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

[Jason Greene]

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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--
Jason T. Greene
WildFly Lead / JBoss EAP Platform Architect
JBoss, a division of Red Hat