Explain interface composition and give a practical backend example.

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Accepted Answer

// Small, focused interfaces
type UserReader interface {
    FindByID(ctx context.Context, id string) (*User, error)
    FindByEmail(ctx context.Context, email string) (*User, error)
}

type UserWriter interface {
    Create(ctx context.Context, u *User) error
    Update(ctx context.Context, u *User) error
    Delete(ctx context.Context, id string) error
}

// Composed interface for services that need full access
type UserRepository interface {
    UserReader
    UserWriter
}

// Handler only needs to read — accepts narrow interface
type UserHandler struct {
    repo UserReader // not UserRepository
}

// Admin service needs full access
type AdminService struct {
    repo UserRepository
}

// Test mock only needs to implement UserReader — less work
type mockUserReader struct{}
func (m *mockUserReader) FindByID(_ context.Context, id string) (*User, error) {
    return &User{ID: id, Name: "Test"}, nil
}
func (m *mockUserReader) FindByEmail(_ context.Context, _ string) (*User, error) {
    return nil, errors.New("not found")
} Keep interfaces small — the io.Reader/io.Writer pattern (one or two methods) is idiomatic Define interfaces in the package that uses them, not the package that implements them Compose interfaces at the point where a broader contract is genuinely needed Narrow interfaces make unit testing easy — mocks only need to implement what is used Accept interfaces, return concrete types is a common Go guideline

Explain interface composition and give a practical backend example.

Go interfaces can embed other interfaces to build larger contracts from smaller ones, following the Interface Segregation Principle and making mocking in tests trivial.

Explain interface composition and give a practical backend example.

Go interfaces can embed other interfaces to build larger contracts from smaller ones, following the Interface Segregation Principle and making mocking in tests trivial.

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