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Aspect Oriented Programming style Caching with Castle Windsor

We were doing some work on caching at work the other day, sprinkling calls to cache values here and there in our methods. I couldn't shake the feeling that this was creating a spaghetti-mess where what a method should do got dilluted with the caching logic. I then remembered that this was exactly the problem Aspect Oriented Programming set out to solve. Wouldn't it be nice if you could decorate a method with an attribute and all the caching logic would magically be applied to the output of the method. Like this:

[Cached]
public string SomeMethod(int someArgument)
{
    //some logic
    return "someValue";
}

Challenge Accepted!

Our IoC-container du-coix, Castle Windsor actually has excellent support for Aspect Oriented Programming. Each time you resolve a service with Castle Windsor, you don't just get the component you registered. You actually get a Proxy Object (provided by Castle Dynamic Proxy) wrapped around your component. This gives you complete control over what happens when a method or propety of a component is called. We can intercept a call to a method and just redirect it to a completely different implementation.

The center piece in this is the IInvocation interface. It's signature goes like this:

public interface IInterceptor
{
    void Intercept(IInvocation invocation);
}

The IInvocation parameter describes the intercepted call. It gives us information about the method and allows us to execute it. This means we can implement a CacheInterceptor as follows:

public class CacheInterceptor : IInterceptor
{
    var cacheAttribute = invocation.Method.GetAttribute<CachedAttribute>();
    if(cacheAttribute == null)
    {
        invocation.Proceed();
        return;
    }
    
    var cacheKey = String.Concat(invocation.TargetType.FullName, ".", invocation.Method.Name, "(", String.Join(", ", invocation.Arguments), ")");
    if (MemoryCache.Default.Contains(cacheKey, cacheAttribute.CacheRegion))
    {
        invocation.ReturnValue = MemoryCache.Default.Get(cacheKey, cacheAttribute.CacheRegion);
    }
    else
    {
        invocation.Proceed();
        MemoryCache.Default.Add(cacheKey, invocation.ReturnValue);
    }
}

Let's break down this code:

1. First we check if the method that is intercepted is correctly attributed. If not, we execute the code of the method (invocation.Proceed();) and exit.
2. If the code is attributed, we need to create a cachekey to uniquely identify our cached value. We do this by concatenating the methodname and the values of all the arguments.
3. With this cachekey, we check if we have this item in our chache.
   1. If the item is already cached, we set the returnvalue of the invocation to the value we found in the cache.
   2. If the item is not cached, we proceed with the invocation and add the returnvalue to the cache.

Now, how do we tell Castle Windsor to use this interceptor for the proxies it generates? One way to do this is by defining the Interceptor with each component you register in the container:

container.Register(
    Component.For<IService>().ImplementedBy<Service>().Interceptors<CacheInterceptor>()
);

But I don't like this very much. Instead of littering your methods with Cache-code, you're littering you container setup code. Caching is part of your infrastructure and I like infrastructure setup to be as automagic as possible. Luckely, Castle Windsor lets us do this. With the IContributeComponentModelConstruction interface, we can modify how Castle Windsor creates its Components. It gives us a hook where we can modify the Proxy that is created for a component:

public class RequireCachingContributor : IContributeComponentModelConstruction
{
    public void ProcessModel(IKernel kernel, ComponentModel model)
    {
        var cachedMethods = model.Implementation.GetMethods().Where(m => AttributesUtil.GetAttribute<CachedAttribute>(m) != null).ToList();

        if (cachedMethods.Any())
        {
            model.Interceptors.AddIfNotInCollection(InterceptorReference.ForType<CacheInterceptor>());
        }
    }
}

Basically, this checks if the Component has methods attributed with our CacheAttribute and if so, it adds our interceptor. All we need to do now is to add this to the container before we start adding components:

var container = new WindsorContainer();
container.Kernel.ComponentModelBuilder.AddContributor(new RequireCachingContributor());

Our RequireCachingContributor will now check every Component that is registered with our container. If the class contains a method with the CacheAttribute, it will add our CacheInterceptor. When we ask our container for a component with cached methods, we will now recieve a proxy with our Interceptor attached. Pretty neat.

I've read a few blogs by people with Opinions stating that you don't need an IoC container. Certainly not a fat one like Castle Windsor. While this is true that not every project needs an IoC container, being able to do the things I described here is pretty handy. There is so much you can do with a decent IoC container that most of us only scratch the surface.

You can find the code for this little experiment on github. I've added some extra features there like custom CacheKey providers and properties to control the lifetime of the cache.