[jira] [Commented] (ZOOKEEPER-1486) A couple of bugs in the tutorial code

["Alexander Shraer (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-1486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13419625#comment-13419625 ] 

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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