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https://issues.apache.org/jira/browse/ZOOKEEPER-1486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13419625#comment-13419625
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Alexander Shraer commented on ZOOKEEPER-1486:
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The problem above appears in the barrier java example
(http://zookeeper.apache.org/doc/r3.1.2/zookeeperTutorial.html#sc_barriers).
The pseudocode
(http://zookeeper.apache.org/doc/r3.1.2/recipes.html#sc_recipes_eventHandles)
creates a special "ready" node to deal with the issue, but this doesn't appear
in the java example for some reason.
> A couple of bugs in the tutorial code
> -------------------------------------
>
> Key: ZOOKEEPER-1486
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1486
> Project: ZooKeeper
> Issue Type: Bug
> Components: documentation
> Reporter: Dmitri Perelman
> Priority: Minor
>
> Hi,
> There are two problems with the barrier example code in the tutorial:
> 1) A znode created by a process in the function enter() is created with
> SEQUENTIAL suffix, however, the name of a znode deleted in the function
> leave() doesn't have this suffix. Actually, the leave() function tries to
> delete a nonexistent node => a KeeperException is thrown, which is caught
> silently => the process terminates without waiting for the barrier.
> 2) It seems that the very idea of leaving the barrier by deleting ephemeral
> nodes is problematic. Consider the following scenario: there are two clients:
> C1 and C2.
> - C1 enters the barrier, creates a znode /b1/C1, checks that it's alone and
> starts waiting for the second client to come.
> - C2 enters the barrier and creates a znode /b1/C2 - the notification to C1
> is sent but still not delivered.
> - C2 observes that there are enough children to /b1, enters the barrier,
> executes its own operations and invokes leave() procedure.
> - during the leave() procedure C2 removes its znode /b1/C2 and exits.
> - when the notification about C2's arrival finally arrives to C1, C1 checks
> the children of /b1 and doesn't find C2's znode: C1 is stuck.
> The solution to this data race would be to create special znodes for leaving
> the barrier, similarly to the way they are created for entering the barrier.
> Thanks,
> Dima
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