Allow custom cron expression for cleanup task

[ajthom90]

Updated Description

Use spring.session.cleanup.cron.expression property to override the cleanup task cron expression.

Original Description

I have a Spring Boot application using Spring Session 1.2.1.RELEASE. I have two servers running the application behind a load balancer, and the session information is stored in MySQL databases that use master-master replication. When the Scheduled task runs to delete sessions (JdbcOperationsSessionRepository.cleanUpExpiredSessions()), it runs on both web servers, which then tries to run on both databases and the replication runs into issues.

It would be nice to be able to set a system property so the scheduled cleanUpExpiredSessions() method will only run on one database, and MySQL replication will take care of the rest behind the scenes.

[rwinch]

Thanks for the report!

I'm not sure it is quite that simple since you need fail-over. What happens when one of the server instances crashes? Nothing is cleaning up expired sessions then.

[ajthom90]

Ahh, very good point! Hadn't thought about what would happen if the server in charge of cleaning up sessions goes down.

[vpavic]

@rwinch We could expose the cron expression used by JdbcOperationsSessionRepository#cleanUpExpiredSessions as a configurable parameter. Users could then adjust the expression to ensure the executions on multiple nodes do not overlap.

[ajthom90]

I created Pull Request #621 with @vpavic's suggestion about exposing the cron expression. Let me know what you think

(sorry it took me so many commits to get a passing build... apparently I decided to ignore Checkstyle today)

[vpavic]

@ajthom90 Having in mind this situation can be viewed as an edge case, perhaps it's not worth the complexity it would add to the configuration support. IMO it falls in the same category as custom SQL queries which will be introduced as a part of #613. Setting custom SQL will require users to manually declare JdbcOperationsSessionRepository bean.

That being said, you could easily work around your problem with a custom JdbcOperationsSessionRepository like this example:

public class CustomJdbcOperationsSessionRepository
        extends JdbcOperationsSessionRepository {

    public CustomJdbcOperationsSessionRepository(JdbcOperations jdbcOperations,
            PlatformTransactionManager transactionManager) {
        super(jdbcOperations, transactionManager);
    }

    @Override
    @Scheduled(cron = "desired cron expression")
    public void cleanUpExpiredSessions() {
        super.cleanUpExpiredSessions();
    }

}

When registering the custom repository, make sure the bean is named sessionRepository so it overrides the bean defined in JdbcHttpSessionConfiguration. Here's an example of such config:

@EnableJdbcHttpSession
public class HttpSessionConfig {

    @Bean
    public CustomJdbcOperationsSessionRepository sessionRepository(
            @Qualifier("springSessionJdbcOperations") JdbcOperations jdbcOperations,
            PlatformTransactionManager transactionManager) {
        return new CustomJdbcOperationsSessionRepository(
                jdbcOperations, transactionManager);
    }

}