Mediator

Definition

The Mediator pattern centralizes interactions between components via an intermediary object. Components do not communicate directly; they send messages to the mediator, which routes them to the appropriate recipients.

In Granit, Wolverine is the central mediator: it routes commands, events, and queries to handlers, manages transactions, retries, and the Outbox.

Diagram

flowchart TD
    H1[HTTP Handler] -->|command| W[Wolverine Bus]
    H2[Background Job] -->|command| W
    H3[Event Publisher] -->|event| W

    W -->|route| C1[CommandHandler A]
    W -->|route| C2[CommandHandler B]
    W -->|route| E1[EventHandler X]
    W -->|route| E2[EventHandler Y]

    W -->|manages| TX[Transactions]
    W -->|manages| RT[Retries]
    W -->|manages| OB[Outbox]
    W -->|manages| DLQ[Dead Letter Queue]

    style W fill:#4a9eff,color:#fff

Implementation in Granit

Component	File	Role
GranitWolverineModule	src/Granit.Wolverine/GranitWolverineModule.cs	Mediator configuration
AddGranitWolverine()	src/Granit.Wolverine/Extensions/WolverineHostApplicationBuilderExtensions.cs	Registers policies (retry, validation, DLQ)
WolverineMessagingOptions	src/Granit.Wolverine/WolverineMessagingOptions.cs	Retry delays: 5s, 30s, 5min

Policies managed by the mediator

• FluentValidation: invalid messages go to DLQ immediately
• Exponential retry: ValidationException goes to DLQ, others retry at 5s/30s/5min
• Outbox: IIntegrationEvent messages persisted atomically
• Local queue: IDomainEvent messages processed in-process

Rationale

Without a mediator, each handler would need to know which other handlers to call and manage its own transactions and retries. Wolverine centralizes this logic and keeps handlers pure and testable.

Usage example

// Handlers do not know each other -- Wolverine routes messages
public static class CreatePatientHandler
{
    public static IEnumerable<object> Handle(
        CreatePatientCommand cmd,
        AppDbContext db)
    {
        Patient patient = new() { /* ... */ };
        db.Patients.Add(patient);

        // Wolverine routes to the correct handler automatically
        yield return new PatientCreatedOccurred { PatientId = patient.Id };
        yield return new SendWelcomeEmailCommand { Email = cmd.Email };
    }
}
// PatientCreatedOccurred -> local queue -> domain handler (same tx)
// SendWelcomeEmailCommand -> Outbox -> background handler (after commit)

Further reading

• Mediator — refactoring.guru