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Peng, I think you are right. We should have a common behavior among the
drivers, and move the implementation to the proper methods like
post-commits, do the validation on the pre-commits, etc, etc.

Second phase to tackle the out-of-sync could be the "revision number"
approach from networking-ovn.

Cheers

On Mon, Dec 11, 2017 at 4:32 PM, Peng Liu <pliu at redhat.com> wrote:

> Hi Michael,
>
> Yes, it's a similar issue but different aspect. Actually, the case for
> l2gw is worse, considering we have to deal with 2 existing back-end driver
> which have different understanding for the interfaces. But I think the
> proposed approach for networking-ovn is inspiring and helpful for l2gw.
>
> Thanks,
>
> On Fri, Dec 8, 2017 at 11:59 PM, Michael Bayer <mbayer at redhat.com> wrote:
>
>> On Wed, Dec 6, 2017 at 3:46 AM, Peng Liu <pliu at redhat.com> wrote:
>> > Hi,
>> >
>> > During working on this patch[0], I encounter some DB out-of-sync
>> problem. I
>> > think maybe the design can be improved. Here is my thought, all
>> comments are
>> > welcome.
>>
>>
>> see also https://review.openstack.org/#/c/490834/ which I think is
>> dealing with a similar (if not the same) issue.
>>
>> >
>> > In plugin code, I found all the resource operations follow the pattern
>> in
>> > [1]. It is a very misleading design compare to [2].
>> >
>> > "For every action that can be taken on a resource, the mechanism driver
>> > exposes two methods - ACTION_RESOURCE_precommit, which is called within
>> the
>> > database transaction context, and ACTION_RESOURCE_postcommit, called
>> after
>> > the database transaction is complete."
>> >
>> > In result, if we focus on the out-of-sync between plugin DB and
>> > driver/backend DB:
>> >
>> > 1) In RPC driver, only methods Action_Resource and are implemented.
>> Which
>> > means the action is token before it was written in plugin DB. In case of
>> > action partial succeed (especially for update case) or plugin DB
>> operation
>> > failure, it will cause DB out-of-sync.
>> > 2) In ODL driver, only methods Action_Resource_postcommit are
>> implemented,
>> > which means there is no validation in ODL level before the record is
>> written
>> > in the plugin DB. In case of, ODL side failure, there is no rollback for
>> > plugin DB.
>> >
>> > So, to fix this issue is costly. Both plugin and driver sides need to be
>> > altered.
>> >
>> > The other side of this issue is a period db monitor mechanism between
>> plugin
>> > and drivers, and it is another story.
>> >
>> >
>> > [0] https://review.openstack.org/#/c/516256
>> > [1]
>> > ...
>> >     def Action_Resource
>> >         self.validate_Resource_for_Action
>> >         self.driver.Action_Resource
>> >         with context.session.begin(subtransactions=True):
>> >             super.Action_Resource
>> >             self.driver.Action_Resource_precommit
>> >         try:
>> >             self.driver.Action_Resource_postcommit
>> > ...
>> > [2] https://wiki.openstack.org/wiki/Neutron/ML2
>> >
>> > --
>> > Peng Liu
>> >
>> > ____________________________________________________________
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>> >
>>
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>
>
>
> --
> Peng Liu | Senior Software Engineer
>
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-- 
Ricardo Noriega

Senior Software Engineer - NFV Partner Engineer | Office of Technology  |
Red Hat
irc: rnoriega @freenode
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