> >>So the concurrent request to same resource will be racy.
> >>There is a window after pre_xxx_yyy before post_xxx_yyy() where
> >>other requests can be handled.
> >>
> >>The state machine needs to be enhanced, I think. (plugins need
> >>modification)
> >>For example, adding more states like pending_{create, delete, update}.
> >>Also we would like to consider serializing between operation of ports
> >>and subnets. or between operation of subnets and network depending on
> >>performance requirement.
> >>(Or carefully audit complex status change. i.e.
> >>changing port during subnet/network update/deletion.)
> >>
> >>I think it would be useful to establish reference locking policy
> >>for ML2 plugin for SDN controllers.
> >>Thoughts or comments? If this is considered useful and acceptable,
> >>I'm willing to help.
> >>
> >>thanks,
> >>Isaku Yamahata
> >>
> >>> -Bob
> >>>
> >>> > Basically, if concurrent API calls are sent to Neutron, all of them
> >>>are
> >>> > sent to the plug-in level where two actions have to be made:
> >>> >
> >>> > 1. DB transaction ? No just for data persistence but also to collect
> >>>the
> >>> > information needed for the next action
> >>> > 2. Plug-in back-end implementation ? In our case is a call to the
> >>>python
> >>> > library than consequentially calls PLUMgrid REST GW (soon SAL)
> >>> >
> >>> > For instance:
> >>> >
> >>> > def create_port(self, context, port):
> >>> > with context.session.begin(subtransactions=True):
> >>> > # Plugin DB - Port Create and Return port
> >>> > port_db = super(NeutronPluginPLUMgridV2,
> >>> > self).create_port(context,
> >>> >
> >>> port)
> >>> > device_id = port_db["device_id"]
> >>> > if port_db["device_owner"] == "network:router_gateway":
> >>> > router_db = self._get_router(context, device_id)
> >>> > else:
> >>> > router_db = None
> >>> > try:
> >>> > LOG.debug(_("PLUMgrid Library: create_port()
> >>>called"))
> >>> > # Back-end implementation
> >>> > self._plumlib.create_port(port_db, router_db)
> >>> > except Exception:
> >>> > Š
> >>> >
> >>> > The way we have implemented at the plugin-level in Havana (even in
> >>> > Grizzly) is that both action are wrapped in the same "transaction"
> >>>which
> >>> > automatically rolls back any operation done to its original state
> >>> > protecting mostly the DB of having any inconsistency state or left
> >>>over
> >>> > data if the back-end part fails.=.
> >>> > The problem that we are experiencing is when concurrent calls to the
> >>> > same API are sent, the number of operation at the plug-in back-end
> >>>are
> >>> > long enough to make the next concurrent API call to get stuck at the
> >>>DB
> >>> > transaction level, which creates a hung state for the Neutron Server
> >>>to
> >>> > the point that all concurrent API calls will fail.
> >>> >
> >>> > This can be fixed if we include some "locking" system such as
> >>>calling:
> >>> >
> >>> > from neutron.common import utile
> >>> > Š
> >>> >
> >>> > @utils.synchronized('any-name', external=True)
> >>> > def create_port(self, context, port):
> >>> > Š
> >>> >
> >>> > Obviously, this will create a serialization of all concurrent calls
> >>> > which will ends up in having a really bad performance. Does anyone
> >>>has a
> >>> > better solution?
> >>> >
> >>> > Thanks,
> >>> >
> >>> > Edgar
> >>> >
> >>> >
> >>> > _______________________________________________
> >>> > OpenStack-dev mailing list
> >>> > OpenStack-dev@lists.openstack.org
> >>> > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
> >>> >
> >>>
> >>>
> >>> _______________________________________________
> >>> OpenStack-dev mailing list
> >>> OpenStack-dev@lists.openstack.org
> >>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
> >>
> >>--
> >>Isaku Yamahata <isaku.yamah...@gmail.com>
> >>
> >>_______________________________________________
> >>OpenStack-dev mailing list
> >>OpenStack-dev@lists.openstack.org
> >>http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
> >
> >
> >_______________________________________________
> >OpenStack-dev mailing list
> >OpenStack-dev@lists.openstack.org
> >http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>
>
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