If nothing in the architecture can just be a free-floating procedure, not even a pure function, the system architecture will evolve around this
- Store data
- Simple convenience functions that tell me about its state
- Side-effect-free mutations (or data massaging) that have no external dependencies other than function params
- Simple validation, if there are no external dependencies
Everything else goes in a service/function.
One anti-pattern/code smell I often see is finding classes in a service which should just be a functionally pure pipeline (eg. no internal state is managed)