[jboss-as7-dev] Graceful Shutdown (Revision 2)

[David M. Lloyd]

On 6/26/12 7:13 PM, Bill Burke wrote:
> Are you sure explicity granting "permits" through a "Shutdown Manager"
> the way to go?

Yes, we are 100% sure.

> Having to interact with a central service that grants
> permits per-request is very worrisome.  The subsystem may already have a
> better way to do graceful shutdown, or just cannot follow a permit pattern.

No, we went into great detail to work this out.  But if you have a 
specific case that you think warrants another look, feel free to raise it.

> Is it not simpler and gives more flexibility to a subsystem to drive
> this via events rather than explicit callbacks to a Shutdown Manager?

No, because the "suspended" state isn't really a state, and it is 
certainly not detectable from each subsystem.

For example, there may be no remaining EJB sessions.  However, that does 
not mean that EJB can refuse new session creation.  In particular, if 
you are in a suspended state and deploy a singleton EJB which has a 
@PostConstruct on it, and that interceptor causes a session to be 
created, this is allowed as deployment must always gain a permit (else 
it'd fail).

Note, people, that permits are a *conceptual* entity.  There is no need 
to have permit objects, nor even a semaphore, nor even a non-blocking 
counter.  I do know a thing or two about eliminating concurrency 
bottlenecks.

> i.e.
>
> class Suspending extends ShutdownEvent
> {
>     void iFinishedSuspending(); // the callback to the Shutdown Manager
> }
>
> GIves the subsystem flexibility on how best to implement a graceful
> shutdown.
>
>
> On 6/26/12 11:00 AM, Jason T. Greene wrote:
>> *EDITED*
>>
>> We've worked out the rough outline of how graceful shutdown will work in
>> AS7.
>>
>> The process of graceful shutdown actually is reflected by a number of
>> states:
>>
>> 1. Running - all services acting normally
>> 2. Suspending - services refuse new "permits" (see below), existing
>> permits are allowed to be retained (and threads running under such a
>> permit may still acquire new permits)
>> 3. Suspended - no permits are present and none may be issued
>> 4. Shutting Down - our existing server stop process / reload admin mode
>>
>> The following transitions are allowed:
>>
>> 1. Running → Suspending: Transition occurs at user request (to suspend
>> or gracefully shut down).
>> 2. Suspending → Suspended: Transition occurs when all permits are cleared.
>> 3. Suspending → Running: Transition occurs at user request (to exit
>> suspend mode or cancel graceful shutdown before it completes).
>> 4. Suspended → Running: Transition occurs at user request (to exit
>> suspend mode).
>> 5. Suspended → Shutting Down: Transition occurs automatically (if a
>> graceful shutdown was requested) or at user request (if a shut down
>> request of any kind is entered in the Suspended state).
>> 6. Running → Shutting Down: Transition occurs at user request (to shut
>> down the server "un-gracefully").
>> 7. Suspending → Shutting Down (User aborts a graceful shutdown)
>>
>> These "permits" are issued by the "Shutdown Manager", whose job is to
>> manage these states.  They are issued corresponding to the following events:
>>
>> 1. The invocation of an EJB method
>> 2. The creation of a web session
>> 3. A creation of a transaction
>> 4. MessageEndpoint and WorkManager aquire permit allowing for release()
>> from a thirdparty to indicate connection close.
>>
>> When a permit cannot be issued due to the server shutting down, a
>> standard exception message should be produced so that the user can see a
>> familiar error message regardless of what mechanism is used to access
>> the server.
>>
>


-- 
- DML