Allow using a system property to set cron task of cleaning up expired JDBC sessions

[ajthom90]

Issue #616

Allow using a system property to set CRON expression for cleaning up expired JDBC sessions. It looks like the @Scheduled annotation wouldn't allow a placeholder to be resolved with a system property, so adding the task to the task registrar is now handled in the JdbcHttpSessionConfiguration. Also, there is now a property on @EnableJdbcHttpSession to set the CRON expression.

[rwinch]

Thanks for the PR! I'm interested in why you find this change necessary? Generally speaking I don't think this would really need to change.

[ajthom90]

Hi @rwinch, thanks for the feedback!! As I described in Issue #616, I'm running into issues in environments with multiple application servers connected to multiple databases. At the top of the minute, both servers are kicking off the JdbcOperationsSessionRepository#cleanUpExpiredSessions() method to clean up sessions on their databases. When the databases try to delete the data on the other servers through replication, it runs into errors because the data it is expecting to be there is actually not there. This change would allow a system property to be set on each server for the Cron expression, so, for example, one server could run the cleanUpExpiredSessions() method at the top of every minute, and the other server could easily be set to run cleanUpExpiredSessions() at the 30 second point of every minute. Sorry, I'm not the best at explaining things sometimes, so I hope this makes sense! Let me know if you have any other questions.

In my app, I've essentially set up the solution that @vpavic suggested in his comment on Issue #616 yesterday, but I was just adding a pull request in case others thought it may be useful!

[rwinch]

I'm closing this in favor of 5ecf390 which uses a property with a default value like:

@Scheduled(cron = "${spring.session.cleanup.cron.expression:0 * * * * *}")