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From "Flavio Paiva Junqueira (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-79) Document jacob's leader election on the wiki recipes page
Date Wed, 23 Jul 2008 20:55:31 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-79?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12616257#action_12616257
] 

Flavio Paiva Junqueira commented on ZOOKEEPER-79:
-------------------------------------------------

I have added a description to the wiki. Comments are definitely welcome.

> Document jacob's leader election on the wiki recipes page
> ---------------------------------------------------------
>
>                 Key: ZOOKEEPER-79
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-79
>             Project: Zookeeper
>          Issue Type: New Feature
>          Components: documentation
>            Reporter: Patrick Hunt
>            Assignee: Flavio Paiva Junqueira
>
> The following discussion occurred on the zookeeper-user list. We need to formalize this
recipe and document on the wiki recipes page:
> ---------------------from jacob ----------------
> Avinash
>  
> The following protocol will help you fix the observed misbehavior. As Flavio points out,
you cannot rely on the order of nodes in getChildren, you must use an intrinsic property of
each node to determine who is the leader. The protocol devised by Runping Qi and described
here will do that.
>  
> First of all, when you create child nodes of the node that holds the leadership bids,
you must create them with the EPHEMERAL and SEQUENCE flag. ZooKeeper guarantees to give you
an ephemeral node named uniquely and with a sequence number larger by at least one than any
previously created node in the sequence. You provide a prefix, like "L_" or your own choice,
and ZooKeeper creates nodes named "L_23", "L_24", etc. The sequence number starts at 0 and
increases monotonously.
>  
> Once you've placed your leadership bid, you search backwards from the sequence number
of *your* node to see if there are any preceding (in terms of the sequence number) nodes.
When you find one, you place a watch on it and wait for it to disappear. When you get the
watch notification, you search again, until you do not find a preceding node, then you know
you're the leader. This protocol guarantees that there is at any time only one node that thinks
it is the leader. But it does not disseminate information about who is the leader. If you
want everyone to know who is the leader, you can have an additional Znode whose value is the
name of the current leader (or some identifying information on how to contact the leader,
etc.). Note that this cannot be done atomically, so by the time other nodes find out who the
leader is, the leadership may already have passed on to a different node.
>  
> Flavio
>  
> Might it make sense to provide a standardized implementation of leader election in the
library code in Java?
>  
> --Jacob
>  
> From: zookeeper-user-bounces@lists.sourceforge.net [mailto:zookeeper-user-bounces@lists.sourceforge.net]
On Behalf Of Flavio Junqueira
> Sent: Friday, July 11, 2008 1:02 AM
> To: zookeeper-user@lists.sourceforge.net
> Cc: zookeeper-user@hadoop.apache.org
> Subject: Re: [Zookeeper-user] Leader election
>  
> Hi Avinash, getChildren returns a list in lexicographic order, so if you are updating
the children of the election node concurrently, then you may get a different first node with
different clients. If you are using the sequence flag to create nodes, then you may consider
stripping the prefix of the node name and using the sufix value to determine order.
> Hope it helps.
> -Flavio
>  
> ----- Original Message ----
> From: Avinash Lakshman <avinash.lakshman@gmail.com>
> To: zookeeper-user@lists.sourceforge.net
> Sent: Friday, July 11, 2008 7:20:06 AM
> Subject: [Zookeeper-user] Leader election
> Hi
> I am trying to elect leader among 50 nodes. There is always one odd guy who seems to
think that someone else distinct from what some other nodes see as leader. Could someone please
tell me what is wrong with the following code for leader election:
> public void electLeader()
>         {           
>             ZooKeeper zk = StorageService.instance().getZooKeeperHandle();
>             String path = "/Leader";
>             try
>             {
>                 String createPath = path + "/L-";                               
>                 LeaderElector.createLock_.lock();
>                 while( true )
>                 {
>                     /* Get all znodes under the Leader znode */
>                     List<String> values = zk.getChildren(path, false);
>                     /*
>                      * Get the first znode and if it is the
>                      * pathCreated created above then the data
>                      * in that znode is the leader's identity.
>                     */
>                     if ( leader_ == null )
>                     {
>                         leader_ = new AtomicReference<EndPoint>( EndPoint.fromBytes(
zk.getData(path + "/" + values.get(0), false, null) ) );
>                     }
>                     else
>                     {
>                         leader_.set( EndPoint.fromBytes( zk.getData(path + "/" + values
.get(0), false, null) ) );
>                         /* Disseminate the state as to who the leader is. */
>                         onLeaderElection();
>                     }
>                     logger_.debug("Elected leader is " + leader_ + " @ znode " + ( path
+ "/" + values.get(0) ) );                  
>                     Collections.sort(values);
>                     /* We need only the last portion of this znode */
>                     String[] peices = pathCreated_.split("/");
>                     int index = Collections.binarySearch(values, peices[peices.length
- 1]);                  
>                     if ( index > 0 )
>                     {
>                         String pathToCheck = path + "/" + values.get(index - 1);
>                         Stat stat = zk.exists(pathToCheck, true);
>                         if ( stat != null )
>                         {
>                             logger_.debug("Awaiting my turn ...");
>                             condition_.await();
>                             logger_.debug("Checking to see if leader is around ...");
>                         }
>                     }
>                     else
>                     {
>                         break;
>                     }
>                 }
>             }
>             catch ( InterruptedException ex )
>             {
>                 logger_.warn(LogUtil.throwableToString(ex));
>             }
>             catch ( KeeperException ex )
>             {
>                 logger_.warn(LogUtil.throwableToString(ex));
>             }
>             finally
>             {
>                 LeaderElector.createLock_.unlock();
>             }
>         }
>     }
> Thanks
> Avinash
>  
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