Question

I am building a traditional webapp that do database CRUD operations through JDBC. And I am wondering if it is good to put jdbc operations into actors, out of current request processing thread. I did some search but found no tutorials or sample applications that demo this.

So What are the cons and pros? Will this asynchonization improve the capacity of the appserver(i.e. the concurrent request processed) like nio?

Answer 1

Whether putting JDBC access in actors is ‘good’ or not greatly depends upon the rest of your application.

Most web applications today are synchronous, thanks to the Servlet API that underlies most Java (and Scala) web frameworks. While we’re now seeing support for asynchronous servlets, that support hasn’t worked its way up all frameworks. Unless you start with a framework that supports asynchronous processing, your request processing will be synchronous.

As for JDBC, JDBC is synchronous. Realistically there’s never going to be anything done about that, given the burden that would place on modifying the gazillion JDBC driver implementations that are out in the world. We can hope, but don’t hold your breath.

And the JDBC implementations themselves don’t have to be thread safe, so invoking an operation on a JDBC connection prior to the completion of some other operation on that same connection will result in undefined behavior. And undefined behavior != good.

So my guess is that you won’t see quite the same capacity improvements that you see with NIO.

Edit: Just discovered adbcj; an asynchronous database driver API. It’s an experimental project written for a master’s thesis, very early, experimental. It’s a worthy experiment, and I hope it succeeds. Check it out!

But, if you are building an asynchronous, actor-based system, I really like the idea of having data access or repository actors, much in the same way your would have data acccess or repository objects in a layered OO architecture.

Actors guarantee that messages are processed one at a time, which is ideal for accessing a single JDBC connection. (One word of caution: most connection pools default to handing out connection-per-thread, which does not play well with actors. Instead you’ll need to make sure that you are using a connection-per-actor. The same is true for transaction management.)

This allows you to treat the database like the asynchronous remote system we ought to have been treating it as all along. This also means that results from your data access/repository actors are futures, which are composable. This makes it easier to coordinate data access with other asynchronous activities.

So, is it good? Probably, if it fits within the architecture of the rest of your system. Will it improve capacity? That will depend on your overall system, but it sounds like a very worthy experiment.