go-interface

Design small, consumer-level interfaces in the package where they're used rather than in a shared layer. Define a UserRepository interface in your service package and depend on it, not the concrete implementation. This reduces coupling and makes testing easier by letting you inject mock implementations.

Create focused interfaces with minimal method sets tailored to what your code actually needs. Small interfaces are easier to mock in tests. Place interface definitions where they're consumed, allowing you to define only the methods required for that specific use case.

Compose smaller, focused interfaces together rather than creating god interfaces with many methods. This follows Go idioms and the standard library pattern, making your code more flexible, maintainable, and easier to test with targeted mocks.

Study how the Go standard library defines interfaces—they're typically small and focused, like io.Reader or io.Writer. Apply the same principles: keep method sets minimal, name interfaces idiomatically (often with an -er suffix), and ensure they solve a specific problem.

Yes. Define repository interfaces in the service or domain package that depends on them, not in a shared layer. Keep each interface focused on specific operations needed by that service, enabling you to swap implementations and simplify testing.