Thanks Ritesh for the detailed explanation.
With FiFO client order and Asynchronous Linearizability as you mentioned, Can I
say that even the same client sends two request A and B asynchronously , the
server can still guarantee to process the request A ahead of B? If this is
true, the client must do something to help achieve this Asynchronous
Linearizability
Thanks.
bit1...@163.com
From: Ritesh Jaltare
Date: 2014-10-24 04:54
To: user
CC: bit1...@163.com
Subject: Re: RE: A question about sequential order consistency that zookeeper
garantees
Hi bit1129
Zookeeper provides 2 basic ordering guarantees:
1) Linearizable writes: What this means is all updates that change the state of
zookeeper are serializable and respect the precedence order.
2) FIFO client order: All requests from a given client are executed in the
order that they were sent by the client.
Now let us take your scenario and see how these 2 ordering guarantees interact.
. Suppose Client 1 sends request-1A and request-1B in order and Client2 sends
request-2A and request-2B in order. These requests can reach the zookeeper
server in any order. What zookeeper guarantees is that request-1A and
request-1B would retain the same order in the processing pipeline. That is,
request-1B would be processed by zookeeper only after request-1A. And similarly
for Client 2 , request-2B would be processed by zookeeper only after request-2A.
So the possible valid orderings are
Request-2A -> Request-1A -> Request 2B -> Request-1B
Request-1A -> Request-2A -> Request 2B -> Request-1B
Request-1A -> Request-2A -> Request 1B -> Request-2B
and so on...
In each of thee orderings, you can see that zookeeper maintains a fifo client
order. What zookeeper offers is Asynchronous Linearizability and not sequential
consistency. Asynchronous Linearizability is a stonger condition that
sequential consistency.
Thanks
Ritesh Jaltare, Pivotal
On Wed, Oct 22, 2014 at 10:35 PM, Rakesh R <rake...@huawei.com> wrote:
Yes.
-Rakesh
From: bit1...@163.com [mailto:bit1...@163.com]
Sent: 23 October 2014 10:43
To: Rakesh R
Subject: Re: RE: A question about sequential order consistency that zookeeper
garantees
Thanks Rakesh for the quick reply. So, I understand that the time that clients
send the request doesn't matter,but the server will "queue" the request and
process the request.
________________________________
bit1...@163.com<mailto:bit1...@163.com>
From: Rakesh R<mailto:rake...@huawei.com>
Date: 2014-10-23 12:33
To: user@zookeeper.apache.org<mailto:user@zookeeper.apache.org>;
bit1...@163.com<mailto:bit1...@163.com>
Subject: RE: A question about sequential order consistency that zookeeper
garantees
Hi,
Sequential Order Guarantee:
---------------------------
Server view:- It will process the requests in sequential fashion.
Say, there are two clients connected with the server, client-1 and client-2.
Assume application had created a znode /myapp and there is a logic to do
distributed locking mechanism. Now both these clients will try to acquire lock
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 simplicity, 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. First, 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 next
request-2(create /myapp/lock), while executing this will throw exception saying
NodeExistsException.
ZooKeeper maintains global ordering but how? - Every write is assigned a
globally unique id, sequentially ordered identifier called a zxid, or ZooKeeper
transaction id. This guarantees a global order to all updates in a ZooKeeper
ensemble.
Regards,
Rakesh
-----Original Message-----
From: bit1...@163.com<mailto:bit1...@163.com> [mailto:bit1...@163.com]
Sent: 23 October 2014 08:46
To: user
Subject: A question about sequential order consistency that zookeeper garantees
Hi,Zookeepers,
I'd like to ask a question about sequential order consistency that zookeeper
garantees.
Say, 2 zookeeper clients want to update the same znode data one after the
other.Client1 sends the request 1 second before Client2.
What would be the end result? There is no order gurantee here, dure to network
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 does
sequential order consistencymeans, what does zookeeper exactly garantee?
Thanks.
bit1...@163.com<mailto:bit1...@163.com>
--
Ritesh Jaltare, Pivotal
