Mediator Design Pattern with Cortex.Mediator

Cortex.Mediator is an implementation of the Mediator design pattern tailored for CQRS-based systems. It centralizes the sending of commands, querying for data, and publishing of notifications, while supporting pluggable pipeline behaviors (e.g., transactions, logging, validation).

Key Concepts in Cortex.Mediator

Commands

• Purpose: Change system state with or without (Cortex.Mediator.Unit) returning a value (e.g., create or update actions).
• Marker Interface: ICommand<TResult>
• Handler Interface: ICommandHandler<TCommand, TResult>
• Pipeline Behavior: ICommandPipelineBehavior<TCommand, TResult>

Queries

• Purpose: Retrieve or compute data without modifying system state.
• Marker Interface: IQuery<TResult>
• Handler Interface: IQueryHandler<TQuery, TResult>
• Pipeline Behavior: IQueryPipelineBehavior<TQuery, TResult>

Notifications

• Purpose: Broadcast events to one or more handlers without returning values.
• Marker Interface: INotification
• Handler Interface: INotificationHandler<TNotification>

IMediator

• Core Interface: IMediator
  • Task<TResult> SendCommandAsync<TCommand, TResult>(TCommand command, CancellationToken cancellationToken = default)
  • Task<TResult> SendQueryAsync<TQuery, TResult>(TQuery query, CancellationToken cancellationToken = default)
  • Task PublishAsync<TNotification>(TNotification notification, CancellationToken cancellationToken = default)

Pipeline Behaviors

• Use Cases: Cross-cutting concerns (logging, validation, transactions, etc.) around command and query execution.

• Examples in this library:

  • LoggingCommandBehavior – logs command handling and exceptions.

  Coming with version v1.8

  • ValidationCommandBehavior – runs FluentValidation checks on the command/query before handling.

sequenceDiagram
    participant Client as Client Code
    participant Mediator as IMediator
    participant Behaviors as Pipeline Behaviors
    participant Handler as ICommandHandler
    Client->>Mediator: SendAsync(TCommand)
    activate Mediator
    Mediator->>Behaviors: Invoke pipeline behaviors
    activate Behaviors
    Behaviors->>Handler: handler.Handle(command)
    activate Handler
    Handler-->>Behaviors: Execution completes
    deactivate Handler
    Behaviors-->>Mediator: Return from behaviors
    deactivate Behaviors
    Mediator-->>Client: Return (task completes)
    deactivate Mediator