Synchronous and Asynchronous communication between services.
sequenceDiagram
participant Client
participant API
participant UserSvc
participant EmailSvc
Client->>API: Register User
API->>UserSvc: Create User
UserSvc-->>API: User ID
UserSvc-->>EmailSvc: Async Event: UserCreated
EmailSvc-->>Client: Send Welcome Email
API-->>Client: Success ResponseSequence