Datasource Microsoft JDBC Driver for SQL Server (AlwaysOn Availability Groups)

Question

I have a question related to the scenario when connecting from a Java application using the Microsoft JDBC Driver 4.0 to a SQL Server 2014 with AlwaysOn Availability Groups set up for high availability.

With this set up, we will be connecting to an availability group listener (specified in the db connecting string instead of any particular instance), so that the DB fail-over etc. is handled gracefully by the listener and it tries to connect to the next available instance behind the scenes if current primary goes down in the AG cluster.

Question(s) I have is,

1. In the data-source that is configured on the j2ee application server side (we use WebSphere), what happens to those connections already pooled by the data-source?

2. When a database goes down, though the AG listener would try to reconnect on the db side to the next available DB, will the AG Listener also through the jdbc driver send an event or something to the data-source created on the app server and make sure those connections that are already pooled by the datasource to be discarded and have it create new ones so that transactions on the application side wont fail (though they might for a while till new connections are created and fail over is successful) or the java application has to find out only after requesting it from the datasource?

Answer 1

Version 4 of that SQL Server JDBC driver is old and doesn't know anything about the always on feature.

Any data source connection pool can be configured to check the status of the connection from the pool prior to doling it out to the client. If the connection cannot be used the pool will create a new one. That's true of all vendors and versions. I believe that's the best you can do.

Answer 2

WebSphere Application Server is able to cope with bad connections and removes them from the pool. Exactly when this happens depends on some configurable options and on how fully the Microsoft JDBC driver takes advantage of the javax.sql.ConnectionEventListener API to send notifications to the application server. In the ideal case where a JDBC driver sends the connectionErrorOccurred event immediately for all connections, WebSphere Application Server responds by removing all of these connections from the pool and by marking any connection that is currently in-use as bad so that it does not get returned to the pool once the application closes the handle. Lacking this, WebSphere Application Server will discover the first bad connection upon next use by the application. It is discovered either by a connectionErrorOcurred event that is sent by the JDBC driver at that time, or lacking that, upon inspecting the SQLState/error code of an exception for known indicators of bad connections. WebSphere Application Server then goes about purging bad connections from the pool according to the configured Purge Policy. There are 3 options:

1. Purge Policy of Entire Pool - all connections are removed from the pool and in-use connections marked as bad so that they are not pooled.
2. Purge Policy of Failing Connection Only - only the specific connection upon which the error actually occurred is removed from the pool or marked as bad and not returned to the pool
3. Purge Policy of Validate All Connections - all connections are tested for validity (Connection.isValid API) and connections found to be bad are removed from the pool or marked as bad and not returned to the pool. Connections found to be valid remain in the pool and continue to be used.

I'm not sure from your description if you are using WebSphere Application Server traditional or Liberty. If traditional, there is an additional option for pre-testing connections as they are handed out of the pool, but be aware that turning this on can have performance implications. That said, the one thing to be aware of is that regardless of any of the above, your application will always need to be capable of handling the possibility of errors due to bad connections (even if the connection pool is cleared, connections can go bad while in use) and respond by requesting a new connection and retrying the operation in a new transaction.