OpenStack Kilo Design Summit

Concurrent resource access in cinder is a problem that has caused resource corruption when simultaneous resources are mutated on by multiple cinder entrypoints (api and manager for example). In Icehouse there has been some addition & usage of locks around manager functions to queue up those requests when a resource is being simultaneous worked on by multiple functions (this stops one of those operations from concurrently mutating the underlying resource). Sadly this is more of a *sledgehammer* approach and hides the symptoms of the problem and makes it non-obvious when debugging what other requests are queued up behind the lock (or why dead-locking is occurring, if and when it does).

To help alleviate and hopefully solve this problem we will try to attack some of these issues in a different manner, integrating a *allowed* state transition table into the ``create_volume` workflow and doing *strategic* state transitions and aborting/erroring out when these state transitions are not allowed. In the future this will help create a concrete set of well defined states and transitions for other workflows as well (and will make it clear while looking at code and during debugging which transitions are allowed at the same time and what transitions are actively occurring).