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From "Vishal Kathuria (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-1148) Multi-threaded handling of reads
Date Thu, 04 Aug 2011 19:52:27 GMT
Multi-threaded handling of reads 

                 Key: ZOOKEEPER-1148
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1148
             Project: ZooKeeper
          Issue Type: Improvement
          Components: server
            Reporter: Vishal Kathuria
             Fix For: 3.5.0

This improvement is to take advantage of multiple cores in the machines that typically run
ZooKeeper servers to get higher read throughput. 

The challenge with multiple threads is read/write ordering guarantees that ZooKeeper provides.

One way of handling these is to let readOnly clients use the multiple threads, and the read/write
clients continue to use the same single CommitProcessor thread for both reads and writes.
For this to work, a client would have to declare its readOnly intent through a flag at connect
time. (We already have a readOnly flag, but its intent is a bit different).

Another way of honoring the read/write guarantee is to let all sessions start as readOnly
sessions and have them use the multi-threaded reads until they do their first write. Once
a session performs a write, it automatically flips from a read/write session to a read only
session and starts using the single threaded CommitProcessor. This is a little tricky as one
has to worry about in flight reads when the write comes and we have to make sure those reads
finish before the write goes through.

I would like to get the community's feedback on whether it would be useful to have this and
whether an automatic discovery of readOnly or read/write intent is critical for this to be
useful. For us, the clients know at connect time whether they will ever do a write or not,
so an automatic detection is of limited use.

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