CommandR

NOTE: Project is inactive. Please use MediatR as per SSW Rule - Do you keep business logic out of the presentation layer?

I wrote this library from scratch in an attempt to really understand the basics of the CQRS (Command Query Responsibility Segregation) Pattern.

Top 5 tips to be a good CommandR

1. Do not reuse Command or Query objects inside each other
2. Make your Command and Query objects immutable
3. Use Unit as a response type when dealing with fire-and-forget (eventual consistency) Commands
4. Use Events and IEventHandlers to reduce dependencies and complexity of a Command
5. Tests are easy to write! so write them for all your commands and queries!

Implemeting a Command

1. Create a new class that implements Command<TResponse>
2. Give your command some arguments and persist them as immutable fields (get only)
3. Implement a sealed and nested ICommandHandler<TCommand, TResponse>
4. Implement the body of the Handle method
5. Publish domain events
6. Return a response (or Unit)

public class CreateMovieCommand : Command<int>
{
	public CreateMovieCommand(string movieName)
	{
		Name = movieName;
	}
	
	public string MovieName { get; }

	// Sealed & Nested class that implements the Command Handler for the CreateMovieCommand and returns the new movie ID (as an int)
	public sealed class Handler : ICommandHandler<CreateMovieCommand, int>
	{
		private readonly ICommander _commander;
		private readonly MovieRepository _repo;
		public Handler(ICommander commander, MovieRepository repo)
		{
			_commander = commander;
			_repo = repo;
		}

		public async Task<int> Handle(CreateMovieCommand command, CancellationToken cancellationToken)
		{
			Movie newMovie = await _repo.AddAsync(
				new Movie()
				{
					Name = command.Name
				});

			await _commander.Publish(new MovieCreatedEvent(newMovie.Id, newMovie.Name), cancellationToken);

			return newMovie.Id;
		}
	}
}

Executing a Command

You can Execute your command via an instance of the ICommander. The ICommander will take care of resolving the correct Command Handler for you along with injecting all the required dependencies that the handler requires.

public class MoviesController : Controller
{
	private readonly ICommander _commander;

	public MoviesController(ICommander commander)
	{
		_commander = commander;
	}

	[HttpPost]
	public async Task<IActionResult> Post([FromBody]string movieName)
	{
		// instantiate your command with the required arguments
		var cmd = new CreateMovieCommand(movieName);

		// execute the command via the ICommander
		int newMovieId = await _commander.Execute(cmd);

		// return Ok with the new id
		return Ok(newMovieId);
	}
}

TODO's

This is a short todo list of things that I want and are going to be adding to this library.

• Command/Query processing pielines to deal with cross-cutting concerns (authorization, logging, instrumentation, caching)
• Documentation
• Tests
• Better tests
• Azure Functions Wrapper
• Nugetize