Quick DDD Primer

Here is a list of DDD tactical terms and how they map to Actor Systems:

  • Entity -> Unit of State with Unique Identity: A distinct "thing" in your system that you track individually over time.

    • Actor Mapping: Often represented by the state within an Actor, with the Actor's address or ID representing the Entity's unique identity.

  • Value Object -> Immutable Data Structure: Data that describes something but has no own identity; defined only by its values.

    • Actor Mapping: Data carried as the content of Actor Messages or stored as immutable fields within an Actor's state.

  • Aggregate -> Consistent State Cluster (managed by a Coordinator): A group of related data that must be kept valid together. One point controls all changes to this group.

    • Actor Mapping: Naturally maps to a stateful Actor. The Actor is the coordinator, encapsulating and managing the state cluster, processing messages sequentially to ensure internal consistency.

  • Domain Service -> Domain Process/Operation: Logic for a significant action or calculation that doesn't belong to just one "thing."

    • Actor Mapping: Implemented by dedicated Actors responsible for specific domain processes or orchestrations, possibly interacting with stateful Actors.

  • Application Service -> Use Case Coordinator: Code that directs steps to perform a specific action requested by the user or another system; uses the domain logic but isn't the logic itself.

    • Actor Mapping: Implemented by Actors that serve as the entry point for commands or requests, orchestrating interactions by sending messages to other Actors (Domain Services, Aggregates).

  • Repository -> State Loader/Saver: Gets and saves the "Consistent State Clusters" (Aggregates) from/to storage.

    • Actor Mapping: Used by Actors to manage their persistent state. In some frameworks, persistence is integrated into the Actor model itself.

  • Domain Event -> Something Significant Happened: A notification or record that a key occurrence took place in the system.

    • Actor Mapping: Often implemented directly as Actor Messages that are emitted by Actors and can be subscribed to by other Actors or external systems.