ddd-domain-service

One-line summary

A domain service is a stateless operation in the domain layer that doesn't naturally sit on a single entity or value object — usually because it coordinates multiple aggregates or involves a domain computation that isn't anchored to one thing.

When to use this skill

• A cohesive domain operation involves multiple aggregates (e.g., transferring money between two Account aggregates).
• A calculation involves domain types but isn't naturally a method on any of them (e.g., currency conversion, given exchange rates).
• Forcing the behavior onto one of the participating entities would make that entity "know too much" about the others.

When NOT to use this skill

• The logic fits cleanly on a single entity — put it there.
• The work is application-layer orchestration (handling a use case, opening a transaction, calling repositories) — that is an application service, not a domain service.
• The work is infrastructure (HTTP calls, database access, message sending) — that is an infrastructure service.

Core content

Three kinds of services often coexist in a DDD codebase. Keep them distinct:

Domain service. Lives in the domain layer. Operates on domain types only. Stateless. Expresses a verb in the ubiquitous language — TransferService.transfer(fromAccount, toAccount, amount), not AccountUtils.handleTransfer(...).

Application service. Orchestrates a use case. Opens transactions, calls repositories, invokes domain services, dispatches events. Stateless. Lives in the application layer.

Infrastructure service. Talks to the outside world — sends HTTP, writes to a database, publishes to a queue. Lives in the infrastructure layer.

A domain service is named for what it does in domain terms. TransferService and ExchangeRateConverter are good names. TransferManager and TransferHelper are smells — they hide the verb behind technical noise.

Decision heuristics

• Prefer entity behavior. Reach for a domain service only when the operation involves multiple aggregates or has no natural home.
• If a domain service has state, it is almost certainly misclassified — make it a domain entity, a value object, or move the state into one of the participating aggregates.
• If a domain service grows to dozens of methods, it is probably a "transaction script" in disguise — split it or push behavior back onto entities.

Anti-patterns

• *Service suffix on every class — the domain becomes a collection of transaction scripts over data DTOs (see anemic-domain-model in ddd-anti-patterns).
• Putting infrastructure (database, HTTP) inside a domain service — couples the domain to deployment concerns.
• Domain services that mutate the entities they are given without going through the entity's methods — bypasses invariants.
• Stateful "services" that hold caches, sessions, or other process state.

See also

• ddd-entity — first preference is to put behavior here.
• ddd-aggregate — domain services often coordinate across aggregates.
• ddd-anti-patterns — anemic-model and service-explosion smells.

References

• Evans, Domain-Driven Design (2003), Ch. 5 (A Model Expressed in Software).