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.

Interface composition in a backend service

Best practices

1
Keep interfaces small — the io.Reader/io.Writer pattern (one or two methods) is idiomatic
2
Define interfaces in the package that uses them, not the package that implements them
3
Compose interfaces at the point where a broader contract is genuinely needed
4
Narrow interfaces make unit testing easy — mocks only need to implement what is used
5
Accept interfaces, return concrete types is a common Go guideline