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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-13035) Add states INITING and STARTING to YARN Service model to cover in-transition states.
Date Tue, 03 May 2016 10:28:12 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-13035?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15268487#comment-15268487
] 

Steve Loughran commented on HADOOP-13035:
-----------------------------------------

One thing to bear in mind about that state change lock is that as the {{serviceInit}} and
{{serviceStart}} methods are called holding the lock, code of these methods can enter a new
state while it's in progress. This is why the {{init()}} and {{start()}} methods have checks
for whether they are still in the required state on exit. This is important because it allows
{{serviceInit}} and {{serviceStart}} to invoke stop() and not block.

Anything playing with a new state change field would have to handle similar cases, especially
{{serviceInit}} and calling {{start()}} . ( I don't know of anything which does that; I'm
not sure if we should allow it, but it's there. stop can/may happen, which really complicates
things if something like a contained service ever directly/indirectly called it's parent {{stop()}}
call. There's also some unwinding in {{CompositeService.serviceStop()}} which only calls {{stop()}}
on children which are explicitly in the {{STARTED}} state...we'd have to make sure that includes
in-transition conditions (probably) or explicitly decided to ignoring the starting stuff.

The more I think about this, an expanded state model, with the existing accessor/enum retained
as is, is probably the way to manage this. Users of the old API would get the STARTED state
when the service was in STARTING/STARTED; users of the new API would get the detailed value.
This would allow the inner state model to be complete, covering the logic of the various state
transitions without having to also rely on some transitionary states. 


There's also some fun when we consider that today, INIT and STARTED are idempotent
{code}
if (enterState(STATE.INITED) != STATE.INITED) { ...}
{code}

That would actually get more complex with the extra states; perhaps {{enterState()}} would
be changed to return a bool if a state change has *really* occurred, and both INITED/INITING
and STARTED/STARTING treated as equivalent from the perspective of idempotent transitions.
That is, STARTED.enterState(STARTING) => STARTED, STARTING.enterState(STARTED) => STARTED
is needed, because it's how the start() operation would transit to its final state. Whatever
patch gets in, making sure it is fully idempotent here will have to be looked at carefully


> Add states INITING and STARTING to YARN Service model to cover in-transition states.
> ------------------------------------------------------------------------------------
>
>                 Key: HADOOP-13035
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13035
>             Project: Hadoop Common
>          Issue Type: Bug
>            Reporter: Bibin A Chundatt
>         Attachments: 0001-HADOOP-13035.patch, 0002-HADOOP-13035.patch, 0003-HADOOP-13035.patch
>
>
> As per the discussion in YARN-3971 the we should be setting the service state to STARTED
only after serviceStart() 
> Currently {{AbstractService#start()}} is set
> {noformat} 
>      if (stateModel.enterState(STATE.STARTED) != STATE.STARTED) {
>         try {
>           startTime = System.currentTimeMillis();
>           serviceStart();
> ..
>  }
> {noformat}
> enterState sets the service state to proposed state. So in {{service.getServiceState}}
in {{serviceStart()}} will return STARTED .



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