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From: Amirhossein Kiani <amirhkiani@gmail.com>
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Subject: Re: Getting data after the watch
Date: Fri, 2 Mar 2012 11:23:33 -0800
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Many thanks Patrick for pointing me to the new documentation. I just =
found the other one from Google somehow.

So what I think is happening is actually impossible: to do getData() on =
a node and see the OLD data. in other words, I do not need to loop on a =
getData() to get the actual new data after being notified about the data =
change.
The reason that I'm saying that is that's the behavior I'm seeing in my =
code, but it might be just a bug on my side...

Cheers,
Amir

On Mar 2, 2012, at 9:54 AM, Patrick Hunt wrote:

> On Thu, Mar 1, 2012 at 4:11 PM, Amirhossein Kiani =
<amirhkiani@gmail.com> wrote:
>> Hi ZooKeepers,
>=20
> Hi Amir.
>=20
>> I just recently had an issue about getting the a node's updated data =
after the watch was actually fired. Being pretty confused, I read the =
documentation page again and sure enough this has been stated:
>>=20
>> "A client will see a watch event for a znode it is watching before =
seeing the new data that corresponds to that znode."
>> http://zookeeper.apache.org/doc/r3.1.2/zookeeperProgrammers.html
>>=20
>=20
> Those docs are pretty old. Try using the current version:
> http://zookeeper.apache.org/doc/current/zookeeperProgrammers.html
>=20
>> Now, I'm confused about how a data watch could actually be useful... =
But aside from that, is this statement also true about the node's list =
of Children? I.e. are a node's children it's "data" too?
>>=20
>=20
> Watches let you know something changed. Then you can take actions.
> Typically this means that the data of a znode changed, or the znode
> itself changed (created/deleted, children added/removed). There are
> two separate calls related to your question - getData and getChildren,
> they set watches on the data an child list respectively.
>=20
>> Because if this is the case, wouldn't the recipe for barriers hang if =
it gets the watch for a new node addition goes off and in:
>>=20
>>  if (list.size() < size) {
>>                        mutex.wait();
>>=20
>>  list.size() is still smaller than size?
>>=20
>> Or is this statement about watches only true for a data on the node?
>=20
> No. It's true for both cases. If you set a data watch then it's
> guaranteed that if you getData after being notified you will see the
> updated data. Same for getChildren and it's child list.
>=20
> Regards,
>=20
> Patrick


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