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https://issues.apache.org/jira/browse/ZOOKEEPER-80?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12614466#action_12614466
 ] 

Doug Cutting commented on ZOOKEEPER-80:
---------------------------------------

Whoa!  Big issue description!  Perhaps you could have gone with a link to the 
mail archive?  Descriptions are included in every message about the issue...

In any case, I think perhaps each recipe deserves its own code-tree, and should 
hence be a separate contrib module.  Perhaps, instead of 'contrib/' these 
should just be under 'recipes/', with a separate src/, lib/, doc/, build.xml, 
README.txt, etc. for each?  Multiple language implementations would go in 
different src/ subdirectories.  Does that work?

Also, I am -1 for making these subversion-only.  Only released software is 
fully covered by the Apache license.  Subversion is for internal exchange by 
Apache of works-in-progress, not for end users.



> Document process for client recipe contributions
> ------------------------------------------------
>
>                 Key: ZOOKEEPER-80
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-80
>             Project: Zookeeper
>          Issue Type: Task
>          Components: documentation
>            Reporter: Patrick Hunt
>            Assignee: Patrick Hunt
>
> How do we accept zk client recipe contributions? Initiated by the following 
> discussion on the mailing list:
> ------ ben reed wrote --------
> Excellent proposal. The only thing I would add is that there should be
> an english description of the recipe in subversion. That way if someone
> wanted to do a compatible binding they can do it. If the recipe is on
> the wiki it would be hard to keep it in sync, so it is important that it
> is in subversion. My preference would be that the doc would be in the
> same contrib subdirectory as the source for ease of maintenance.
> ben
> Patrick Hunt wrote:
> > > James, thanks for the contribution! Tests and everything.  :-) 
> > >
> > > Jacob sent some mail to the list recently (attached) that details a
> > > protocol that he's used successfully (and picked up by some zk users).
> > > I have a todo item to document this protocol on the recipes wiki page,
> > > haven't gotten to it yet. Not sure how/if this matches what you've
> > > done but we should sync up (also see below).
> > > https://issues.apache.org/jira/browse/ZOOKEEPER-79
> > >
> > > There has been some discussion on client side helper code in the past
> > > however this is the first contribution. We need to make some decisions
> > > and outline what/how we will accept.
> > >
> > > 1) I think we should have a
> > > "contrib/recipes/{java/{main,test}/org/apache/zookeeper/... ,c/,...}"
> > > hierarchy for contributions that implement recipes, including any
> > > helper code
> > >
> > > 2) We should first document recipes on the wiki, then implement them
> > > in the code
> > > http://wiki.apache.org/hadoop/ZooKeeper/ZooKeeperRecipes
> > > The code should fully document the api/implementation, and refer to
> > > wiki page for protocol specifics.
> > >
> > > 3) What should we do relative to ZK releases. Are recipes included in
> > > a release? Will bugs in recipes hold up a release?
> > >
> > > My initial thought is that contrib is available through svn, but not
> > > included in the release. If users want to access/use this code they
> > > will be required to checkout/build themselves. (at least initially)
> > >
> > > 4) We will not require "parody" btw the various client languages.
> > > Currently we support Java/C clients, we will be adding various
> > > scripting languages soon. Contributions will be submitted for various
> > > clients (James' submission is for java), that will be placed into
> > > contrib, if someone else contributes C bindings (etc...) we will add
> > > those to contrib/recipes as well.
> > >
> > > 5) Implementations should strive to implement similar recipe protocols
> > > (see 2 above, a good reason to document before implement). There may
> > > be multiple, different, protocols each with their own implementation,
> > > but for a particular protocol the implementations should be the same.
> > >
> > > We may want to stress 5 even more - if multiple clients
> > > implementations (c/java/...) are participating in a single instance of
> > > leader election it will be CRITICAL for them to be inter-operable.
> > >
> > >
> > > Comments, questions, suggestion?
> > >
> > > Patrick
> > >
> > > James Strachan wrote:
> >> >> So having recently discovered ZooKeeper, I'm really liking it - good
> >> >> job folks!
> >> >>
> >> >> I've seen discussions of building high level features from the core ZK
> >> >> library and had not seen any available on the interweb so figured I'd
> >> >> have a try creating a simple one. Feel free to ignore it if a ZK ninja
> >> >> can think of a neater way of doing it - I've basically followed the
> >> >> protocol defined in the recent ZK presentation...
> >> >> http://developer.yahoo.com/blogs/hadoop/2008/03/intro-to-zookeeper-video.html
> >> >>
> >> >>
> >> >> I've submitted the code as a patch here...
> >> >> https://issues.apache.org/jira/browse/ZOOKEEPER-78
> >> >>
> >> >> I figured the Java Client might as well come with some helper code to
> >> >> make doing things like exclusive locks or leader elections easier; we
> >> >> could always spin them out into a separate library if and when
> >> >> required etc. Right now its one fairly simple class  :) 
> >> >>
> >> >> Currently its a simple class where you can register a Runnable to be
> >> >> invoked when you have the lock; or you can just keep asking if you
> >> >> have the lock now and again as you see fit etc.
> >> >>
> >> >> WriteLock locker = new WriteLock(zookeeper, "/foo/bar");
> >> >> locker.setWhenOwner(new Runnable() {...}); // fire this code when
> >> >> owner...
> >> >>
> >> >> // lets try own it
> >> >> locker.acquire();
> >> >>
> >> >> // I may or may not have the lock now
> >> >> if (locker.isOwner()) {....}
> >> >>
> >> >> // time passes
> >> >> locker.close();
> >> >>
> >> >>
> >> >> Thoughts?
> >> >>
> > >
> > > ------------------------------------------------------------------------
> > >
> > > Subject:
> > > Re: [Zookeeper-user] Leader election
> > > From:
> > > "Jacob Levy" <[EMAIL PROTECTED]>
> > > Date:
> > > Fri, 11 Jul 2008 10:42:33 -0700
> > > To:
> > > "Flavio Junqueira" <[EMAIL PROTECTED]>,
> > > <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>
> > >
> > > To:
> > > "Flavio Junqueira" <[EMAIL PROTECTED]>,
> > > <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>
> > > CC:
> > > [EMAIL PROTECTED]
> > >
> > >
> > > *Avinash*
> > >
> > >  
> > >
> > > The following protocol will help you fix the observed misbehavior. As
> > > Flavio points out, you cannot rely on the order of nodes in
> > > getChildren, you must use an intrinsic property of each node to
> > > determine who is the leader. The protocol devised by Runping Qi and
> > > described here will do that.
> > >
> > >  
> > >
> > > First of all, when you create child nodes of the node that holds the
> > > leadership bids, you must create them with the EPHEMERAL and SEQUENCE
> > > flag. ZooKeeper guarantees to give you an ephemeral node named
> > > uniquely and with a sequence number larger by at least one than any
> > > previously created node in the sequence. You provide a prefix, like
> > > "L_" or your own choice, and ZooKeeper creates nodes named "L_23",
> > > "L_24", etc. The sequence number starts at 0 and increases monotonously.
> > >
> > >  
> > >
> > > Once you've placed your leadership bid, you search backwards from the
> > > sequence number of **your** node to see if there are any preceding (in
> > > terms of the sequence number) nodes. When you find one, you place a
> > > watch on it and wait for it to disappear. When you get the watch
> > > notification, you search again, until you do not find a preceding
> > > node, then you know you're the leader. This protocol guarantees that
> > > there is at any time only one node that thinks it is the leader. But
> > > it does not disseminate information about who is the leader. If you
> > > want everyone to know who is the leader, you can have an additional
> > > Znode whose value is the name of the current leader (or some
> > > identifying information on how to contact the leader, etc.). Note that
> > > this cannot be done atomically, so by the time other nodes find out
> > > who the leader is, the leadership may already have passed on to a
> > > different node.
> > >
> > >  
> > >
> > > *Flavio*
> > >
> > >  
> > >
> > > Might it make sense to provide a standardized implementation of leader
> > > election in the library code in Java?
> > >
> > >  
> > >
> > > --Jacob
> > >
> > >  
> > >
> > > ------------------------------------------------------------------------
> > >
> > > *From:* [EMAIL PROTECTED]
> > > [mailto:[EMAIL PROTECTED] *On Behalf Of
> > > *Flavio Junqueira
> > > *Sent:* Friday, July 11, 2008 1:02 AM
> > > *To:* [EMAIL PROTECTED]
> > > *Cc:* [EMAIL PROTECTED]
> > > *Subject:* Re: [Zookeeper-user] Leader election
> > >
> > >  
> > >
> > > Hi Avinash, getChildren returns a list in lexicographic order, so if
> > > you are updating the children of the election node concurrently, then
> > > you may get a different first node with different clients. If you are
> > > using the sequence flag to create nodes, then you may consider
> > > stripping the prefix of the node name and using the sufix value to
> > > determine order.
> > >
> > > Hope it helps.
> > >
> > > -Flavio
> > >
> > >  
> > >
> > > ----- Original Message ----
> > > From: Avinash Lakshman <[EMAIL PROTECTED]>
> > > To: [EMAIL PROTECTED]
> > > Sent: Friday, July 11, 2008 7:20:06 AM
> > > Subject: [Zookeeper-user] Leader election
> > >
> > > Hi
> > >
> > > I am trying to elect leader among 50 nodes. There is always one odd
> > > guy who seems to think that someone else distinct from what some other
> > > nodes see as leader. Could someone please tell me what is wrong with
> > > the following code for leader election:
> > >
> > > public void electLeader()
> > >         {           
> > >             ZooKeeper zk = StorageService.instance().getZooKeeperHandle();
> > >             String path = "/Leader";
> > >             try
> > >             {
> > >                 String createPath = path +
> > > "/L-";                               
> > >                 LeaderElector.createLock_.lock();
> > >                 while( true )
> > >                 {
> > >                     /* Get all znodes under the Leader znode */
> > >                     List<String> values = zk.getChildren(path, false);
> > >                     /*
> > >                      * Get the first znode and if it is the
> > >                      * pathCreated created above then the data
> > >                      * in that znode is the leader's identity.
> > >                     */
> > >                     if ( leader_ == null )
> > >                     {
> > >                         leader_ = new AtomicReference<EndPoint>(
> > > EndPoint.fromBytes( zk.getData(path + "/" + values.get(0), false,
> > > null) ) );
> > >                     }
> > >                     else
> > >                     {
> > >                         leader_.set( EndPoint.fromBytes(
> > > zk.getData(path + "/" + values .get(0), false, null) ) );
> > >                         /* Disseminate the state as to who the leader
> > > is. */
> > >                         onLeaderElection();
> > >                     }
> > >                     logger_.debug("Elected leader is " + leader_ + " @
> > > znode " + ( path + "/" + values.get(0) ) );                  
> > >                     Collections.sort(values);
> > >                     /* We need only the last portion of this znode */
> > >                     String[] peices = pathCreated_.split("/");
> > >                     int index = Collections.binarySearch(values,
> > > peices[peices.length - 1]);                  
> > >                     if ( index > 0 )
> > >                     {
> > >                         String pathToCheck = path + "/" +
> > > values.get(index - 1);
> > >                         Stat stat = zk.exists(pathToCheck, true);
> > >                         if ( stat != null )
> > >                         {
> > >                             logger_.debug("Awaiting my turn ...");
> > >                             condition_.await();
> > >                             logger_.debug("Checking to see if leader
> > > is around ...");
> > >                         }
> > >                     }
> > >                     else
> > >                     {
> > >                         break;
> > >                     }
> > >                 }
> > >             }
> > >             catch ( InterruptedException ex )
> > >             {
> > >                 logger_.warn(LogUtil.throwableToString(ex));
> > >             }
> > >             catch ( KeeperException ex )
> > >             {
> > >                 logger_.warn(LogUtil.throwableToString(ex));
> > >             }
> > >             finally
> > >             {
> > >                 LeaderElector.createLock_.unlock();
> > >             }
> > >         }
> > >     }
> > >
> > > Thanks
> > > Avinash
> > >
> > >  
> > >
> > > ------------------------------------------------------------------------
> > >
> > > -------------------------------------------------------------------------
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> > > ------------------------------------------------------------------------
> > >
> > > _______________________________________________
> > > Zookeeper-user mailing list
> > > [EMAIL PROTECTED]
> > > https://lists.sourceforge.net/lists/listinfo/zookeeper-user
> > >   

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