[jira] Commented: (ZOOKEEPER-78) added a high level protocol/feature - for easy Leader Election or exclusive Write Lock creation

["Tom White (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-78?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12675792#action_12675792 ] 

Tom White commented on ZOOKEEPER-78:
------------------------------------

Mahadev,

I'm glad this is being worked on again. Some comments:

* Not sure if a Runnable is right for whenOwner, as Runnable implementations are often one-shot and can't be re-run, whereas it looks like WriteLock can be locked and unlocked repeatedly (is this right?). There's also no way to tell when the lock has been released. Might be better to have a standard listener interface called LockListener, with lockAcquired and lockReleased methods. Writing an implementation to run a Runnable would be trivial.
* I worry a little about WriteLock implementing java.util.concurrent.locks.Lock, since Lock seems oriented to single-process programs. By making WriteLock a Lock we make it easy for programmers to drop it in to programs that use Lock, without having to think about distributed error recovery. For example, Lock#lock() cannot throw checked exceptions, so we would have to wrap KeeperException in an unchecked exception, which could too readily be ignored. Perhaps the answer is to make the method naming and semantics as close as possible, without actually implementing Lock proper? (I think this is similar to the discussion in ZOOKEEPER-104.)
* Lots of classes and methods are public when they don't need to be. Really only WriteLock should be public, with public methods lock(), unlock(), getDir() (better named as getLockPath() or similar?), getWhenOwner(), setWhenOwner() (or whatever replaces the last two).


> added a high level protocol/feature - for easy Leader Election or exclusive Write Lock creation
> -----------------------------------------------------------------------------------------------
>
>                 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
>            Assignee: james strachan
>             Fix For: 3.2.0
>
>         Attachments: patch_with_including_Benjamin's_fix.patch, using_zookeeper_facade.patch, ZOOKEEPER-78.patch, ZOOKEEPER-78.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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