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.