[ZOOKEEPER-1486] A couple of bugs in the tutorial code

• Type: Bug
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Component/s: documentation
• Assignee: Unassigned
• Reporter: Dmitri Perelman
• Created: 12/Jun/12 21:07
• Updated: 20/Jul/12 22:11

Description

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