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From Flavio Junqueira <...@yahoo-inc.com>
Subject Re: group messaging, empheral nodes on zookeeper
Date Tue, 06 Jan 2009 17:48:41 GMT
If I understand it correctly, you propose two mechanisms:

1- Have one single node, and modify the data of that znode;
2- Have a znode, say "/broadcast", and have clients creating a new  
child znode under "/broadcast" for every new message they want to  
broadcast.

In case 1), if you are proposing to overwrite the content of the  
znode, then you would need first to make sure that all receivers have  
already received the previous message. This doesn't seem a good  
solution to me because a client that wants to broadcast a message  
would have to wait until all others flag that they have received the  
previous message. Appending new messages to the current content of the  
znode doesn't seem a great solution either because clients would have  
to make sure that they are overwriting the correct version.

Solution 2) sounds better to me because it is wait-free: no client  
willing to broadcast a message contends with other clients. Now, as  
you say, you have to use the sequence flag to make sure that all  
messages are delivered in the same order (if you want total order).  
You may still want to have a mechanism for clients to flag which  
messages they have already delivered so that you remove messages that  
everyone has already delivered. If your application generates a large  
number of messages and you don't remove them, you might end up with an  
unnecessarily large number of znodes. Finally, I'm not sure why you  
want to use ephemeral znodes for messages. I think that ephemeral  
znodes may cause you trouble in this case. Suppose that a client  
broadcasts a message by creating an ephemeral znode and crashes before  
all other clients deliver the message. You may end up with two clients  
delivering different sequences of messages.

-Flavio

On Jan 6, 2009, at 6:28 PM, Kevin Burton wrote:

>>
>> 1. Group Messaging:
>> Is it possible to do group messaging with zookeeper 3.0.1 ?
>> If not, are there any plans to add group messaging (say w/ casual  
>> ordering
>> or even better total ordering) to future releases ?
>> What would be the best approach if I need to do group messaging using
>> zookeeper right now, should I use a separate group messaging or  
>> messaging
>> broker service ?
>>
>
> You could just have a node named '/service/foo'
>
> and then have all members in your group listen to this node.  When  
> you need
> to push a messages you can write to that node.
>
> Or you could use a directory with ephemeral sequence files, once per
> message.
>
>
>
> -- 
> Founder/CEO Spinn3r.com
> Location: San Francisco, CA
> AIM/YIM: sfburtonator
> Skype: burtonator
> Work: http://spinn3r.com


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