Go Interface Design

Go interfaces are implicit. This is the single most important design feature of the language, and most people coming from Java or C# get it wrong at first.

1. The Cardinal Rule: Define Interfaces at the Consumer

The consumer of a behavior defines the interface, NOT the provider:

// ❌ Wrong — producer defines interface (Java thinking)
// package store
type UserStore interface {      // defined alongside implementation
    GetByID(ctx context.Context, id string) (*User, error)
    Create(ctx context.Context, user *User) error
    // ... 15 more methods
}

type PostgresStore struct { ... }