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https://issues.apache.org/jira/browse/ZOOKEEPER-79?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12614556#action_12614556
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Flavio Paiva Junqueira commented on ZOOKEEPER-79:
-------------------------------------------------
This idea of having clients watching on different nodes when waiting for
application events is crucial to avoid the herd effect, and it has applications
other than leader election. Ben and I, for example, used a similar idea to
implement barriers. With a barrier, clients need to wait until all other
clients are done with their part of a computation, and we can use the existence
of a znode to express the fact that a particular client hasn't finished.
> 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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