Re: [Zookeeper-user] Leader election

[Patrick H. <ph...@ap...>]

I don't see leader election documented on the ZK "recipies" wiki:
http://zookeeper.wiki.sourceforge.net/ZooKeeperRecipes

Jacob, would you mind updating the wiki page, documenting this recipe?

Thanks!

Patrick

Jacob Levy wrote:
> *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:* zoo...@li... 
> [mailto:zoo...@li...] *On Behalf Of 
> *Flavio Junqueira
> *Sent:* Friday, July 11, 2008 1:02 AM
> *To:* zoo...@li...
> *Cc:* zoo...@ha...
> *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 <avi...@gm...>
> To: zoo...@li...
> 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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