Separating Concerns with Pipes & Filters
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How do you separate concerns when processing a request? Typical concerns such as Logging, Validation, Exception Handling, Retries, and many more. One way is to build a request pipeline for separating concerns by using the Pipes and Filters pattern. You can also build a pipeline using the Russian Doll model that allows you to short circuit at any point throughout the pipeline.
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Pipes & Filters
Often time when processing a request you need to handle various concerns. The Pipes & Filters pattern allows you to break up these various concerns into a series of steps that create a pipeline. A request can be an HTTP request but can also be a message that’s being processed. If you’re using ASP.NET Core, you’re already using the Pipes & Filters pattern!
In the diagram above, the sender is sending a request to the receiver, however, there are filters in between. These filters in between are what can handle various cross-cutting concerns.
The request will pass through the various filters until it finally reaches the Receiver. Both Sender and Receiver are unaware that the request passed through the filters.
As mentioned, the filters can be cross-cutting concerns such as logging, validation, caching, etc.
One important note is that filters are independent. They should be composable with various types of requests. As you can assume a logging filter could be used in many different types of requests.
You should be able to decide per request type, which filters you want to use. They should be plug-and-play.
This allows the receiver of the request to really focus on the behavior that it needs to perform and not other concerns.
A good example of where you may have used this is with ASP.NET Core Action Filters.
Russian Doll
Another way of implementing Pipes & Filters is with the Russian Doll Model. The concept is the same that you have a Sender and a Receiver with Filters in between. The difference however is that each filter calls the next filter in the pipeline.
Each filter is still independent but because the pipeline is using a uniform interface, each filter can be provided the next filter to be invoked.
Since each filter is calling the next filter, this means that you can short circuit the pipeline at any point.
In the diagram above, the Validation filter may choose to short circuit the request if validation fails. Because it’s not calling the next filter in the pipeline the Receiver will never get executed to handle the request.
A good example of where you maybe have used this is with ASP.NET Core Middleware.
Messaging
Processing a message is often very similar to processing any other type of request, except it often doesn’t have an in-process response. There are still various concerns when processing a message that creating a pipeline helps with.
As an example using the Brighter messaging library, which supports Pipes & Filters using the Russian Doll model using attributes. This feels very similar to action filters in ASP.NET Core.
using System; using Paramore.Brighter;
namespace PipesAndFilters { public class PlaceOrderCommand : IRequest { public PlaceOrderCommand(Guid id) { Id = id; }
public Guid Id { get; set; } }
public class PlaceOrderHandler : RequestHandler<PlaceOrderCommand> { [Logging(1)] [Filter(2, typeof(RetryHandler<>))] [Filter(3, typeof(ValidationHandler<>))] public override PlaceOrderCommand Handle(PlaceOrderCommand command) { Console.WriteLine($"Placing Order: {command.Id}"); return base.Handle(command); } } public class LoggingAttribute : RequestHandlerAttribute { public LoggingAttribute(int step) : base(step) { }
public override object[] InitializerParams() { return new object[] { Timing }; }
public override Type GetHandlerType() { return typeof(LoggingHandler<>); } }
public class LoggingHandler<TRequest> : RequestHandler<TRequest> where TRequest : class, IRequest { public override TRequest Handle(TRequest command) { Console.WriteLine($"Generic Logging {typeof(TRequest)}"); return base.Handle(command); } }
public class RetryHandler<TRequest> : RequestHandler<TRequest> where TRequest : PlaceOrderCommand { public override TRequest Handle(TRequest command) { Console.WriteLine("Retry Handler Executing");
try { return base.Handle(command); } catch (InvalidOperationException) { return base.Handle(command); } } }
public class ValidationHandler<TRequest> : RequestHandler<TRequest> where TRequest : PlaceOrderCommand { public override TRequest Handle(TRequest command) { Console.WriteLine($"Validating {nameof(PlaceOrderCommand)}");
if (command.Id == Guid.Empty) { throw new InvalidOperationException("Invalid Order Id."); }
return base.Handle(command); } } }
In the example, the PlaceOrderHandler has 3 different filters that will be executed before it is executed. Logging, Retry, Validation. I’ve implemented logging as a generic filter that can be used in any type of request. While Retry and Validation are specific to the PlaceOrderCommand.
Pipes & Filters
Creating a pipeline can be a powerful way to separate various concerns when processing a request. It allows you to focus on each individual task as a task (filter) within the pipeline.
You can create generic filters that can be used with any type of request or create very specific filters that are only used for a very specific request Also leverage the Russian Doll model to short circuit your request pipeline.
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