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Golang / GoLang Production Patterns and Web Standards Interview Questions

How do you document a Go REST API and maintain an OpenAPI specification?

Documentation is a first-class concern for production APIs. Go projects adopt either code-first (generate OpenAPI from code annotations) or design-first (write OpenAPI spec, generate server stubs) approaches.

// Approach 1: code-first with swaggo/swag annotations
// go install github.com/swaggo/swag/cmd/swag@latest
// swag init -g cmd/server/main.go  → generates docs/swagger.json

// @Summary Get user by ID
// @Description Returns a single user by their unique identifier
// @Tags users
// @Accept json
// @Produce json
// @Param id path int true "User ID"
// @Success 200 {object} User
// @Failure 404 {object} ProblemDetail
// @Failure 500 {object} ProblemDetail
// @Router /users/{id} [get]
func getUserHandler(w http.ResponseWriter, r *http.Request) {
    // handler implementation
}

// Approach 2: design-first with oapi-codegen
// Write openapi.yaml first, then generate:
// oapi-codegen -package api -generate types,server openapi.yaml > api/api.gen.go

// Generated interface — implement this in your handler
type StrictServerInterface interface {
    GetUser(ctx context.Context, request GetUserRequestObject) (GetUserResponseObject, error)
    CreateUser(ctx context.Context, request CreateUserRequestObject) (CreateUserResponseObject, error)
}

// Serve Swagger UI and spec
mux.Handle("GET /docs/",
    http.StripPrefix("/docs/", swaggerui.Handler("openapi.yaml")))
mux.Handle("GET /openapi.yaml", func(w http.ResponseWriter, r *http.Request) {
    http.ServeFile(w, r, "openapi.yaml")
})

Design-first advantages: the API contract is established before implementation — frontend and backend teams can work in parallel. Type safety is enforced by generated code. Breaking changes are visible in spec diffs before any code runs.

What is the main advantage of the design-first (OpenAPI spec → code generation) approach?
What does 'swag init' do in a Go project?

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