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From "Stephen Yuan Jiang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-16233) Procedure V2: Support acquire/release shared table lock concurrently
Date Fri, 15 Jul 2016 01:58:20 GMT

    [ https://issues.apache.org/jira/browse/HBASE-16233?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15378745#comment-15378745
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Stephen Yuan Jiang commented on HBASE-16233:
--------------------------------------------

Several approaches were discussed with [~mbertozzi]:

- Solution 1: as long as isSingleSharedLock() is false, we don't call zk to acquire shared
lock, because one shared lock in zk is good enough; same as released the lock, if isSingleSharedLock()
is false, no need to call zk to release the lock.
Solution 1 looks like a hacked solution, but it should work and the change is simple (we do
need to move the expensive zk lock acquire/release inside the synchronization block.  - Note:
we have already used isSingleSharedLock() to make decision on 'reset' parameter of release
lock - I think there is a bug that acquire/release zk lock not inside synchronization block,
because isSingleSharedLock() could change.)

- Solution 2: make a 'private HashMap<procedureId, TableLock > tableLock' to replace
'private TableLock tableLock' - now we track all locks.  The draw back is that when exclusive
lock is used, a little bit more overhead by looking at hash table.
Solution 2 is more robust as we creates multiple shared lock znode to track each procedure.
 But it is a little complicated and really there is no need to over-complicate the part of
code that might not exist in a long term.  

According to [~mbertozzi], [~Abby] is looking to removing the zklock in Apache HBASE 2.0.
 He would open a JIRA for the work.  

In the mean time, the V1 patch is using the proposal of Solution 1.

> Procedure V2: Support acquire/release shared table lock concurrently
> --------------------------------------------------------------------
>
>                 Key: HBASE-16233
>                 URL: https://issues.apache.org/jira/browse/HBASE-16233
>             Project: HBase
>          Issue Type: Sub-task
>          Components: proc-v2
>            Reporter: Stephen Yuan Jiang
>            Assignee: Stephen Yuan Jiang
>             Fix For: 2.0.0
>
>         Attachments: HBASE-16233.v1-master.patch
>
>
> {{MasterProcedureScheduler.TableQueue}} class only has one single instance of TableLock
({{private TableLock tableLock = null;}}) to track exclusive/shared table lock from TableLockManager.
 
> When multiple shared lock request comes, the later shared lock request would overwrite
the lock acquired from earlier shared lock request, and hence, we will get some weird error
when the second or later release lock request comes, because we lose track of the lock.
> The issue can be reproduced in the unit test of HBASE-14552.  [~mbertozzi] also comes
up with a UT without using any real procedure to repro the problem:
> {code}
> @Test
>   public void testSchedWithZkLock() throws Exception {
>     MiniZooKeeperCluster zkCluster = new MiniZooKeeperCluster(conf);
>     int zkPort = zkCluster.startup(new File("/tmp/test-zk"));
>     Thread.sleep(10000);
>     conf.set("hbase.zookeeper.quorum", "localhost:" + zkPort);
>     ZooKeeperWatcher zkw = new ZooKeeperWatcher(conf, "testSchedWithZkLock", null, false);
>     queue = new MasterProcedureScheduler(conf,
>       TableLockManager.createTableLockManager(
>         conf, zkw, ServerName.valueOf("localhost", 12345, 1)));
>     final TableName tableName = TableName.valueOf("testtb");
>     TestTableProcedure procA = new TestTableProcedure(1, tableName,
>           TableProcedureInterface.TableOperationType.READ);
>     TestTableProcedure procB = new TestTableProcedure(2, tableName,
>           TableProcedureInterface.TableOperationType.READ);
>     assertTrue(queue.tryAcquireTableSharedLock(procA, tableName));
>     assertTrue(queue.tryAcquireTableSharedLock(procB, tableName));
>     queue.releaseTableSharedLock(procA, tableName);
>     queue.releaseTableSharedLock(procB, tableName);
>   }
> {code}



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