1.3. Lifecycle and Mode🔗

Each engine has a lifecycle status (MailboxActors.EngineStatus), an inductive type with three constructors:

• ready f — accepting messages filtered by f : Msg_i → Bool,

• busy m — currently processing message m,

• terminated — stopped, awaiting cleanup.

The operational mode (MailboxActors.EngineMode) distinguishes processing engines (process) from mailbox engines (mail). The mode is fixed at creation and opaque to the engine's behaviour.