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From "M. C. Srivas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4087) HBaseAdmin should perform validation of connection it holds
Date Thu, 14 Jul 2011 14:13:00 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4087?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13065278#comment-13065278

M. C. Srivas commented on HBASE-4087:

First, waiting for reference count to reach zero wasn't part of the original semantics of
obtaining connection. Suppose there were two clients A and B. Client A is waiting in the new
while loop for reference count to reach zero. What if client B had a bug and crashed ?

If many threads share a connection, the reference count protects the connection from being
destroyed until the last reference is dropped. We need to continue to preserve that guarantee.
With the change being proposed, if any thread calls HConnectionManager#dropStaleConnection,
the connection gets destroyed from underneath other threads. The sequence of steps that causes
this is as follows:

1. threads T1 and T2 both get connections using the same conf, thus are returned the same
2. T1 gets a failure, so it deletes the stale connection and creates a new one (thus HBASE_INSTANCES.get()
will return the new good connection, since the HConnectionKey is identical).
3. T2 still has a handle on the old one, and gets a failure, and calls deleteStaleConnection.
4. deleteStaleConnection destroys the good connection created in step 2, from underneath T1.

So either we wait for the last ref to become zero, or set some sort of connection#state ==
INVALID if you'd rather not wait. I think the second choice (not waiting) seems better.

I didn't follow your second comment about client A and client B. Both are in the same JVM,
so if one crashes, the other would too, would it not?

> HBaseAdmin should perform validation of connection it holds
> -----------------------------------------------------------
>                 Key: HBASE-4087
>                 URL: https://issues.apache.org/jira/browse/HBASE-4087
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Ted Yu
>            Assignee: Ted Yu
>            Priority: Critical
>             Fix For: 0.92.0
>         Attachments: 4087-v2.txt, 4087-v3.txt, 4087.txt
> Through HBASE-3777, HConnectionManager reuses the connection to HBase servers.
> One challenge, discovered in troubleshooting HBASE-4052, is how we invalidate connection(s)
to server which gets restarted.
> There're at least two ways.
> 1. HConnectionManager utilizes background thread(s) to periodically perform validation
of connections in HBASE_INSTANCES and remove stale connection(s).
> 2. Allow HBaseClient (including HBaseAdmin) to provide feedback to HConnectionManager.
> The solution can be a combination of both of the above.

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