Too many connections on zookeper server - apache-kafka

Question

Environment: HDP 2.6.4
Ambari – 2.6.1
3 zookeeper server
23.1.35.185 - is the IP of the first zookeeper server
hi all,
In the first zookeeper server it seems that even after closing the connection to zookeeper is not getting closed,
which causes the maximum number of client connections to be reached from a host - we have maxClientCnxns as 60 in zookeeper config
As a result when a new application comes and tries to create a connection it fails.
Example when Connections are:
echo stat | nc 23.1.35.185 2181
Latency min/avg/max: 0/71/399
Received: 3031 Sent: 2407
Connections: 67
Outstanding: 622
Zxid: 0x130000004d
Mode: follower
Node count: 3730
But after some time when connection comes to ~70 we see
echo stat | nc 23.1.35.185 2181
Ncat: Connection reset by peer.
And We can see also many CLOSE_WAIT
java 58936 zookeeper 60u IPv6 381963738 0t0 TCP Zookeper_server.sys54.com:eforward->zookeper_server.sys54.com:44983 (CLOSE_WAIT)
From the zookeeper log
2018-12-26 02:50:46,382 [myid:1] - WARN [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxnFactory#193]
- Too many connections from /23.1.35.185 - max is 60
In the ambari we can see also
Connection failed: [Errno 104] Connection reset by peer to zookeper_server.sys54.com.:2181
I must to say that this not happening on zookeeper servers 2 and 3
NOTE - if we increase the maxClientCnxns to 300 , its not help because after some time we get more the 300 connections ( CLOSE_WAIT ) and then we see from the log
2018-12-26 02:50:49,375 [myid:1] - WARN [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxnFactory#193] - Too many connections from /23.1.35.187 - max is 300
so any hint why the connection are CLOSE_WAIT ?

Answer 1

CLOSE_WAIT means that the local end of the connection has received a FIN from the other end, but the OS is waiting for the program at the local end to actually close its connection.
The problem is your program running on the local machine is not closing the socket. It is not a TCP tuning issue. A connection can (and quite correctly) stay in CLOSE_WAIT forever while the program holds the connection open.
Once the local program closes the socket, the OS can send the FIN to the remote end which transitions you to LAST_ACK while you wait for the ACK of the FIN. Once that is received, the connection is finished and drops from the connection table (if your end is in CLOSE_WAIT you do not end up in the TIME_WAIT state).
There is a kernel level property to reuse the connection and reduce the CLOSE_WAIT time.
I suggest you to follow this tutorial http://www.linuxbrigade.com/reduce-time_wait-socket-connections/
This should probably solve your problem.