I agree with Ben. Can we have something like?

Contrib/recipes/leaderelection
Or 
Contrib/recipes/locking/

And a README inside each of these directories to explain what its doing?


mahadev

> -----Original Message-----
> From: Benjamin Reed [mailto:[EMAIL PROTECTED]
> Sent: Thursday, July 17, 2008 10:33 AM
> To: zookeeper-dev@hadoop.apache.org
> Subject: Re: Recipe contrib -- was Re: [PATCH] a simple Leader
Election or
> exclusive Write Lock protocol/policy
> 
> 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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