Document jacob's leader election on the wiki recipes page
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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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