The Senile Coder: oop

Thursday, August 10, 2017

Building a Pipeline (the Template Pattern)

Building a Simple Pipeline

A lot of my development experience has been building back-end systems. There's been plenty of times where something I've built needed to process a request which was composed of a number of tasks. These tasks had to be performed in the correct sequence forming an algorithm. I've heard these things called many different names including Pipelines.

Unfortunately, many times this need leads to a class which contains both the steps of the algorithm, and the logic to complete the steps. These classes quickly turn into huge monsters. They also tend to become a real pain to test. It turns out, there's a GoF pattern for them: Template Method Design Pattern.

In other words, I wanted to express these simple algorithms like this:

public class FileCreator : IRequestHandler<CreatePackage>
{
    private readonly IPipeline<CreatePackage> pipeline;
    
    public FileCreator(IPipeline<CreatePackage> pipeline)
    {
        this.pipeline = pipeline;
    }
    
    public void Handle(CreatePackage request)
    {
        pipeline.Subject(request);
        pipelline.Do<BoxTheItems>();
        pipelline.Do<CreateTheShippingLabel>();
        pipeline.Do<ShipTheBox>();
        pipeline.Do<SendShippingNotification>();
    }
}

PS - MediatR is awesome. That's where IRequestHandler<T> comes from.

Enter LittlePipeline

LittlePipeline is a small library, or set of example code on how one can build a template that is easy to read, easy to test, and can work with an IoC container. It's designed to be a starting point, not an end-all solution to the problem. The gist of using the library (or classes if you want to copy/paste the code) is simple...

Start with a subject class. This class is the guy who holds the data necessary to process the request. It can be as dumb (or smart) as you need. The only requirement is the subject be a reference object (a class).

public class CreatePackage
{
    public int OrderId { get; set; }
    public Address ShipTo { get; set; }
    public string TrackingNumber { get; set; }
}

Create any number of tasks. These tasks should implement the ITask<T> interface where T is the subject type you created earlier.

public class SendShippingNotification : ITask<CreatePackage>
{
    private readonly IBus bus;
    
    public SendShippingNotification(IBus bus)
    {
        this.bus = bus;
    }

    public void Run(CreatePackage subject)
    {
        var notification = new ShippingNotification { TrackingNumber = subject.TrackingNumber };
        bus.Publish(notification);
    }
}

Use the baked-in pipeline creator, or your favorite IoC container to register the tasks, and create the pipeline. Then, use it like in the class above. Here's the built-in, no frills, no guaranties example.

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var pipeline = MakePipeline.ForSubject<FirstTestSubject>()
    .With<Increment>(() => new Increment())
    .Build();

Oh, and this is what testing a pipeline looks like (with FakeItEasy):

public class PipelineTestExample
{
    [Test]
    public void ThePipelineCanBeTested()
    {
        var pipeline = A.Fake<IPipeline<FirstTestSubject>>();
        var example = new ThingThatUsesThePipeline(pipeline);

        var subject = new FirstTestSubject();
        example.Run(subject);

        A.CallTo(() => pipeline.Subject(subject)).MustHaveHappened()
            .Then(A.CallTo(() => pipeline.Do<Increment>()).MustHaveHappened())
            .Then(A.CallTo(() => pipeline.Do<Square>()).MustHaveHappened());
    }
}

public class ThingThatUsesThePipeline
{
    private readonly IPipeline<FirstTestSubject> pipeline;

    public ThingThatUsesThePipeline(IPipeline<FirstTestSubject> pipeline)
    {
        this.pipeline = pipeline;
    }

    public void Run(FirstTestSubject subject)
    {
        pipeline.Subject(subject);
        pipeline.Do<Increment>();
        pipeline.Do<Square>();
    }
}

That's It

There you have it, a simple template class that (hopefully) helps clean up some code by separating the algorithm steps from their implementations. The code is on github. The ReadMe.md file gives some information about how it might be tested.

Tuesday, July 22, 2014

Basics: Removing the 'if' (Using Polymorphism)

Preface

Performing transformations of one object type to another type is a very common task in programming. It might be publishing an event based on a command, or an externally known DTO from an internal DTO. It's pretty common to see some use of the if or switch keywords to determine the code flow. I thought I'd take a minute to show how we can go from a typical implementation using if statements to one which uses Linq and AutoMapper to reduce the coupling in the implementation.

The example code uses the following tools:

NUnit
NUnit Test Adapter for VS2012 and VS2013
Fluent Assertions
FakeItEasy
AutoMapper

The Interface

For this example, we'll have three different publishers. Each will implement a common interface: IPublisher. The implementations will be responsible for accepting a Command object and publishing the associated Event object. We'll be using two commands and events: Start -> Started, Stop -> Stopped.

The Publish method on the IPublisher interface is intentionally not using a generic declaration.

Overloading

The first Publisher accepts a command. It checks the type of the command received, and calls the appropriate overload. Each overloaded method creates the appropriate event, and publishes it.

This works, but it has a few problems. It both uses an if to determine which type to publish, and manually maps the inbound command to the outbound event. That means this class is responsible for both determining what kind of event to publish and creating that event.

Adding AutoMapper

AutoMapper removes the responsibility of creating the event from the publisher class. AutoMapper Profiles could be used to map more complex associations, but the DynamicMap method works just fine here. Our publisher class is relieved of this responsibility, limiting it to just sorting out the type of event to be published.

It still has the problem of using the if statement to determine the type of command received (and event to be be published).

Removing the 'if'

Introducing a map from the commands and a Linq query allows us to remove the if statements. The class is still responsible for selecting the appropriate action. The concept of associating commands to events is distilled into the dictionary. This leaves the class' methods to simply select the appropriate action and execute it.

The Dictionary was left inside the publisher class to keep everything in once class. It wouldn't take much to move the mappings out of the class. This would further reduce the coupling on the Publisher. More complex mappings could be introduced by changing the Dictionary out for a custom type.

Wrapping It Up

This was a quick demonstration of removing two concerns from a class. The manual mapping of one class to another by introducing AutoMapper. The if statement was removed by introducing a map between the two types. I hope this helped describe a different was of building classes with reduced responsibilities.