Merq 2.0.0-rc.3

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Mercury > Merq-ry > Merq

Merq brings the Message Bus pattern together with a command-oriented interface for an extensible and decoupled in-process application architecture.

These patterns are well established in microservices and service oriented architectures, but their benefits can be applied to apps too, especially extensible ones where multiple teams can contribute extensions which are composed at run-time.

The resulting improved decoupling between components makes it easier to evolve them independently, while improving discoverability of available commands and events. You can see this approach applied in the real world in VSCode commands and various events such as window events. Clearly, in the case of VSCode, everything is in-process, but the benefits of a clean and predictable API are pretty obvious.

Merq provides the same capabilities for .NET apps.

Events

Events can be any type, there is no restriction or interfaces you must implement. Nowadays, C# record types are a perfect fit for event data types. An example event could be a one-liner such as:

public record ItemShipped(string Id, DateTimeOffset Date);

The events-based API surface on the message bus is simple enough:

public interface IMessageBus
{
    void Notify<TEvent>(TEvent e);
    IObservable<TEvent> Observe<TEvent>();
}

By relying on IObservable<TEvent>, Merq integrates seamlessly with more powerful event-driven handling via System.Reactive or the more lightweight RxFree. Subscribing to events with either of those packages is trivial:

IDisposable subscription;

// constructor may use DI to get the dependency
public CustomerViewModel(IMessageBus bus)
{
    subscription = bus.Observe<ItemShipped>().Subscribe(OnItemShipped);
}

void OnItemShipped(ItemShipped e) => // Refresh item status

public void Dispose() => subscription.Dispose();

Commands

Commands can also be any type, and C# records make for concise definitions:

record CancelOrder(string OrderId) : IAsyncCommand;

Unlike events, command messages need to signal the invocation style they require for execution:

// perhaps a method invoked when a user 
// clicks/taps a Cancel button next to an order
async Task OnCancel(string orderId)
{
    await bus.ExecuteAsync(new CancelOrder(orderId), CancellationToken.None);
    // refresh UI for new state.
}

An example of a synchronous command could be:

// Command declaration
record SignOut() : ICommand;

// Command invocation
void OnSignOut() => bus.Execute(new SignOut());

// or alternatively, for void commands that have no additional data:
void OnSignOut() => bus.Execute<SignOut>();

There are also ICommand<TResult> and IAsyncCommand<TResult> interfaces to signal that the execution produces a result.

While these marker interfaces on the command messages might seem unnecessary, they are actually quite important. They solve a key problem that execution abstractions face: whether a command execution is synchronous or asynchronous (as well as void or value-returning) should not be abstracted away since otherwise you can end up in two common anti-patterns (i.e. async guidelines for ASP.NET), known as sync over async and async over sync.

The marker interfaces on the command messages drive the compiler to only allow the right invocation style on the message bus, as defined by the command author:

public interface IMessageBus
{
    // sync void
    void Execute(ICommand command);
    // sync value-returning
    TResult Execute<TResult>(ICommand<TResult> command);
    // async void
    Task ExecuteAsync(IAsyncCommand command, CancellationToken cancellation);
    // async value-returning
    Task<TResult> ExecuteAsync<TResult>(IAsyncCommand<TResult> command, CancellationToken cancellation);
}

For example, to create a value-returning async command that retrieves some value, you would have:

record FindDocuments(string Filter) : IAsyncCommand<IEnumerable<string>>;

class FindDocumentsHandler : IAsyncCommandHandler<FindDocument, IEnumerable<string>>
{
    public bool CanExecute(FindDocument command) => !string.IsNullOrEmpty(command.Filter);
    
    public Task<IEnumerable<string>> ExecuteAsync(FindDocument command, CancellationToken cancellation)
        => // evaluate command.Filter across all documents and return matches
}

In order to execute such command, the only execute method the compiler will allow is:

IEnumerable<string> files = await bus.ExecuteAsync(new FindDocuments("*.json"));

If the consumer tries to use Execute, the compiler will complain that the command does not implement ICommand<TResult>, which is the synchronous version of the marker interface. Likewise, mistakes cannot be made when implementing the handler, since the handler interfaces define constraints on what the commands must implement:

// sync
public interface ICommandHandler<in TCommand> : ... where TCommand : ICommand;
public interface ICommandHandler<in TCommand, out TResult> : ... where TCommand : ICommand<TResult>;

// async
public interface IAsyncCommandHandler<in TCommand> : ... where TCommand : IAsyncCommand;
public interface IAsyncCommandHandler<in TCommand, TResult> : ... where TCommand : IAsyncCommand<TResult>

This design choice also makes it impossible to end up executing a command implementation improperly.

In addition to execution, the IMessageBus also provides a mechanism to determine if a command has a registered handler at all via the CanHandle<T> method as well as a validation mechanism via CanExecute<T>, as shown above in the FindDocumentsHandler example.

Commands can notify new events, and event observers/subscribers can in turn execute commands.

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