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From "Jonathan Gray (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-798) Provide Client API to explicitly lock and unlock rows
Date Tue, 12 Aug 2008 18:49:44 GMT

    [ https://issues.apache.org/jira/browse/HBASE-798?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12621929#action_12621929

Jonathan Gray commented on HBASE-798:

Thanks for review Jim!

1) Agreed.
2) Okay.  I was using the identical structure used to keep track of scanners:

  Map<String, InternalScanner> scanners =
    Collections.synchronizedMap(new HashMap<String, InternalScanner>());

Should that be changed as well?

3) Yes, I had it that way before but changed it for some reason.  I think because I need to
do a check at the end, but this checks the passed-in argument which is unchanged by the getLock
function call.  So you are correct, will change.

4) Yup.

Will wait to hear back regarding synchronizedMap(new Hashmap()) versus ConcurrentHashMap()
before posting new patch.

Thanks again, Jim!

> Provide Client API to explicitly lock and unlock rows
> -----------------------------------------------------
>                 Key: HBASE-798
>                 URL: https://issues.apache.org/jira/browse/HBASE-798
>             Project: Hadoop HBase
>          Issue Type: New Feature
>          Components: client, ipc, regionserver
>            Reporter: Jonathan Gray
>            Assignee: Jonathan Gray
>            Priority: Minor
>             Fix For: 0.3.0
>         Attachments: hbase-798-v1.patch, hbase-798-v2.patch, hbase-798-v3.patch, hbase-798-v4.patch
> We need to be able to perform a series of reads from and writes to a single row without
any potential interference from other clients.
> Unfortunately this is a bit involved because normal reads currently do not acquire row
locks, so it requires adding additional get/getRow calls that obtain and release a row lock.
> In addition, there will be two additional client calls, lockRow/unlockRow, which actually
acquire and release the locks.  Though each lock is associated with an HRegion, this will
be tracked within the HRegionServer.  When a lock is acquired from the client, it is handled
much like a Scanner.  We obtain the row lock from the HRegion, store the region name and lock
identifier in a synchronized Map, and also obtain a lease to ensure that the lock will eventually
be released even if the client dies.
> This also required adding a RowLockListener (implements LeaseListener) private class
in HRS to handle row lock lease expiration.
> HRS.lockRow will return a long lockId (as openScanner does) that will be used in subsequent
client calls to reuse this existing row lock.  These calls will check that the lock is valid
and perform the operations without any locking  (wrappers around get*, new versions of batchUpdate,
openScanner, etc).
> This is going to really add some noise to the list of available HTable/client methods
so I'm not sure if it's something people would want to commit into a normal release.  Regardless
this does provide some very convenient functionality that may be useful to others.
> We are also looking into Clint Morgan's HBASE-669, but one major downside is that there
is a significant amount of overhead involved.  This row locking is already built in and this
will only extend the API to allow clients to work with them directly.  There is little to
no overhead at all.  The only (obvious) performance consideration is that this should only
used where necessary as rows will not be locked and unlocked as quickly with round-trip client
calls.  In our design, we will have specific notes in our schema about which tables (or even
which families or columns) must be accessed with row locks at all times and which do not.
> This is my first attempt at adding any additional functionality, so comments, criticism,
code reviews are encouraged.
> I should have a patch up tomorrow.

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