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From "Jonathan Gray (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HBASE-798) Provide Client API to explicitly lock and unlock rows
Date Fri, 08 Aug 2008 00:22:46 GMT

     [ https://issues.apache.org/jira/browse/HBASE-798?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Jonathan Gray updated HBASE-798:

    Attachment: hbase-798-v1.patch

Here's my first go.

This adds lockRow and unlockRow methods to the client/HTable.  The return type of lockRow
is a new client object RowLock (contains long lockid and byte[] row).  unlockRow takes a RowLock
as argument.

Also adds new versions of commit, deleteAll, and deleteFamily that also can take RowLock's.

Within HRS I created new versions of the same functions which take the actual lock ids: long
in HRS (randomized id as with scannerids, used for leases, etc), Integer in HR (existing type
used for row locks).  Existing functions, which do not explicitly take locks, now call the
same HR functions but with nulls as row locks.

Internally HR checks if it was passed a valid row lock or not.  If it's a null, it obtains
a row lock and releases it at the end of the call.  If it's valid, it makes use of the lock,
and does nothing at the end.  Otherwise an exception is thrown.

Code has not been completely cleaned up, but this demonstrates how the client row locks are
being implemented.

> 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
>            Priority: Minor
>         Attachments: hbase-798-v1.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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