integration-events

Define a sealed record contract in Shared.Contracts, publish it from a DomainEvent handler in the source module via MassTransit IPublishEndpoint, and consume it with an IConsumer implementation in the destination module. Modules never reference each other directly.

Check whether the event was already processed before applying business logic, using the EventId or a business key like InvoiceId. If a duplicate is detected, log a warning and return without reprocessing, so handling the same message N times yields the same result.

Use UsingInMemory for a modular monolith where all modules run in one process. Switch to UsingRabbitMq when you extract a module into a separate service, configuring the host through application configuration.

Publish after the transaction commits, never inside it. A TransactionBehavior pipeline commits first and then lets domain event handlers publish integration events, preventing consumers from reacting to rolled-back changes.

Use the pattern {Entity}{PastTense}IntegrationEvent, such as InvoiceApprovedIntegrationEvent, placed in the namespace Shared.Contracts.Events.{Module}. Contracts must be sealed records containing only primitive types or other records, never domain entities.