Go Web Expert

Go Web Expert System

Five non-negotiable rules for production-quality Go web applications. Every handler, every service, every line of code must satisfy all five.

Quick Reference

Rules of Engagement

Hard gates (new HTTP handler)

Apply in order. Do not treat the next step as done until the Pass when for the current step is satisfied (objective evidence on disk or in test output—not “I checked mentally”).

1. Dependencies (Rule 1) — Pass when: the handler is a method on a struct that holds every mutable dependency (db, logger, HTTP clients, caches); any new package-level var is only in the allowlist under What Is Allowed at Package Level. Evidence: constructor wires deps; no new forbidden globals from that list.

2. Boundary (Rule 3) — Pass before calling service/domain code: Pass when: the request decodes into a tagged struct and validate.Struct (or equivalent) runs; invalid JSON and validation failures have defined HTTP status bodies (e.g. 400/422). Evidence: decode + validate.Struct appear in the handler; tests or manual run show 422/400 for bad input.

3. Errors (Rule 2) — Pass when: no _ discards on the handler path; json.NewEncoder(w).Encode errors are handled; errors passed up or logged use wrapping (%w) or mapped AppError as this skill prescribes. Evidence: review the diff for ignored errors and bare return err without context where wrapping is required.

4. Tests (Rule 4) — Pass when: a _test.go exists for the handler package and calls ServeHTTP with httptest, including at least one success case and one non-2xx case (validation, not found, or domain error). Evidence: test file path exists; go test for that package passes.

5. Documentation (Rule 5) — Pass when: every new or changed exported identifier in the change has a doc comment whose first line starts with that identifier’s name. Evidence: go doc <pkg> or the IDE/doc preview shows summaries for new exports.

Rule 1: Zero Global State

All handlers must be methods on a server struct. No package-level var for databases, loggers, clients, or any mutable state.

// FORBIDDEN
var db *sql.DB
var logger *slog.Logger

func handleGetUser(w http.ResponseWriter, r *http.Request) {
    user, err := db.QueryRow(...)  // global state -- untestable, unsafe
}

// REQUIRED
type Server struct {
    db     *sql.DB
    logger *slog.Logger
    router *http.ServeMux
}

func (s *Server) handleGetUser(w http.ResponseWriter, r *http.Request) {
    user, err := s.db.QueryRow(...)  // explicit dependency
}

What Is Allowed at Package Level

• Constants -- const maxPageSize = 100
• Pure functions -- functions with no side effects that depend only on their arguments
• Sentinel errors -- var ErrNotFound = errors.New("not found")
• Validator instance -- var validate = validator.New() (stateless after init)

What Is Forbidden at Package Level

• Database connections (*sql.DB, *pgxpool.Pool)
• Loggers (*slog.Logger)
• HTTP clients configured with timeouts or transport
• Configuration structs read from environment
• Caches, rate limiters, or any mutable shared resource

Constructor Pattern

func NewServer(db *sql.DB, logger *slog.Logger) *Server {
    s := &Server{
        db:     db,
        logger: logger,
        router: http.NewServeMux(),
    }
    s.routes()
    return s
}

func (s *Server) routes() {
    s.router.HandleFunc("GET /api/users/{id}", s.handleGetUser)
    s.router.HandleFunc("POST /api/users", s.handleCreateUser)
}

func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    s.router.ServeHTTP(w, r)
}

Rule 2: Explicit Error Handling

Never ignore errors. Every error must be wrapped with context describing what was being attempted when the error occurred.

// FORBIDDEN
result, _ := doSomething()
json.NewEncoder(w).Encode(data)  // error ignored

// REQUIRED
result, err := doSomething()
if err != nil {
    return fmt.Errorf("doing something for user %s: %w", userID, err)
}

if err := json.NewEncoder(w).Encode(data); err != nil {
    s.logger.Error("encoding response", "err", err, "request_id", reqID)
}

Error Wrapping Convention

Format: "<verb>ing <noun>: %w" -- lowercase, no period, provides call-chain context.

// Good wrapping -- each layer adds context
return fmt.Errorf("creating user: %w", err)
return fmt.Errorf("inserting user into database: %w", err)
return fmt.Errorf("hashing password for user %s: %w", email, err)

// Bad wrapping
return fmt.Errorf("error: %w", err)           // no context
return fmt.Errorf("Failed to create user: %w", err) // uppercase, verbose
return err                                      // no wrapping at all

Structured Error Type for HTTP APIs

type AppError struct {
    Code    int    `json:"-"`
    Message string `json:"error"`
    Detail  string `json:"detail,omitempty"`
}

func (e *AppError) Error() string {
    return fmt.Sprintf("%d: %s", e.Code, e.Message)
}

// Map domain errors to HTTP errors in one place
func handleError(w http.ResponseWriter, r *http.Request, err error) {
    var appErr *AppError
    if errors.As(err, &appErr) {
        writeJSON(w, appErr.Code, appErr)
        return
    }

    slog.Error("unhandled error",
        "err", err,
        "path", r.URL.Path,
    )
    writeJSON(w, 500, map[string]string{"error": "internal server error"})
}

Common Mistakes

// MISTAKE: not checking Close errors on writers
defer f.Close()  // at minimum, log Close errors for writable resources

// BETTER for writable resources:
defer func() {
    if err := f.Close(); err != nil {
        s.logger.Error("closing file", "err", err)
    }
}()

// OK for read-only resources where Close rarely fails:
defer resp.Body.Close()

Rule 3: Validation First

Use go-playground/validator for all incoming JSON. Validate at the boundary, trust internal data.

import "github.com/go-playground/validator/v10"

var validate = validator.New()

type CreateUserRequest struct {
    Name  string `json:"name"  validate:"required,min=1,max=100"`
    Email string `json:"email" validate:"required,email"`
    Age   int    `json:"age"   validate:"omitempty,gte=0,lte=150"`
}

func (s *Server) handleCreateUser(w http.ResponseWriter, r *http.Request) error {
    var req CreateUserRequest
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        return &AppError{Code: 400, Message: "invalid JSON", Detail: err.Error()}
    }

    if err := validate.Struct(req); err != nil {
        return &AppError{Code: 422, Message: "validation failed", Detail: formatValidationErrors(err)}
    }

    // From here, req is trusted
    user, err := s.userService.Create(r.Context(), req.Name, req.Email)
    if err != nil {
        return fmt.Errorf("creating user: %w", err)
    }

    writeJSON(w, http.StatusCreated, user)
    return nil
}

Validation Error Formatting

func formatValidationErrors(err error) string {
    var msgs []string
    for _, e := range err.(validator.ValidationErrors) {
        msgs = append(msgs, fmt.Sprintf("field '%s' failed on '%s'", e.Field(), e.Tag()))
    }
    return strings.Join(msgs, "; ")
}

Validation Boundary Rule

• Validate at the edge -- HTTP handlers, message consumers, CLI input
• Trust internal data -- service layer receives already-validated types
• Never validate twice -- if the handler validated, the service does not re-validate the same fields

See references/validation.md for custom validators, nested struct validation, slice validation, and cross-field validation.

Rule 4: Testability

Every handler must have a corresponding _test.go file using httptest. Test through the HTTP layer, not by calling handler methods directly.

func TestServer_handleGetUser(t *testing.T) {
    mockStore := &MockUserStore{
        GetUserFunc: func(ctx context.Context, id string) (*User, error) {
            if id == "123" {
                return &User{ID: "123", Name: "Alice"}, nil
            }
            return nil, ErrNotFound
        },
    }
    srv := NewServer(mockStore, slog.Default())

    tests := []struct {
        name       string
        path       string
        wantStatus int
        wantBody   string
    }{
        {
            name:       "existing user",
            path:       "/api/users/123",
            wantStatus: http.StatusOK,
            wantBody:   `"name":"Alice"`,
        },
        {
            name:       "not found",
            path:       "/api/users/999",
            wantStatus: http.StatusNotFound,
        },
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            req := httptest.NewRequest("GET", tt.path, nil)
            w := httptest.NewRecorder()

            srv.ServeHTTP(w, req)

            if w.Code != tt.wantStatus {
                t.Errorf("status = %d, want %d", w.Code, tt.wantStatus)
            }
            if tt.wantBody != "" && !strings.Contains(w.Body.String(), tt.wantBody) {
                t.Errorf("body = %q, want to contain %q", w.Body.String(), tt.wantBody)
            }
        })
    }
}

Key Testing Principles

• Test through HTTP -- use httptest.NewRequest and httptest.NewRecorder, call srv.ServeHTTP
• Interface-based mocks -- define narrow interfaces at the consumer, create mock implementations for tests
• Table-driven tests -- one []struct with test cases, one t.Run loop
• Error paths matter -- test 400s, 404s, 422s, and 500s, not just 200s
• No global test state -- each test creates its own server with its own mocks

See references/testing-handlers.md for middleware testing, integration tests with real databases, file upload testing, and streaming response testing.

Rule 5: Documentation

Every exported function, type, method, and constant must have a Go doc comment following standard conventions.

// CreateUser creates a new user with the given name and email.
// It returns ErrDuplicateEmail if a user with the same email already exists.
func (s *UserService) CreateUser(ctx context.Context, name, email string) (*User, error) {
    // ...
}

// Server handles HTTP requests for the user API.
type Server struct {
    // ...
}

// NewServer creates a Server with the given dependencies.
// The logger must not be nil.
func NewServer(store UserStore, logger *slog.Logger) *Server {
    // ...
}

// ErrNotFound is returned when a requested resource does not exist.
var ErrNotFound = errors.New("not found")

Doc Comment Conventions

• Start with the name -- // CreateUser creates... not // This function creates...
• First sentence is the summary -- shown in go doc listings and IDE tooltips
• Mention important error returns -- callers need to know which errors to check
• Don't document the obvious -- // SetName sets the name adds no value
• Document why, not what -- when behavior is non-obvious, explain the reasoning

Package Documentation

// Package user provides user management for the application.
// It handles creation, retrieval, and deletion of user accounts,
// with email uniqueness enforced at the database level.
package user

Cross-Cutting Concerns

The five rules reinforce each other. Here is how they interact.

Zero Global State Enables Testability

Because all dependencies are on the struct, tests can inject mocks:

// Production
srv := NewServer(realDB, prodLogger)

// Test
srv := NewServer(mockStore, slog.Default())

If db were a global var, tests would need to mutate package state, causing race conditions in parallel tests.

Validation First Simplifies Error Handling

When handlers validate at the boundary, the service layer can assume valid input. This means service-layer errors are always unexpected (database failures, network issues), and error handling becomes simpler:

func (s *UserService) Create(ctx context.Context, name, email string) (*User, error) {
    // No need to check if name is empty -- handler already validated
    user := &User{Name: name, Email: email}
    if err := s.store.Insert(ctx, user); err != nil {
        return nil, fmt.Errorf("inserting user: %w", err)
    }
    return user, nil
}

Documentation Makes Error Handling Discoverable

Doc comments that mention error returns tell callers what to handle:

// Delete removes a user by ID.
// It returns ErrNotFound if the user does not exist.
// It returns ErrHasActiveOrders if the user has unfinished orders.
func (s *UserService) Delete(ctx context.Context, id string) error {

Self-Review Checklist

Before considering any handler or service complete, verify all five rules:

Zero Global State

• No package-level var for mutable state (db, logger, clients)
• All handlers are methods on a struct
• Dependencies injected through constructor

Explicit Error Handling

• No _ ignoring returned errors
• All errors wrapped with fmt.Errorf("doing X: %w", err)
• json.NewEncoder(w).Encode(...) error checked or logged
• Structured AppError used for HTTP error responses

Validation First

• All request structs have validate tags
• validate.Struct(req) called before any business logic
• Validation errors return 422 with field-level detail
• Service layer does not re-validate handler-validated data

Testability

• _test.go file exists for every handler file
• Tests use httptest.NewRequest and httptest.NewRecorder
• Table-driven tests cover happy path and error paths
• Mocks implement narrow interfaces, not concrete types

Documentation

• Every exported function has a doc comment starting with its name
• Error return values are documented
• Package has a doc comment

When to Load References

Load validation.md when:

• Adding new request types with validation tags
• Creating custom validators
• Validating nested structs, slices, or maps
• Formatting validation errors for API responses

Load testing-handlers.md when:

• Writing handler tests for the first time in a project
• Testing middleware chains or authentication
• Setting up integration tests with a real database
• Testing file uploads or streaming responses