Tuesday, December 29, 2015

OutputCache & Instrument using Flatwhite

If you are looking for OutputCache for WebApi, go here

What is Flatwhite?

Flatwhite is an AOP library with MVC and WebAPI ActionFilter style using Castle dynamic proxy. There are many libraries out there to help you intercept a method call such as PostSharp, recently CodeCop and they're really cool tools. However, I've been using Castle dynamic proxy for a many years and I think it offers enough needs for my projects. Therefore, Flatwhite is an opinionated library to facilitate usages of Castle dynamic proxy for method interceptions.

Current release only supports Autofac but I think other IOC containers also use Castle dynamic proxy when they come to interception so they will be supported in the future.

You can create MethodFilterAttribute to add custom logic to any methods as soon as it is interceptable by Castle Dynamic Proxy (virtual not final). Flatwhite has a built-in OutputCacheFilter to cache method result which can auto refresh stale content. You can use Flatwhite simply for caching or extending behavior of your code such as profiling, logging by implement MethodFilterAttribute similar to MVC's ActionFilterAttribute

When to use Flatwhite?

You have classes implemented interfaces and registered using Autofac (for now). You have a need to intercept method calls so you possibly have 2 quick options:

  • Use Autofac.Extras and call EnableInterfaceInterceptor() on type registrations then create/register custom IInterceptor.
  • Or use Flatwhite, implement an MethodFilterAttribute and decorate on the methods on your interfaces which you want to intercept.

As mentioned above, Flatwhite has a built-in OutputCacheFilter to cache method output. It works for methods that have a return value both sync and async methods. Beside caching, you can also implement MethodFilterAttribute and ExceptionFilterAttribute to add custom logic to your code.

How to use Flatwhite?

** Required packages: and
For now, Flatwhite needs to be used with Autofac (except Flatwhite.WebApi package). It requires Castle Dynamic proxy to intercept methods so it's a requirement to have public interface or your methods must be virtual and not final to be intercepted.

For caching:

1/ Enable class interceptor
If you modify the class to make the method virtual and decorate the method with OutputCacheAttribute, you will register the class like this:
public class UserService
    [OutputCache(Duration = 2, VaryByParam = "userId")]
    public virtual object GetById(Guid userId) 
        // ...
var builder = new ContainerBuilder().EnableFlatwhite();

2/ Enable interface interceptor
If the methods are not virtual, but the class implements an interface, you can decorate the methods on the interface with OutputCacheAttribute and register the type like this
public interface IUserService
    [OutputCache(Duration = 2, VaryByParam = "userId")]
    object GetById(Guid userId);

    object GetByEmail(string email);

    IEnumerable<object> GetRoles(Guid userId);

var builder = new ContainerBuilder().EnableFlatwhite();

3/ Quick enable cache on all methods
If you don't want to decorate the OutputCache attribute on the interface, you can do like this to enable cache on all methods
var builder = new ContainerBuilder().EnableFlatwhite();

4/ Choose the method to cache without using Attribute filter
If you want to cache on just some methods, you can selectively do like below. Again, it works only on virtual methods if you are registering class service; interface services are fine.
var builder = new ContainerBuilder().EnableFlatwhite();
                              .ForMember(x => x.GetById(Argument.Any<Guid>()))

                              .ForMember(x => x.GetComments(Argument.Any<Guid>(), Argument.Any<int>()))
                              .WithChangeMonitors((i, context) => 
                                    return new[] {new YourCustomCacheChangeMonitor()};

5/ Enable interceptors on all previous registrations
If you're a fan of assembly scanning, you can decorate the OutputCache attribute on classes & interfaces you want to cache and enable them by RegisterModule FlatwhiteBuilderInterceptModule before building the container
var builder = new ContainerBuilder();

// Register other types normally

// Register FlatwhiteBuilderInterceptModule at the end
var container = builder.Build();

Note that you don't have to call EnableFlatwhite() after creating ContainerBuilder like the other methods.

6/ Auto refresh stale data
Flatwhite can auto refresh the stale content if you set StaleWhileRevalidate with a value greater than 0. This should be used with Duration to indicates that caches MAY serve the cached result in which it appears after it becomes stale, up to the indicated number of seconds The first call comes to the service and gets a stale cache result will also make the cache system auto refresh once in the background. So if the method is not called many times in a short period, it's better to turn on AutoRefresh to make the cache alive and refreshed as soon as it starts to be stale
public interface IBlogService
    // For method with too many cache variations because of VaryByParam settings
    [OutputCache(Duration = 5, VaryByParam = "tag, from, authorId", StaleWhileRevalidate = 5)]
    IEnumerable<object> Search(string tag, DateTime from, Guid authorId);    

    // For method with not many cache variations and data is likely to changed every 5 seconds
    [OutputCache(Duration = 5, VaryByParam = "blogId", StaleWhileRevalidate = 5)]
    object GetById(int blogId);    

    // You can turn on AutoRefresh to keep the cache active if there are limited variations of the cache
    [OutputCache(Duration = 5, VaryByParam = "blogId", StaleWhileRevalidate = 5, AutoRefresh = true)]
    IEnumerable<string> GetBlogCategory();    

7/ Using CacheProfile
public interface IUserService
    object GetById(Guid userId);

Profile setting default file name is cacheProfile.yaml located at the same folder of your app.config/web.config file and has a "yaml like" format:
-- Cache profile settings, everything is case-sensitive
-- Profile name
    -- Profile propertyName:propertyValue, start with a tab or 4+ empty spaces


You can implement another IOutputCacheProfileProvider and set to Global.OutputCacheProfileProvider or simply change the location/name of the yaml file. At the moment, only yaml file is supported.

8/ Revalidate cache
Even though you can use AutoRefresh or StaleWhileRevalidate to auto refresh cache data. Some time you want to remove the cache item after you call a certain method. You can use RevalidateAttribute to remove the cache item or some related cache items. Decorate the attribute on another method and the cache item will be removed once the method is invoked successfully. On example below, when you call method DisableUser, because it has the Revalidate attribute decorated with "User" as the key, all related caches created for method with attribute OutputCache which has RevalidationKey = "User" will be reset.
public interface IUserService
    [OutputCache(Duration = 2, StaleWhileRevalidate = 2, VaryByParam = "userId", RevalidationKey = "User")]
    object GetById(Guid userId);

    [OutputCache(Duration = 2, VaryByParam = "userId", RevalidationKey = "User")]
    Task<object> GetByIdAsync(Guid userId); 

    void DisableUser(Guid userId);  

Unfortunately, this is not working for distributed services. That means the method is called on one server cannot notify the other service instances on remote servers. However, it's technically achievable to extend this filter using queueing or something like that to notify remote system.

For additional logic before/after calling methods

Flatwhite is inspired by WebAPI and ASP.NET MVC ActionFilterAttribute, so it works quite similar. The base filter attribute has following methods. So simply implement your filter class and do whatever you want.
public abstract class MethodFilterAttribute : Attribute

    public virtual void OnMethodExecuting(MethodExecutingContext methodExecutingContext);    
    public virtual Task OnMethodExecutingAsync(MethodExecutingContext methodExecutingContext);   
    public virtual void OnMethodExecuted(MethodExecutedContext methodExecutedContext);    
    public virtual Task OnMethodExecutedAsync(MethodExecutedContext methodExecutedContext);    
If you decorate the filter on async methods, only OnMethodExecutingAsync and OnMethodExecutedAsync are called. During the filters are being executed, if the Result value is set to the MethodExecutingContext, the remaining filters will be ignored.

For error handling

Similar to MethodFilterAttribute, you can implement ExceptionFilterAttribute to provide custom error handling logic. If the property MethodExceptionContext.Handled is true, all remaining ExceptionFilter will be ignored.
public abstract class ExceptionFilterAttribute : Attribute
    public virtual void OnException(MethodExceptionContext exceptionContext);    
    public virtual Task OnExceptionAsync(MethodExceptionContext exceptionContext);       

What's else?

Flatwhite for WebAPI: https://github.com/vanthoainguyen/Flatwhite/wiki/Flatwhite.WebApi
Wiki: https://github.com/vanthoainguyen/Flatwhite/wiki


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