what is java.io.EOFException, Message: Can not read response from server. Expected to read 4 bytes, read 0 bytes

Question

This question has been asked a couple of times in SO and many times in other sites. But I didn\'t get any satisfiable answer.

My problem:
I have a java web app

Answer 1

It most likely means that the database has restarted or the network connection to the database has been broken (e.g. a NAT connection has timed out) ... and your webapp is trying to use a stale database connection.

If the problem persists after restarting the web container, it could be something more serious.

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You asked the following:

How can the NumConnFree become 161 as I have Maximum Pool Size = 100 ?
How can the NumConnUsed become -136, a negative number ?
How can the NumConnDestroyed > NumConnCreated ? 

On the face of it, these don't make sense. However, they could simply be the result of some usage counters being updated in a non-thread-safe way. This is not necessarily related to your original problem.

Answer 2

While I don't have definitive solutions, it seems something is interfering communication between the app server and db. Following are few things you can try to isolate the problems:

• Try to determine wether this is mysql problem or java code problem. Try connect to mysql using the command line tool from the same host as the app server and issue similar SQL to perform login. Test using a simple java code that does a select, deploy it to the same infrastructure, see what happen etc. Also check the mysql server log, see if you can find anything useful

• There are two way an idle connection get closed: by the connection pool code that runs inside app server, or by mysql itself. Make sure you check the config on both sides

• Check if any network infrastructure config has changed recently. Was there any new firewall rule in place interfering with app server <--> mysql connectivity? Was there any settings that prohibits open TCP connection idling longer than X?

• Try a different connection pooling library just to eliminate the possibility it's the connection pooling

Good luck