JDBC Connection Pooling in a Tomcat Cluster Environment

Question

I'm relatively very new to this, but I have a Tomcat cluster set up (using mod_proxy from httpd) with session replication (separate redis server) for fault-tolerance.

I have a couple of questions about this setup:

1. My application (spring/hibernate) has a different database per user. So the problem here is that the data source (using spring along with hibernate for persistence) is created at Tomcat level. Thus, whatever connection pooling I do will be at server level.

   As per the cluster configuration the Tomcat instances will create their own Connection Pool.

   I'd like to know if connection pooling is possible at a cluster level using Tomcat i.e. is there a way to make sure that all the servers in the cluster are using the shared Connection Pool?

2. I do not want to configure a DataSource on every Tomcat instance because of performance issues. Before the cluster setup, the application was deployed on a single server and the DataSource was configured such that it allowed only a few (50) connections in a connection pool per DataSource.

   Now in a clustered environment, I cannot afford to create or split those number of connections on every Tomcat, and also dynamic registration of nodes will create further problems. I'd also like to know is there some alternative solution to this problem if connection pooling is not possible or inefficient?

Answer 1

I'm going to handle your questions in reverse order, since the second one is more simple.

Database connection pooling in Tomcat cannot be configured cluster-wide: you have to configure a separate pool for each node in the cluster. But this doesn't have to be bad news... there's nothing wrong with configuring a node to have 5 or 10 or 100 connections in the connection pool on each node.

It's true, you might end up with a situation where you have too many users connecting to the database at a single time which overwhelms your database, but that could also happen with a single node as well. There isn't anything conceptually different about multiple-nodes that wouldn't also be true for a single node.

the key is to make sure that your cluster balances users appropriately so that you don't have a limit of e.g. 5 database connections per node, but 100 users end up on one node while the other nodes only have 5 users per node. In that case, the popular node (100 users) will have to share those 5 connections while on the other nodes, each user gets a connection all to themselves.

Back to your first item, which is more complicated. If you have a separate database per user, then connection-pooling is an impossible thing to accomplish because you will absolutely have to establish a new connection for every user every time. Those connections aren't poolable, at least not without being quite careful about it. It sounds like you have an architectural issue that you might have to solve before you can identify a technical solution to that issue.