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From Patrick Hunt <ph...@apache.org>
Subject Re: Stale value for read request
Date Mon, 25 Oct 2010 19:33:49 GMT
On Sat, Oct 23, 2010 at 9:03 PM, jingguo yao <yaojingguo@gmail.com> wrote:

> Read requests are handled locally at each Zookeeper server. So it is
> possible for a read request to return a stale value even though a more
> recent update to the same znode has been committed. Does this statement
> still hold if the Zookeeper follower serving the read request is the one
> which has just served the recent update request?
>

It's probably good to start with the explicit guarantees:
http://hadoop.apache.org/zookeeper/docs/current/zookeeperProgrammers.html#ch_zkGuarantees

Yes (it could still get "stale" data from quorum perspective). The leader
may have committed a new change that has not yet been seen by the follower
(ie two changes in quick succession)


> For example, client A connects to follower X. And client A issues a request
> to update znode /a from 0 to 1. After receiving this request, follower X
> forwards this request to the leader. Then the leader broadcasts this update
> proposal to all the Zookeeper servers. After a quorum of the followers
> commit the update request, the update succeeds. Then client A issues a read
> request to get the value of znode /a. And follower X receives this read
> request. So if follower X is not among the quorum and follower X has not
> committed the update to catch up with the leader, it is still possible for
> client A to get a stale value of znode /a. In this case, the return value
> is
> 0.
>
> Is my understanding correct?


That's correct. See the the NOTE in the section (link) I provided above.

Patrick

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