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From "Steve Loughran (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MAPREDUCE-3939) Enhance YARN service model
Date Wed, 29 Feb 2012 10:31:58 GMT
Enhance YARN service model

                 Key: MAPREDUCE-3939
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3939
             Project: Hadoop Map/Reduce
          Issue Type: Improvement
          Components: mrv2
    Affects Versions: 0.24.0, 0.23.2
            Reporter: Steve Loughran
            Assignee: Steve Loughran

Having played the YARN service model, there are some issues
that I've identified based on past work and initial use.

This JIRA issue is an overall one to cover the issues, with solutions pushed out to separate

h2. state model prevents stopped state being entered if you could not successfully start the

In the current lifecycle you cannot stop a service unless it was successfully started, but
* {{init()}} may acquire resources that need to be explicitly released
* if the {{start()}} operation fails partway through, the {{stop()}} operation may be needed
to release resources.

*Fix:* make {{stop()}} a valid state transition from all states and require the implementations
to be able to stop safely without requiring all fields to be non null.

Before anyone points out that the {{stop()}} operations assume that all fields are valid;
and if called before a {{start()}} they will NPE; MAPREDUCE-3431 shows that this problem arises
today, MAPREDUCE-3502 is a fix for this. It is independent of the rest of the issues in this
doc but it will aid making {{stop()}} execute from all states other than "stopped".

MAPREDUCE-3502 is too big a patch and needs to be broken down for easier review and take up;
this can be done with issues linked to this one.
h2. AbstractService doesn't prevent duplicate state change requests.

The {{ensureState()}} checks to verify whether or not a state transition is allowed from the
current state are performed in the base {{AbstractService}} class -yet subclasses tend to
call this *after* their own {{init()}}, {{start()}} & {{stop()}} operations. This means
that these operations can be performed out of order, and even if the outcome of the call is
an exception, all actions performed by the subclasses will have taken place. MAPREDUCE-3877
demonstrates this.

This is a tricky one to address. In HADOOP-3128 I used a base class instead of an interface
and made the {{init()}}, {{start()}} & {{stop()}} methods {{final}}. These methods would
do the checks, and then invoke protected inner methods, {{innerStart()}}, {{innerStop()}},
etc. It should be possible to retrofit the same behaviour to everything that extends {{AbstractService}}
-something that must be done before the class is considered stable (because once the lifecycle
methods are declared final, all subclasses that are out of the source tree will need fixing
by the respective developers.

h2. AbstractService state change doesn't defend against race conditions.

There's no concurrency locks on the state transitions. Whatever fix for wrong state calls
is added should correct this to prevent re-entrancy, such as {{stop()}} being called from
two threads.

h2.  Static methods to choreograph of lifecycle operations

Helper methods to move things through lifecycles. init->start is common, stop-if-service!=null
another. Some static methods can execute these, and even call {{stop()}} if {{init()}} raises
an exception. These could go into a class {{ServiceOps}} in the same package. These can be
used by those services that wrap other services, and help manage more robust shutdowns.

h2. state transition failures are something that registered service listeners may wish to
be informed of.

When a state transition fails a {{RuntimeException}} can be thrown -and the service listeners
are not informed as the notification point isn't reached. They may wish to know this, especially
for management and diagnostics.

*Fix:* extend {{ServiceStateChangeListener}} with a callback such as {{stateChangeFailed(Service
service,Service.State targeted-state, RuntimeException e)}} that is invoked from the (final)
state change methods in the {{AbstractService}} class (once they delegate to their inner {{innerStart()}},
{{innerStop()}} methods; make a no-op on the existing implementations of the interface.

h2. Service listener failures not handled

Is this an error an error or not? Log and ignore may not be what is desired.

*Proposed:* during {{stop()}} any exception by a listener is caught and discarded, to increase
the likelihood of a better shutdown, but do not add try-catch clauses to the other state changes.

h2. Support static listeners for all AbstractServices

Add support to {{AbstractService}} that allow callers to register listeners for all instances.
The existing listener interface could be used. This allows management tools to hook into the

The static listeners would be invoked for all state changes except creation (base class shouldn't
be handing out references to itself at this point).

These static events could all be async, pushed through a shared {{ConcurrentLinkedQueue}};
failures logged at warn and the rest of the listeners invoked.

h2. Add some example listeners for management/diagnostics
* event to commons log for humans.
* events for machines hooked up to the JSON logger.
* for testing: something that be told to fail.

h2.  Services should support signal interruptibility

The services would benefit from a way of shutting them down on a kill signal; this can be
done via a runtime hook. It should not be automatic though, as composite services will get
into a very complex state during shutdown. Better to provide a hook that lets you register/unregister
services to terminate, and have the relevant {{main()}} entry points tell their root services
to register themselves.

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