MSL.Plumber.Pipeline 2.3.1

Build Status

Plumber

Middleware pipelines for host-free projects like AWS Lambda, Console, etc.

Plumber is a request-response pipeline that supports middleware delegates and classes. It provides configuration, dependency injection, and middleware pipeline services. It's useful for AWS Lambdas, Azure Functions, queue event handlers, and similar use cases.

References

Plumber is based on this article: How is the ASP.NET Core Middleware Pipeline Built - Steve Gorden, July 2020

Getting Started

If you're not familiar with middleware pipelines, Microsoft has a good primer on how middleware works in ASP.NET Core.

Installing

To install, use the following command in your terminal or command prompt

dotnet add package MSL.Plumber.Pipeline

Usage

1. Create an IRequestHandlerBuilder<TRequest, TResponse> by calling one of the static RequestHandlerBuilder.Create methods.
2. Use RequestHandlerBuilder.Create(string[] args) to create a builder with the default configuration providers for appsettings.json files, environment variables, command line args, and user secrets.
3. Use RequestHandlerBuilder.Create(string[] args, Action<IConfiguration, string[]> configure) to create a builder with your own set of configuration providers.
4. Handle additional configuration scenarios through the IConfigurationManager Configuration property on the builder.
5. Register services with the IServiceCollection Services property.
6. Use the Build method to create an IRequestHandler<TRequest, TResponse> instance.
7. Configure the request delegate pipeline by calling the Use methods on the request handler.
8. Call the Prepare method on the request handler to compile the pipeline. If you don't call Prepare, the pipeline will be compiled when the first request is processed.
9. Call the InvokeAsync method on the request handler to forward the request to the pipeline.
10. Any class that meets the following criteria can be used as middleware
    • constructor must take RequestMiddleware<TRequest, TResponse> as its first argument
    • must contain an InvokeAsync method
    • InvokeAsync must return a Task
    • InvokeAsync must take RequestContext<TRequest, TResponse> as its first argument
11. Within your middleware delegates or class implementations, always invoke next to pass the request context to the next delegate in the pipeline.
12. Always call context.CancellationToken.ThrowIfCancellationRequested() to check for cancellation requests before processing the request or invoking next.
13. To terminate or "short circuit" the pipeline, don't invoke next.
14. Set the Response property on the RequestContext<TRequest, TResponse> argument to return a value from the pipeline execution.

Sample AWS Lambda Projects

• Samples.Lambda.SQS
• Samples.Lambda.SQS.Tests
• Samples.Lambda.APIGateway
• Samples.Lambda.APIGateway.Tests

Examples

The following examples demonstrate common usage scenarios.

Simplest Example - no config, no services, no middleware

In this sample, we create a request handler that does nothing with no configuration, no service registration, and no user-defined middleware. This is the simplest possible example.

var request = "Hello, World!";

var handler = RequestHandlerBuilder
    .Create<string, string>()
    .Build();

var response = await handler.InvokeAsync(request);

Assert.True(String.IsNullOrEmpty(response));

Middleware Delegate Example

In this sample, we create a request handler with user-defined middleware that converts the request to uppercase.

var request = "Hello, World!";

var handler = RequestHandlerBuilder.Create<string, string>()
    .Build()
    .Use(async (context, next) =>
    {
        context.CancellationToken.ThrowIfCancellationRequested();
        context.Response = context.Request.ToUpperInvariant();
        await next(context); // call next to pass the request context to the next delegate in the pipeline
    });

var response = await handler.InvokeAsync(request);

Assert.Equal(request.ToUpperInvariant(), response);

Middleware Class Example

In this sample, we create a request handler with a user-defined middleware class that converts the request to lowercase.

First, we define the middleware class, which receives the next middleware delegate in the pipeline in its constructor. The middleware is responsible for invoking the next delegate. You will short-circuit the pipeline if you don't invoke the next delegate. An example short-circuit scenario might be a request validation middleware that returns an error response if the request is invalid. The middleware is also responsible for short-circuiting when the pipeline is explicitly canceled via the context.CancellationToken.

Constructor-based dependency injection is supported for middleware implementations, with the condition that the next delegate must be the first argument in the constructor.

internal sealed class ToLowerMiddleware(RequestMiddleware<string, string> next)
{
    public Task InvokeAsync(RequestContext<string, string> context)
    {
        context.CancellationToken.ThrowIfCancellationRequested();
        context.Response = context.Request.ToLowerInvariant();

        // call next to pass the request context to the next delegate in the pipeline
        return next(context);
    }
}

InvokeAsync dependency injection is supported for middleware implementations, with the condition that the RequestContext<TRequest, TResponse> must be the first argument.

internal sealed class ToLowerMiddleware(RequestMiddleware<string, string> next)
{
    public Task InvokeAsync(
        RequestContext<string, string> context, // context is first argument
        IMyFavoriteService service) // second argument is injected by the pipeline's service scope
    {
        context.CancellationToken.ThrowIfCancellationRequested();
        context.Response = service.DoSomethingWonderful(context.Request.ToLowerInvariant());

        // call next to pass the request context to the next delegate in the pipeline
        return next(context); 
    }
}

Next, we register the middleware with the request handler with the Use<T> method.

var request = "Hello, World!";

var handler = RequestHandlerBuilder.Create<string, string>()
    .Build()
    .Use<ToLowerMiddleware>();

var response = await handler.InvokeAsync(request);

Assert.Equal(request.ToLowerInvariant(), response);

Builder Configuration Example

Use the IRequestHandlerBuilder.Configuration property to add configuration providers, like AddInMemory or AddJsonFile.

// default configuration providers are added
// 1. optional appsettings.json
// 2. optional appsettings.{env}.json
// 3. environment variables
// 4. if dev env, then user secrets
// 5. command line args
var builder = RequestHandlerBuilder.Create<string, string>(args);

// extra configuraiton providers can be added
builder.Configuration.AddInMemoryCollection(new Dictionary<string, string> { { "MyKey", "MyValue" } });

var handler = builder.Build();

Provide your own set of configuration providers with the Create method.

// only user specified configuration providers are added
var builder = RequestHandlerBuilder.Create<string, string>(args, (args, configuration) => 
{
    configuration
        .AddUserSecrets("appsettings.json")
        .AddInMemoryCollection(new Dictionary<string, string> { { "MyKey", "MyValue" } });
});

var handler = builder.Build();

Builder Service Registration Example

Use the IRequestHandlerBuilder.Services property to register services.

var builder = RequestHandlerBuilder.Create<string, string>(args);

builder.Services
    .AddSingleton<IMyService, MyService>()
    .AddSerilog();

var handler = builder.Build();

Void Response Example

Some pipelines need to return a response. One example is an AWS Lambda triggered by APIGateway. These handlers receive an APIGatewayHttpApiV2ProxyRequest and return an APIGatewayHttpApiV2ProxyResponse to the APIGateway.

However, many use cases don't require a response. For example, an AWS Lambda triggered by an SQS message. In these cases, the Void type can be used as the generic argument for TResponse.

public readonly struct Void { }

You can use it as the response type for request handlers that don't need to return a response.

var handler = RequestHandlerBuilder.Create<string, Void>() // Void TResponse type
    .Build();