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https://issues.apache.org/jira/browse/ZOOKEEPER-79?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12614448#action_12614448
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james strachan commented on ZOOKEEPER-79:
-----------------------------------------

Ah cool :) Was just checking we were not about to do the same thing separate :).

I've basically followed the same algorithm from the wiki recipe - and the same 
one described in the ZooKeeper tutorial...
http://developer.yahoo.com/blogs/hadoop/2008/03/intro-to-zookeeper-video.html

So AFAIK yes its the same




> 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: Patrick Hunt
>
> 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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Flavio 
> Junqueira
> Sent: Friday, July 11, 2008 1:02 AM
> To: [EMAIL PROTECTED]
> Cc: [EMAIL PROTECTED]
> 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 <[EMAIL PROTECTED]>
> To: [EMAIL PROTECTED]
> 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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