Domain Service

A Domain Service is a piece of stateless domain logic that doesn't belong to any single Aggregate because it operates over several – or because it has no clear single owner.

When a calculation, a check, or a transformation belongs to the domain (the rules of the business apply) but no single Aggregate is the natural home for it, you reach for a Domain Service. Compute the optimal route from A to B given the current traffic. Decide whether two transactions are likely the same fraud pattern. Check whether an account is permitted to perform this Command.

What a Domain Service Is Not

A Domain Service is not infrastructure. Send an email is not a Domain Service; it is an integration concern, modeled as an Event Handler or via an External System.

A Domain Service is also not a stateful coordinator. Wait for these three Events, then emit a Command is a Process Manager, not a Domain Service. The Domain Service is stateless and synchronous: in, compute, out.

Why Model One

Modeling a Domain Service explicitly is a way of capturing that this piece of logic exists, and it isn't owned by any Aggregate. Without that statement, the logic tends to scatter – a copy here, a slightly different copy there – and the model loses fidelity.

A Domain Service in ESDM declares the functions it offers (the operations it performs). The linter checks that the Domain Service is non-empty: a Domain Service with no functions is a Domain Service that does nothing, which is the kind of thing that drifts in.