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https://issues.apache.org/jira/browse/ZOOKEEPER-1011?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16695473#comment-16695473
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Michael K. Edwards commented on ZOOKEEPER-1011:
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Appropriate for 3.5.5?
> fix Java Barrier Documentation example's race condition issue and polish up
> the Barrier Documentation
> -----------------------------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-1011
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1011
> Project: ZooKeeper
> Issue Type: Bug
> Components: documentation
> Reporter: Semih Salihoglu
> Assignee: maoling
> Priority: Major
> Labels: pull-request-available
> Time Spent: 20m
> Remaining Estimate: 0h
>
> There is a race condition in the Barrier example of the java doc:
> http://hadoop.apache.org/zookeeper/docs/current/zookeeperTutorial.html. It's
> in the enter() method. Here's the original example:
> boolean enter() throws KeeperException, InterruptedException{
> zk.create(root + "/" + name, new byte[0], Ids.OPEN_ACL_UNSAFE,
> CreateMode.EPHEMERAL_SEQUENTIAL);
> while (true) {
> synchronized (mutex) {
> List<String> list = zk.getChildren(root, true);
> if (list.size() < size) {
> mutex.wait();
> } else {
> return true;
> }
> }
> }
> }
> Here's the race condition scenario:
> Let's say there are two machines/nodes: node1 and node2 that will use this
> code to synchronize over ZK. Let's say the following steps take place:
> node1 calls the zk.create method and then reads the number of children, and
> sees that it's 1 and starts waiting.
> node2 calls the zk.create method (doesn't call the zk.getChildren method yet,
> let's say it's very slow)
> node1 is notified that the number of children on the znode changed, it checks
> that the size is 2 so it leaves the barrier, it does its work and then leaves
> the barrier, deleting its node.
> node2 calls zk.getChildren and because node1 has already left, it sees that
> the number of children is equal to 1. Since node1 will never enter the
> barrier again, it will keep waiting.
> --- End of scenario ---
> Here's Flavio's fix suggestions (copying from the email thread):
> ...
> I see two possible action points out of this discussion:
>
> 1- State clearly in the beginning that the example discussed is not correct
> under the assumption that a process may finish the computation before another
> has started, and the example is there for illustration purposes;
> 2- Have another example following the current one that discusses the problem
> and shows how to fix it. This is an interesting option that illustrates how
> one could reason about a solution when developing with zookeeper.
> ...
> We'll go with the 2nd option.
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