Document jacob's leader election on the wiki recipes page

                 Key: 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 ----------------


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 


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.




Might it make sense to provide a standardized implementation of leader election 
in the library code in Java?




From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Flavio Junqueira
Sent: Friday, July 11, 2008 1:02 AM
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.



----- Original Message ----
From: Avinash Lakshman <[EMAIL PROTECTED]>
Sent: Friday, July 11, 2008 7:20:06 AM
Subject: [Zookeeper-user] Leader election


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";
                String createPath = path + "/L-";                               
                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) ) );
                        leader_.set( EndPoint.fromBytes( zk.getData(path + "/" 
+ values .get(0), false, null) ) );
                        /* Disseminate the state as to who the leader is. */
                    logger_.debug("Elected leader is " + leader_ + " @ znode " 
+ ( path + "/" + values.get(0) ) );                  
                    /* 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 ...");
                            logger_.debug("Checking to see if leader is around 
            catch ( InterruptedException ex )
            catch ( KeeperException ex )



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