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From "james strachan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-78) added a high level protocol/feature - for easy Leader Election or exclusive Write Lock creation
Date Thu, 24 Jul 2008 09:19:32 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-78?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12616413#action_12616413

james strachan commented on ZOOKEEPER-78:

Thanks for the great comments Benjamin! Have already [added the constructor for you|https://svn.apache.org/repos/asf/activemq/sandbox/zookeeper/zookeeper-protocols/src/main/java/org/apache/zookeeper/protocols/WriteLock.java]

BTW I was pondering about switching the whenOwner from a Runnable to some kinda interface
that invokes you when you become the leader/owner - or when you stop being the leader/owner.
Something like 

public interface WhenOwnerListener {
  void whenOwner();
  void whenNotOwner();

Where only znodes that are the owner would be notified with the whenOwner() method; but then
if a connection fails or session times out, they'd be notified with a call to whenNotOwner();
Spookily - I'd set myself the target today to properly implement the watches so that WriteLock
gets a notification of it no longer being the leader/owner when a connection fails (which
normally auto-reconnects anyway right now in the base ZooKeeper). Then I was gonna add a notification
mechanism so we could notify the leader/owner is no longer the leader/owner when the session
expired exception occurs.

So we're absolutely on the same page; once I'd grokked the proper watch code for dealing with
normal connection failures & reconnects I was hoping to add something vaguely similar
to the ZooKeeperFacade so that higher level protocols can be aware of both when ZooKeeper
reconnects and when ZooKeeperFacade creates a whole new connection.

Does that make sense? I totally understand your concerns at making sure the WriteLock and
associated helper classes like ProtocolSupport/ZooKeeperFacade do the right thing - I want
exactly the same thing :) I'd just not yet had the chance to go through all the different
failure conditions and scenarios and make sure they all work properly :)

> added a high level protocol/feature - for easy Leader Election or exclusive Write Lock
> -----------------------------------------------------------------------------------------------
>                 Key: ZOOKEEPER-78
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-78
>             Project: Zookeeper
>          Issue Type: New Feature
>          Components: java client
>    Affects Versions: 3.0.0
>            Reporter: james strachan
>         Attachments: patch_with_including_Benjamin's_fix.patch, using_zookeeper_facade.patch
> Here's a patch which adds a little WriteLock helper class for performing leader elections
or creating exclusive locks in some directory znode. Note its an early cut; am sure we can
improve it over time. The aim is to avoid folks having to use the low level ZK stuff but provide
a simpler high level abstraction.

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