[wildfly-dev] Discarding the changed management model during operation rollback

[Kabir Khan]

Me too
> On 1 Jul 2015, at 20:03, Jason Greene <jason.greene at redhat.com> wrote:
> 
> I completely agree.
> 
>> On Jul 1, 2015, at 11:43 AM, Brian Stansberry <brian.stansberry at redhat.com> wrote:
>> 
>> tl;dr;
>> 
>> When an operation that has changed the management model is rolling back, 
>> the OperationStepHandlers processing the rollback see the model as it 
>> was at the arbitrary point when the rollback began. I propose changing 
>> this so they see things as they were prior to the first change to the model.
>> 
>> Long version:
>> 
>> When an operation mutates either the management Resource tree, or the 
>> registry of capabilities (together known as the management model), we 
>> clone the management model and thereafter that operation works on the 
>> clone. The clone is invisible to other callers until the operations 
>> successfully commits. When the operation commits, it publishes the clone 
>> and that becomes the official model.
>> 
>> I call this copy-on-write, publish-on-commit.
>> 
>> If the operation rolls back, the changed model is never published and 
>> the clone is just discarded when the operation execution returns.
>> 
>> However, while the operation is rolling back, all the 
>> OperationStepHandlers that are processing the rollback see the modified 
>> clone, not the original model. They are seeing arbitrarily incorrect data.
>> 
>> This hasn't been a problem up to now, as our standard OSHs get a copy of 
>> whatever part of the Resource tree they are going to modify and keep it 
>> for use in rollback. They don't need to re-read the management model to 
>> perform rollback.
>> 
>> But this doesn't work for the capability registry. If an OSH removes a 
>> capability and removes a service, and then in rollback tries to use the 
>> capability to figure out how to restore the service, it fails, as 
>> management model it can see still shows the capability as being removed.
>> 
>> To fix this, I propose discarding the cloned management model as soon as 
>> rollback begins. Thereafter, an OSH processing rollback will see the 
>> model as it was before the first modification. The removed capability 
>> will still be present.
>> 
>> I have a commit that does this at [1]. Running the testsuite with it 
>> shows no regressions. This doesn't surprise me, as our standard OSHs up 
>> to now have had no need to re-read the model during rollback.
>> 
>> [1] 
>> https://github.com/bstansberry/wildfly-core/commit/419f350931d5b7e345cf9aa514de08c01bf976be
>> 
>> -- 
>> Brian Stansberry
>> Senior Principal Software Engineer
>> JBoss by Red Hat
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>> wildfly-dev at lists.jboss.org
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> 
> --
> Jason T. Greene
> WildFly Lead / JBoss EAP Platform Architect
> JBoss, a division of Red Hat
> 
> 
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