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From yl wu <wuyl6...@gmail.com>
Subject Re: What is the impact if hbase master downs?
Date Fri, 04 Jul 2014 01:22:43 GMT
Hi Mikhail,

Thank you so much! Your reply really resolve my confusions.

Best,
Yanglin


2014-07-03 16:31 GMT+08:00 Mikhail Antonov <olorinbant@gmail.com>:

> With HBASE-10569, all masters are implicitly also region servers (both
> active master and backup masterS), and can host regions (depending on load
> balancer settings). Region server process can't become a master, backup
> masters are regular hbase masters, which just happened to not be chosen as
> active masters, so they are waiting for next re-election.
>
> If you look at org.apache.hadoop.hbase.master.HMaster class, in its
> constructor it calls #startActiveMasterManager method, it's where the
> startup logic happens.  An HMaster tries to become active master (unless
> explicitly started with configuration param master.type.backup=true), if it
> succeeded - it calls #finishActiveMasterInitialization, where actual
> activation happens (in this sense, takeover of primary master role by
> backup master isn't different from starting new master on fresh cluster
> first time), otherwise it stalls and waits for the active master to fail.
>
> In finishActiveMasterInitialization() method, primary steps are described
> in javadoc:
>
> /**
>    * Finish initialization of HMaster after becoming the primary master.
>    *
>    * <ol>
>    * <li>Initialize master components - file system manager, server
> manager,
>    *     assignment manager, region server tracker, etc</li>
>    * <li>Start necessary service threads - balancer, catalog janior,
>    *     executor services, etc</li>
>    * <li>Set cluster as UP in ZooKeeper</li>
>    * <li>Wait for RegionServers to check-in</li>
>    * <li>Split logs and perform data recovery, if necessary</li>
>    * <li>Ensure assignment of meta/namespace regions<li>
>    * <li>Handle either fresh cluster start or master failover</li>
>    * </ol>
>
> Hope it helps.
>
> -Mikhail
>
>
>
>
> 2014-07-03 1:06 GMT-07:00 yl wu <wuyl6099@gmail.com>:
>
> > Hi Mikhail,
> >
> > Thank you very much for your quick reply.
> > I understand the process you mentioned. What I am confued is that, how
> this
> > new one takes over the responsibilities in details? By the way, what is
> the
> > physical relationship between backup masters and the region servers? Can
> a
> > region server becomes a master?
> >
> > I'm sorry that maybe my questions are simple. But I have searched answers
> > for a long time.
> >
> > Thank you very much.
> >
> > Best,
> > Yanglin
> >
> >
> >
> >
> >
> > 2014-07-03 15:45 GMT+08:00 Mikhail Antonov <olorinbant@gmail.com>:
> >
> > > Hi Yanglin,
> > >
> > > what exactly do you mean by "system..need to take any failover" - any
> > > manual action by cluster admin? Failover happens automatically based on
> > > zookeeper timeout for current active master. Currently, hbase cluster
> > has 1
> > > active master and several backup masters. When backup masters notice
> > active
> > > master going down, they elect new one, and the new one takes over
> > > responsibilities.
> > >
> > > -Mikhail
> > >
> > >
> > > 2014-07-03 0:40 GMT-07:00 yl wu <wuyl6099@gmail.com>:
> > >
> > > > Hi All,
> > > >
> > > > I've read many articles, including Bertozzi's Who needs a Master,
> but I
> > > am
> > > > still confused about the Hmaster.
> > > >
> > > > If the Hmaster goes down, does the system need to take any failover ?
> > > > Or just elect a new master, and the new one can restore all the
> > > information
> > > > from hbase:meta?
> > > >
> > > > Best regards,
> > > > Yanglin
> > > >
> > >
> > >
> > >
> > > --
> > > Thanks,
> > > Michael Antonov
> > >
> >
>
>
>
> --
> Thanks,
> Michael Antonov
>

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