Github user CheneySun commented on the issue:
https://github.com/apache/zookeeper/pull/312
@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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