Why does Hibernate/JDBC/MySQL drop connections after a day or so?

Question

I have several server processes that once in a while respond to messages from the clients and perform read-only transactions.

After about a few days that the servers are running, they stop working correctly and when I check it turns out that there's a whole bunch of messages about the connection being closed.

When I checked it out, it turned out that hibernate by default works in some sort of development mode where connections are dropped after a few hours, and I started using c3po for connection pooling.

However, even with c3po, I get that problem about 24 hours or so after the servers are started.

Has anyone encountered that problem and knows how to address it? I'm not familiar enough with the intricacies of configuring hibernate.

Answer 1

The MySQL JDBC driver times out after 8 hours of inactivity and drops the connection.

You can set autoReconnect=true in your JDBC URL, and this causes the driver to reconnect if you try to query after it has disconnected. But this has side effects; for instance session state and transactions cannot be maintained over a new connection.

If you use autoReconnect, the JDBC connection is reestablished, but it doesn't automatically re-execute your query that got the exception. So you do need to catch SQLException in your application and retry queries.

Read http://dev.mysql.com/doc/refman/5.0/en/connector-j-reference-configuration-properties.html for more details.

Answer 2

I would suggest that, in almost any client/server set-up, it's a bad idea to leave connections open when they're not needed.

I'm thinking specifically about DB2/z connections but it applies equally to all servers (database and otherwise). These connections consume resources at the server that could be best utilized elsewhere.

If you were to hold connections open in a corporate environment where tens of thousand of clients connect to the database, you would probably even bring a mainframe to its knees.

I'm all for the idea of connection pooling but not so much for the idea of trying to hold individual sessions open for ever.

My advice would be as follows:

1/ Have three sorts of connections in your connection pool:

• closed (so not actually in your pool).
• ready, meaning open but not in use by a client.
• active, meaning in use by a client.

2/ Have your connection pooling maintain a small number of ready connections, minimum of N and maximum of M. N can be adjusted depending on the peak speed at which your clients request connections. If the number of ready connections ever drops to zero, you need a bigger N.

3/ When a client wants a connection, give them one of the ready ones (making it active), then immediately open a new one if there's now less than N ready (but don't make the client wait for this to complete, or you'll lose the advantage of pooling). This ensures there will always be at least N ready connections. If none are ready when the client wants one, they will have to wait around while you create a new one.

4/ When the client finishes with an active connection, return it to the ready state if there's less than M ready connections. Otherwise close it. This prevents you from having more than M ready connections.

5/ Periodically recycle the ready connections to prevent stale connections. If there's more than N ready connections, just close the oldest connection. Otherwise close it and re-open another.

This has the advantage of having enough ready AND youthful connections available in your connection pool without overloading the server.

Answer 3

MySql basically timeouts by default in 8 hours.

I got the same exception & resolved the issue after 3 hectic days.Check if you are using I hibernate3. In this version it is required to explicitly mention the connection class name. Also check if the jar is in classpath. Check steps & comments in below link

http://hibernatedb.blogspot.com/2009/05/automatic-reconnect-from-hibernate-to.html

Remove autoReconnec=true