Revisiting Creating and Updating Entities

Just when I thought I was in a good place to carry forward with retrieving a persisted entity and then modifying its values, I started looking intently into the CQRS Journey on MSDN. While reading through their narative, I thought I was really getting to understand how the code was structured. However, once I dug into the source code - well, I was naively mistaken. The notion of having a Factory was entirely flawed, as that role of validating and placing Commands on the Event Bus was better suited in the user interface layer (be it a Web Controller, a console library, or some other set of code). I supposed that the Factory could fall under "some other set of code"; however, for the purposes of demonstrating the bare minimum needed to declare an action completed, the Factory was adding one too many steps. I realized that the unit test need not create a Factory to then pass a Command to the Bus. Instead, the Unit Test should assume the command was already on the bus.

If I go back to the notion of "User Can Create New Patron", I have realized that I did it entirely wrong. Since what I wanted to demonstrate was the power and ability of using interface design to better focus on what I want to accomplish, the statement I should have been "User Can Command A Patron To Be Created". This provides a better starting place for me to carry that command forward. The CreatePatronCommand has now already been created (ipso facto, it is in the message queue) and now needs an ICommandHandler to turn it into one or more events and to persist the command for historical referencing.

The New Command Interfaces

With this refocus, I now have an ICommand that is handled by an ICommandHandler with access to a ICommandRepository for persistence needs. The narative now becomes:

A Command describes the action that needs to take place. The name of the command (CreatePatronCommand) indicates what should be peformed and the data in the command provides the necessary information to fully execute the command.

using System;

namespace Vigil.Domain.Messaging
{
    public interface ICommand
    {
        /// <summary>Gets the command identifier.
        /// </summary>
        Guid Id { get; }
    }
}

The command, having been put on the bus, is then passed to a CommandHandler which creates one or more events, placing them on the event bus, and persists the command to some storage for archival purposes. Otherwise, the command is of no further use and could be discarded.

namespace Vigil.Domain.Messaging
{
    public interface ICommandHandler
    {
    }

    public interface ICommandHandler<TCommand> : ICommandHandler
        where TCommand : ICommand
    {
        void Handle(TCommand command);
    }
}

In order to persist the command, a CommandRepository is passed to the command handler to presist the command. When needed, this could also fetch data if it is required to do some kind of look-up when creating events.

namespace Vigil.Domain.Messaging
{
    public interface ICommandRepository
    {
        void Save<TCommand>(TCommand command) where TCommand : ICommand;
    }
}