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From "bit1129@163.com" <bit1...@163.com>
Subject Re: Re: A question about sequential order consistency that zookeeper garantees
Date Mon, 03 Nov 2014 10:00:03 GMT
Hi Ritesh,

If your following case(client has multiple concurrent sessions and session expired),you said
that only writeA and writeC requests maybe honored, then where is writeB, gone?



bit1129@163.com
 
From: Ritesh Jaltare
Date: 2014-10-25 02:46
To: bit1129@163.com
CC: user
Subject: Re: Re: A question about sequential order consistency that zookeeper garantees
H bit1129

You are right. Before requesting any read or write operations, client application has to establish
a session with one of the zookeeper servers. The client initially connects to any server in
the ensemble, and only to a single server and uses a TCP connection to communicate with the
server. These sessions are the entities that provide order guarantees. Typically, a client
has only one session open but if a client has multiple concurrent sessions, fifo ordering
may not be preserved across the sessions.  

Lets consider this scenario:

Client c1 sends writeA and writeB requests asynchronously in its session

Session expires

Client c1 establishes another session and sends writeC request.

Here it is possible that only writeA and writeC requests are honored but it would still maintain
fifo order. 

Thanks
Ritesh Jaltare, Pivotal 




On Thu, Oct 23, 2014 at 7:15 PM, bit1129@163.com <bit1129@163.com> wrote:
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.



bit1129@163.com
 
From: Ritesh Jaltare
Date: 2014-10-24 04:54
To: user
CC: bit1129@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 <rakeshr@huawei.com> wrote:

Yes.

-Rakesh

From: bit1129@163.com [mailto:bit1129@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.



________________________________
bit1129@163.com<mailto:bit1129@163.com>

From: Rakesh R<mailto:rakeshr@huawei.com>
Date: 2014-10-23 12:33
To: user@zookeeper.apache.org<mailto:user@zookeeper.apache.org>; bit1129@163.com<mailto:bit1129@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: bit1129@163.com<mailto:bit1129@163.com> [mailto:bit1129@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.


bit1129@163.com<mailto:bit1129@163.com>



-- 
Ritesh Jaltare, Pivotal



-- 
Ritesh Jaltare, Pivotal
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