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From "Suresh Srinivas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-7455) Introduce HA Service Protocol Interface
Date Mon, 19 Sep 2011 23:27:09 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7455?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13108231#comment-13108231
] 

Suresh Srinivas commented on HADOOP-7455:
-----------------------------------------

Sorry, I missed these comments earlier.

The reason why I do not have start*() methods is, this is an interface implemented by a server.
Without a server being available there is no one to field these requests. The way existing
interface will fit in with HA framework is:
start, stop scripts called by HA framework will call existing start and stop scripts.
For the rest, such as monitoring, making a server active/standby, this interface will be invoked
and will affect it by talking to a server.

bq. Neutral state is the protocol which ensures that only one Active Namenode will be present
at a time. The approach says 'Avoid the possibility of having two Active Namenodes; rather
than do fencing later to prevent the inconsistencies that can be caused by two Active Namenodes'.
This is the key to understand the intent behind neutral state.
I have not seen this kind of state in HA frameworks. I do not think there is a reliable way
other than fencing to handle partitions.

bq. Once the Namenode (active / standby) loses the connection to Zookeeper, it will relinquish
it's current role & stays idle by transitioning to neutral state. Once the Zookeeper service
is available again to the Namenode, it conducts the active instance election once again. Based
on the result of the election it decides the new role.
Why should it go to neutral state. Why not standby?


> Introduce HA Service Protocol Interface
> ---------------------------------------
>
>                 Key: HADOOP-7455
>                 URL: https://issues.apache.org/jira/browse/HADOOP-7455
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: util
>            Reporter: Suresh Srinivas
>            Assignee: Suresh Srinivas
>             Fix For: HA Branch (HDFS-1623)
>
>         Attachments: HADOOP-7455.1.patch, HADOOP-7455.2.patch, HADOOP-7455.3.patch, HADOOP-7455.4.patch,
HADOOP-7455.5.patch, HDFS-7454.patch
>
>
> This jira introduces a protocol interface to be implemented by services that provide
HA functionality.

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