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From Shawn Heisey <>
Subject Re: [DBCP] Connection pool not behaving as I expect
Date Fri, 02 Mar 2018 03:46:00 GMT
On 3/1/2018 4:46 PM, Gary Gregory wrote:
> I do not think this is a question I, or anyone here, can answer
> generically. I can read between that lines that you must feel frustrated
> and I certainly empathize with that. I think you might want to debug your
> application and come up with some parameters for us to start helping you. A
> reproducible example is always best but I understand it might be hard to
> provide in this particular case.

There is a lot of frustration.  Until today all of it was directed at 
our developers, for creating programs and configs that make way too many 
connections to the DB.

But then today, I had that small eureka moment, thinking "wait a minute 
... how can this even be happening at all, if the connection pool has 
connections that the DB server says are active and idle?"

Reiterating something I said before: I know you can't help me with the 
pools that the Tomcat servers are creating for our webapps.  So I'll 
limit the rest of the discussion to my own program, which uses DBCP, and 
has the same problems.

Please tell me what information you'd like me to provide. Anything that 
is in my power, I will get it to you.

This is how I set up DBCP in my code:

    * Create a datasource (connection pool) for the master database server.
   ConnectionFactory cfMaster = new 
DriverManagerConnectionFactory(masterUrl, dbUser, dbPass);
   PoolableConnectionFactory pcfMaster = new 
PoolableConnectionFactory(cfMaster, null);
   pcfMaster.setValidationQueryTimeout(Const.FIVE_SECONDS / 1000);
   opMaster = new GenericObjectPool<>(pcfMaster);
   opMaster.setMaxTotal(numShards * 5);
   opMaster.setNumTestsPerEvictionRun(numShards * 5);
   opMaster.setMinEvictableIdleTimeMillis(Const.ONE_MINUTE * 5);
   dsMaster = new PoolingDataSource<>(opMaster);

The JDBC driver we use is MySQL.  As of a few weeks ago, it was the 
newest stable version available, 5.1.something.  Also at that time, I 
was using the latest DBCP and POOL versions.  If any new versions have 
come out very recently, I probably don't have them yet.

Typically the numShards value we're using is 6, to help with 
understanding the code above.

Observations: When the MySQL server has reached its connection limit, at 
least one of the idle connections is from this program using DBCP.  But 
when the program attempts to use the DB, it gets the "Too many 
connections" error response -- which means that it must be opening a 
brand new connection, despite the fact that there SHOULD be at least one 
that is ready and sitting in the pool.

The code that uses the DB is basic JDBC code.  It calls getConnection() 
on the dataSource, verifies that the connection is valid, creates a 
statement, executes it, and if it was a query, processes the resultset.  
Then it closes any resultset, closes the statement, and closes the 
connection.  As I understand it, that close should return the connection 
to the pool, still open, and ready for re-use.  This all happens within 
a single thread.  I went through this code pretty closely for another 
issue on this mailing list.  It's possible that I missed something, but 
it looks very clean.

I was going to add some debug logging to my code, but I can't see any 
way with PoolingDataSource to get the number of active and idle 
connections, just to make SURE that the pool really has what I think it 

I have a code change ready to switch everything to BasicDataSource and 
add the debug logging.  It's generally less verbose code, and looks to 
be just as configurable as PoolingDataSource.  Would that change be a 
good idea?


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