Java – In-use connections equal max-pool-size and expired max-wait-time. Cannot allocate more connections

database connectionjava

I am facing following problem with an HTTP 500 status.

Message description:

The server encountered an internal error () that prevented it from
fulfilling this request.

Exception:

javax.servlet.ServletException: java.sql.SQLException: Error in
allocating a connection. Cause: In-use connections equal max-pool-size
and expired max-wait-time. Cannot allocate more connections.

Root cause:

java.sql.SQLException: Error in allocating a connection. Cause: In-use
connections equal max-pool-size and expired max-wait-time. Cannot
allocate more connections.

note The full stack traces of the exception and its root causes are
available in the GlassFish v3 logs. GlassFish v3

Best Answer

Seems that you have a connection leak somewhere in your application.

When you borrow a connection from the connection pool and forget to properly close it, it remains marked as unavailable, causing the exception you are experiencing.

To locate the leak, you should monitor the connection usage through the JMX connector surely exposed by your application server.

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Java – change the max-pool-size for connections, and what determines what I should set it to

How to know how much connection the application is currently using

You don't give enough information to answer that. Most appservers will have some sort of JMX reporting on things like this. Alternatively, depending on the database, you could find the number of currently open connections.

what should be the optimal connection pool settings for a heavily traffic applicaiton

Higher than what you've got?

The above of course assumes that you're not mishandling connections. By that I mean if you're using them directly in code you should always use this idiom:

Connection conn = null;
try {
  conn = ... ; // get connection
  // do stuff
} finally {
  if (conn != null) try { conn.close(); } catch (Exception e) { }
}

If you're not released connections back to the pool and are waiting for the garbage collector to clean them up and release them you're going to use way more connections that you actually need.

Java – way to release a saturated connection pool

Before doing so, figure out why the connections are not released properly. Sounds like there is a single place where this is forgotten (do you have all close() in finally clauses?).

I can strongly recommend Michael Nygards "Release It!" for techniques to make software production ready.

EDIT#1: If I understand your description correctly your backend programs go in MSGW in QSYSOPR which results in a hanging connection until the response is given which in your case is close to never. Is it an option to use a profile with a default reply of "C" which allows the fault to propagate to you as an exception?

Otherwise you may be able to set a timeout period for either the connections or for the whole server of 24 hours? Then at least the connections will eventually be closed. This solution does not scale though, but may make development easier.

Please note, it is also possible to have a separate surveillance thread which looks for MSGW's regularily and send them an answer automatically AFTER grabbing the call back stack for post mortem analysis.

Best Answer

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