HDFS-HA customized callback support

[Azuryy Yu <az...@gmail.com>]

I just submitted the following patch, welcome review it.
---------- Forwarded message ----------
From: "Fengdong Yu (JIRA)" <ji...@apache.org>
Date: Mar 25, 2013 6:07 PM
Subject: [jira] [Created] (HDFS-4631) Support customized call back method
during failover automatically.
To: <hd...@hadoop.apache.org>
Cc:

Fengdong Yu created HDFS-4631:
---------------------------------

             Summary: Support customized call back method during failover
automatically.
                 Key: HDFS-4631
                 URL: https://issues.apache.org/jira/browse/HDFS-4631
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: ha
    Affects Versions: 3.0.0
            Reporter: Fengdong Yu
             Fix For: 3.0.0


ZKFC add HealthCallbacks bu default, this can do quiteElection at least.
but we often want to be alerted if there is fail over occurring(such as
send email, short message), especially for prod cluster.

There is a configured fence script. maybe we can put all these logics in
the script. but in reasonable, fence script does only one thing: fence :)

So I added this patch, we can configure customized HM callback method, if
there is no configration, then only HealthCallbacks is added.


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