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From: Rakesh R <rakeshr@huawei.com>
To: "user@zookeeper.apache.org" <user@zookeeper.apache.org>,
        "bit1129@163.com"
	<bit1129@163.com>
Subject: RE: A question about sequential order consistency that zookeeper
 garantees
Thread-Topic: A question about sequential order consistency that zookeeper
 garantees
Thread-Index: AQHP7m+ql7xiVJRUvEq/49CaFRPBm5w9DApQ
Date: Thu, 23 Oct 2014 04:33:09 +0000
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 <C2496325850AA74C92AAF83AA9662D26458DFA01@szxeml561-mbx.china.huawei.com>
References: <201410231115497082978@163.com>
In-Reply-To: <201410231115497082978@163.com>
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Hi,

Sequential Order Guarantee:
---------------------------

Server view:- It will process the requests in sequential fashion.=20

Say, there are two clients connected with the server, client-1 and client-2=
.=20
Assume application had created a znode /myapp and there is a logic to do di=
stributed locking mechanism. Now both these clients will try to acquire loc=
k by creating '/myapp/lock' znode. I'm calling these requests as request-1(=
create /myapp/lock) and request-2(create /myapp/lock). Again assume, server=
 receives the requests in the order of request-2 and then request-1. For si=
mplicity, you can imagine there is a Queue maintained at the server and all=
 these requests will be put into this Queue and executes in FIFO order. Fir=
st, server will pick request-2 and creates /myapp/lock znode. Also, it will=
 send watcher notifications if anyone has registered. Then move on to the n=
ext request-2(create /myapp/lock), while executing this will throw exceptio=
n saying NodeExistsException.

ZooKeeper maintains global ordering but how? - Every write is assigned a gl=
obally unique id, sequentially ordered identifier called a zxid, or ZooKeep=
er transaction id. This guarantees a global order to all updates in a ZooKe=
eper ensemble.


Regards,
Rakesh
-----Original Message-----
From: bit1129@163.com [mailto:bit1129@163.com]=20
Sent: 23 October 2014 08:46
To: user
Subject: A question about sequential order consistency that zookeeper garan=
tees

Hi,Zookeepers,

I'd like to ask a question about sequential order consistency that zookeepe=
r garantees.

Say, 2 zookeeper clients want to update the same znode data one after the o=
ther.Client1 sends the request 1 second before Client2.
What would be the end result? There is no order gurantee here, dure to netw=
ork delay, it is likely that clients1 send the request before client2, but =
the zookeeper servers first execute the request from Client1 before client2

If this is not what sequential order consistency means, then what exactly d=
oes sequential order consistencymeans, what does zookeeper exactly garantee=
?

Thanks.


bit1129@163.com