Asynchronous messaging patterns for .NET applications. Covers Wolverine and MassTransit, outbox pattern, saga and choreography, and broker configuration for RabbitMQ and Azure Service Bus. Load this skill when implementing event-driven communication, background processing, module-to-module messaging

Messaging Core Principles 1. Wolverine is the recommended default — MIT licensed, combines mediator + messaging in one library with built in outbox, saga support, and convention based handlers. MassTransit is an alternative but requires a commercial license from v9. 2. Outbox pattern for reliability — Always use the transactional outbox to ensure messages are published only when the database transaction succeeds. 3. Choreography for simple flows, saga for complex — If a workflow has 2 3 steps, use event choreography. If it has compensating actions or complex state, use a saga. 4. Messages are contracts — Put message types in a shared contracts project. Keep them as simple records with primitive types. Patterns Wolverine Setup Why : UseWolverine() registers handler discovery, transport, and outbox in one place. AutoProvision() eliminates manual broker setup during development. Publishing Events Wolverine supports two publishing styles: cascading messages (return values) and explicit publishing. Why : Cascading messages (tuple return) are simpler and testable — the handler is a pure function. Use explicit IMessageBus when publishing is conditional or requires multiple events. Consuming Events Wolverine uses convention based handlers — no interface, no base class. Just a Handle method with the message type as the first parameter. Why : Convention based handlers have zero ceremony. Wolverine discovers them by signature: any public method named Handle / HandleAsync / Consume / ConsumeAsync with the message type as the first parameter. Transactional Outbox Ensures messages are only published if the database transaction succeeds. Why : AddDbContextWithWolverineIntegration + AutoApplyTransactions wraps every handler in a transaction that includes outbox writes. Messages are only sent after the transaction commits — no dual write problem. Saga (Stateful Orchestration) Wolverine sagas use a Saga<T base class with Start and Handle methods. Cascading messages drive the saga forward. Why : Wolverine sagas use simple C methods instead of a state machine DSL. Each handler returns cascading messages to drive the workflow. MarkCompleted() cleans up the saga state. Alternative: MassTransit MassTransit is a mature alternative with a commercial license requirement from v9+. Key API surface: License note : MassTransit v9+ requires a commercial license for production use. Wolverine (MIT) is the recommended default for new projects. Anti patterns Don't Publish Events Without Outbox Don't Put Complex Logic in Message Contracts Don't Use Fire and Forget for Important Events Decision Guide Scenario Recommendation Module to module communication (new project) Wolverine with events (MIT, free) Module to module communication (existing MassTransit) MassTransit (commercial license required from v9) Reliable event publishing Transactional outbox (both Wolverine and MassTransit support this) Simple 2 3 step workflow Event choreography Complex workflow with compensation Wolverine saga or MassTransit saga Local development broker RabbitMQ (via Docker or Aspire) Production cloud broker Azure Service Bus or RabbitMQ Want single lib for mediator + messaging Wolverine (replaces both Mediator and MassTransit)