Controller (Spring Managed Bean) Scope Question: Singleton, Request or Session?

Question

The question is a bit long since it's conceptual. I hope it's not a bad read :)

I'm working in a performance critical Spring MVC/Tiles web-app (10,000 users typical load). We load an update employee screen, where we load an employee details screen (bound to an employee business object) for updates via a MultiActionController. There are multiple tabs on this screen, but only tab1 has the updatabale data. Rest of the tabs are read-only stuff, for reference basically.

Needless to say, we've decided to load these read-only tabs in a lazy manner, i.e., when each tab is activated, we fire an ajax call (one-time) for fetch the data from the server. We don't load everything via the update view loading method. Remember: this is one time, read-only data.

Now, I'm in a dilemma. I've made another multiaction controller, named "AjaxController" for handling these ajax calls. Now, my questions:

1. What should be the best scope for this controller?

Thoughts: If I make it request scoped, then 10,000 users together can create 10,000 instances of this bean: memory problem there. If I make it session scoped, then one will be created per user session. That means, when 10,000 users log in to the app, regardless of whether they hit the AjaxController methods, they will each have a bean in possession.

1. Then, is singleton the best scope for this controller?

Thoughts: A singleton bean will be created when spring boots, and this very instance will be provided throughout. Sounds good.

1. Should the handler methods (like fetchTab7DataInJsonFormat) be static and attached to the class?

Thoughts: In this case, can havign static methods semantically conflict with the scope? For example: does scope="session"/"request" + static methods make sense? I ask because even though each user session has its own AjaxController bean, the handler methods are actually attached to the class, and not the instances. Also, does scope="singleton" + static handler methods make sense?

1. Can I implement the singleton design pattern into AjaxController manually?

Thoughts: What if I control the creation: do the GoF singleton basically. Then what can the scope specification do? Scope session/request surely can't create multiple instances can they?

1. If, by whatever mechanism (bean specification/design pattern/static methods), I do manage to have one single instance of AjaxController: Will these STATIC methods need to be synchronized? I think not, because even if STATIC handler methods can talk to services (which talk to DB/WS/MQ etc.) which take time, I think each request thread entering the static methods will be returned by their thread Id's right? It's not like user1 enters the static method, and then user2 enters the static method before user1 has been returned, and then they both get some garbled data? This is probably silly, but I want to be sure.

I'm confused. I basically want exactly one single instance of the controller bean servicing all requests for all clients.

Critical Note: The AjaxController bean is not INJECTED anywhere else, it exists isolated. It's methods are hit via ajax calls.

Answer 1

If I were doing this, I would definitely make the LazyLoadController singleton without having static methods in it and without any state in it.

Also, you definitely shouldn't instantiate singletons manually, it's better to use Spring's common mechanism and let the framework control everything.

The overall idea is to avoid using any static methods and/or persistent data in controllers. The right mechanism would be use some service bean for generating data for request, so controller acts as request parameter dispatcher to fetch the data out into the view. No mutable state or concurrently unsafe stuff should be allowed in controller. If some components are user-specific, Spring's AOP system provides injection of the components based on session/request.

That's about good practice in doing thing like that. There's something to clarify to give more specific answer for your case. Did I understand it right that typical use case for will be that AjaxController will pass some of requests to LazyLoadController to get tab data? Please provide details about that in comment or your question, so I may update my answer.

The thing that is wrong with having static methods in controller is that you have to manage concurrent safety by yourself which is not just error-prone but will also reduce overall performance. Spring runs every request in its own thread, so if two concurrent calls need to use some static method and there are shared resources (so you need to use synchronize statement or locks), one of threads will have to wait for another one to complete working in protected block. From the other hand, if you use stateless services and avoid having data that may be shared for multiple calls, you get increased performance and no need to deal with concurrent data access.