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From Patrick Hunt <ph...@apache.org>
Subject Re: Simpler ZooKeeper event interface....
Date Tue, 06 Jan 2009 22:02:19 GMT
I should have been more clear on 3c - in this case you will get notified 
in the callback of CONNECTIONLOSS for any pending async requests, but as 
you are ignoring rc it may cause problems.


Patrick Hunt wrote:
> Hi Kevin, a couple of issues I noticed while looking at the pastebin:
> 1) you are ignoring the result codes in the callbacks, this could get 
> you into trouble (say you do a getData on a node that has been deleted 
> ie someone changes then immed. deletes the node)
> 2) I'm confused by one of your comments, you mention:
>   //read the current value.  NOTE that this could easily be a blocking
>   //read here but we might as well have one code path.
>   zk.getData( event.getPath(), true, this, null );
> however you are using an async API call, what blocking are you referring 
> to? If you are ok w/blocking use the synchronous API, your code would be 
> simpler (no callbacks!)
> 3) it's possible for your code to get notified of a change, but never 
> process the change. This might happen if:
>  a) a node changed watch fires
>  b) your client code runs an async getData
>  c) you are disconnected from the server
> Patrick
> Kevin Burton wrote:
>> Hey guys.
>> I think I'm finally in the position to push ZK into production for a 
>> while
>> to test it out.
>> My biggest feedback (other than the small bugs I found) was that the API
>> could be a bit simpler.
>> I codified my thoughts here:
>> http://pastebin.com/f2ecea8c7
>> http://pastebin.com/f62a01e9
>> Basically, I was thinking that one could receive an onData event to 
>> receive
>> the initial value.
>> Then all future events would call onData.....
>> I was thinking that an onExists() method might also be nice.
>> The current API could be made cleaner with:
>>  - one or two standalone Listener interfaces with onFoo methods for each
>> event type.
>>  - the processResults() method is the same for each interface right now
>> which is somewhat confusing.  Using onFoo is more self documenting.
>>  - using the main thread by using poll() to wait for events from 
>> ZooKeeper.
>> I use a ConcurrentLinkedQueue in my implementation.
>> Also, is there a race condition between when the client receives an event
>> for an update and before it can request a new one?  I was thinking 
>> session
>> local based events would solve this problem (you register your watch once
>> per session and then get all events until it is unregistered).
>> I think this can be solved in my code by reading the current version 
>> of the
>> value from the getData() method I call when I register the new watch and
>> comparing it to the last version I saw. If it was incremented then I 
>> would
>> call onData again.
>> The problem here is that I might miss two updates (but at least I would
>> receive the last stable value).
>> Kevin

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