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From CheneySun <...@git.apache.org>
Subject [GitHub] zookeeper issue #312: ZOOKEEPER-1669: Operations to server will be timed-out...
Date Fri, 21 Jul 2017 08:53:04 GMT
Github user CheneySun commented on the issue:

    @hanm The issue is raised while tens thousands of clients try to reconnect ZooKeeper service.
Actually, we came across the issue during maintaining our HBase cluster, which used a  5-server
ZooKeeper cluster. The HBase cluster was composed of many many regionservers (in thousand
order of magnitude), and connected by tens thousands of clients to do massive reads/writes.
Because the r/w throughput is very high, ZooKeeper zxid increased quickly as well. Basically,
each two or three weeks, Zookeeper would make leader relection triggered by the zxid roll
over. The leader relection will cause the clients(HBase regionservers and HBase clients) disconnected
and reconnected with Zookeeper servers in the mean time, and try to renew the sessions. 
    In current implementation of session renew, NIOServerCnxnFactory will clone all the connections
at first in order to avoid race condition in multi-threads and go iterate the cloned connection
set one by one to find the related session to renew. It's very time consuming. In our case
(described above), it caused many region servers can't  successfully  renew session before
session timeout, and eventually the HBase cluster lose these region servers and affect the
HBase stability. 
    The change is to make refactoring to the close session logic and introduce a ConcurrentHashMap
to store session id and connection map relation, which is a thread-safe data structure and
eliminate the necessary to clone the connection set at first. 

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