Building high-performance web servers and REST APIs
Developing microservices architectures with gRPC or HTTP
Implementing concurrent processing with goroutines and channels
Creating real-time systems requiring high throughput
Building database-backed applications with connection pooling
Developing cloud-native applications for containerized deployment
Writing performance-critical backend services
Building distributed systems with service discovery
Implementing event-driven architectures
Creating CLI tools and system utilities with networking capabilities
Developing WebSocket servers for real-time communication
Building data processing pipelines with concurrent workers

Go excels at:

Network programming and HTTP services
Concurrent processing with lightweight goroutines
System-level programming with garbage collection
Cross-platform compilation
Fast compilation times for rapid development
Built-in testing and benchmarking

Core Concepts

1. Goroutines: Lightweight Concurrency

Goroutines are lightweight threads managed by the Go runtime. They enable concurrent execution with minimal overhead.

Key Characteristics:

Extremely lightweight (start with ~2KB stack)
Multiplexed onto OS threads by the runtime
Thousands or millions can run concurrently
Scheduled cooperatively with integrated scheduler

Basic Goroutine Pattern:

func main() {
    // Launch concurrent computation
    go expensiveComputation(x, y, z)
    anotherExpensiveComputation(a, b, c)
}

The go keyword launches a new goroutine, allowing expensiveComputation to run concurrently with anotherExpensiveComputation. This is fundamental to Go's concurrency model.

Common Use Cases:

Background processing
Concurrent API calls
Parallel data processing
Real-time event handling
Connection handling in servers

2. Channels: Safe Communication

Channels provide type-safe communication between goroutines, eliminating the need for explicit locks in many scenarios.

Channel Types:

// Unbuffered channel - synchronous communication
ch := make(chan int)

// Buffered channel - asynchronous up to buffer size
ch := make(chan int, 100)

// Read-only channel
func receive(ch <-chan int) { /* ... */ }

// Write-only channel
func send(ch chan<- int) { /* ... */ }

Synchronization with Channels:

func computeAndSend(ch chan int, x, y, z int) {
    ch <- expensiveComputation(x, y, z)
}

func main() {
    ch := make(chan int)
    go computeAndSend(ch, x, y, z)
    v2 := anotherExpensiveComputation(a, b, c)
    v1 := <-ch  // Block until result available
    fmt.Println(v1, v2)
}

This pattern ensures both computations complete before proceeding, with the channel providing both communication and synchronization.

Channel Patterns:

Producer-consumer
Fan-out/fan-in
Pipeline stages
Timeouts and cancellation
Semaphores and rate limiting

3. Select Statement: Multiplexing Channels

The select statement enables multiplexing multiple channel operations, similar to a switch for channels.

Timeout Implementation:

timeout := make(chan bool, 1)
go func() {
    time.Sleep(1 * time.Second)
    timeout <- true
}()

select {
case <-ch:
    // Read from ch succeeded
case <-timeout:
    // Operation timed out
}

Context-Based Cancellation:

select {
case result := <-resultCh:
    return result
case <-ctx.Done():
    return ctx.Err()
}

4. Context Package: Request-Scoped Values

The context.Context interface manages deadlines, cancellation signals, and request-scoped values across API boundaries.

Context Interface:

type Context interface {
    // Done returns a channel closed when work should be canceled
    Done() <-chan struct{}

    // Err returns why context was canceled
    Err() error

    // Deadline returns when work should be canceled
    Deadline() (deadline time.Time, ok bool)

    // Value returns request-scoped value
    Value(key any) any
}

Creating Contexts:

// Background context - never canceled
ctx := context.Background()

// With cancellation
ctx, cancel := context.WithCancel(context.Background())
defer cancel()

// With timeout
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()

// With deadline
deadline := time.Now().Add(10 * time.Second)
ctx, cancel := context.WithDeadline(context.Background(), deadline)
defer cancel()

// With values
ctx = context.WithValue(parentCtx, key, value)

Best Practices:

Always pass context as first parameter: func DoSomething(ctx context.Context, ...)
Call defer cancel() immediately after creating cancelable context
Propagate context through call chain
Check ctx.Done() in long-running operations
Use context values only for request-scoped data, not optional parameters

5. WaitGroup: Coordinating Goroutines

sync.WaitGroup waits for a collection of goroutines to finish.

Basic Pattern:

var wg sync.WaitGroup

for i := 0; i < 10; i++ {
    wg.Add(1)
    go func(id int) {
        defer wg.Done()
        // Do work
    }(i)
}

wg.Wait()  // Block until all goroutines complete

Common Use Cases:

Waiting for parallel tasks
Coordinating worker pools
Ensuring cleanup completion
Synchronizing shutdown

6. Mutex: Protecting Shared State

When shared state is necessary, use sync.Mutex or sync.RWMutex for protection.

Mutex Pattern:

var (
    service   map[string]net.Addr
    serviceMu sync.Mutex
)

func RegisterService(name string, addr net.Addr) {
    serviceMu.Lock()
    defer serviceMu.Unlock()
    service[name] = addr
}

func LookupService(name string) net.Addr {
    serviceMu.Lock()
    defer serviceMu.Unlock()
    return service[name]
}

RWMutex for Read-Heavy Workloads:

var (
    cache   map[string]interface{}
    cacheMu sync.RWMutex
)

func Get(key string) interface{} {
    cacheMu.RLock()
    defer cacheMu.RUnlock()
    return cache[key]
}

func Set(key string, value interface{}) {
    cacheMu.Lock()
    defer cacheMu.Unlock()
    cache[key] = value
}

7. Concurrent Web Server Pattern

Go's standard pattern for handling concurrent connections:

for {
    rw := l.Accept()
    conn := newConn(rw, handler)
    go conn.serve()  // Handle each connection concurrently
}

Each accepted connection is handled in its own goroutine, allowing the server to scale to thousands of concurrent connections efficiently.

Web Server Development

HTTP Server Basics

Simple HTTP Server:

package main

import (
    "fmt"
    "net/http"
)

func main() {
    http.HandleFunc("/", handler)
    http.ListenAndServe("localhost:8080", nil)
}

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprint(w, "Hello!")
}

Request Handling Patterns

Handler Functions:

func handler(w http.ResponseWriter, r *http.Request) {
    // Read request
    method := r.Method
    path := r.URL.Path
    query := r.URL.Query()

    // Write response
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(http.StatusOK)
    fmt.Fprintf(w, `{"message": "success"}`)
}

Handler Structs:

type APIHandler struct {
    db *sql.DB
    logger *log.Logger
}

func (h *APIHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    // Access dependencies
    h.logger.Printf("Request: %s %s", r.Method, r.URL.Path)
    // Handle request
}

Middleware Pattern

Logging Middleware:

func loggingMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        start := time.Now()
        next.ServeHTTP(w, r)
        log.Printf("%s %s %v", r.Method, r.URL.Path, time.Since(start))
    })
}

// Usage
http.Handle("/api/", loggingMiddleware(apiHandler))

Authentication Middleware:

func authMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        token := r.Header.Get("Authorization")
        if !isValidToken(token) {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        next.ServeHTTP(w, r)
    })
}

Chaining Middleware:

handler := loggingMiddleware(authMiddleware(corsMiddleware(apiHandler)))
http.Handle("/api/", handler)

Context in HTTP Handlers

HTTP Request with Context:

func handleSearch(w http.ResponseWriter, req *http.Request) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    // Check query parameter
    query := req.FormValue("q")
    if query == "" {
        http.Error(w, "missing query", http.StatusBadRequest)
        return
    }

    // Perform search with context
    results, err := performSearch(ctx, query)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    // Render results
    renderTemplate(w, results)
}

Context-Aware HTTP Request:

func httpDo(ctx context.Context, req *http.Request,
            f func(*http.Response, error) error) error {
    c := &http.Client{}

    // Run request in goroutine
    ch := make(chan error, 1)
    go func() {
        ch <- f(c.Do(req))
    }()

    // Wait for completion or cancellation
    select {
    case <-ctx.Done():
        <-ch  // Wait for f to return
        return ctx.Err()
    case err := <-ch:
        return err
    }
}

Routing Patterns

Custom Router:

type Router struct {
    routes map[string]http.HandlerFunc
}

func (r *Router) Handle(pattern string, handler http.HandlerFunc) {
    r.routes[pattern] = handler
}

func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
    if handler, ok := r.routes[req.URL.Path]; ok {
        handler(w, req)
    } else {
        http.NotFound(w, req)
    }
}

RESTful API Structure:

// GET /api/users
func listUsers(w http.ResponseWriter, r *http.Request) { /* ... */ }

// GET /api/users/:id
func getUser(w http.ResponseWriter, r *http.Request) { /* ... */ }

// POST /api/users
func createUser(w http.ResponseWriter, r *http.Request) { /* ... */ }

// PUT /api/users/:id
func updateUser(w http.ResponseWriter, r *http.Request) { /* ... */ }

// DELETE /api/users/:id
func deleteUser(w http.ResponseWriter, r *http.Request) { /* ... */ }

Concurrency Patterns

1. Pipeline Pattern

Pipelines process data through multiple stages connected by channels.

Generator Stage:

func gen(nums ...int) <-chan int {
    out := make(chan int)
    go func() {
        for _, n := range nums {
            out <- n
        }
        close(out)
    }()
    return out
}

Processing Stage:

func sq(in <-chan int) <-chan int {
    out := make(chan int)
    go func() {
        for n := range in {
            out <- n * n
        }
        close(out)
    }()
    return out
}

Pipeline Usage:

func main() {
    // Set up pipeline
    c := gen(2, 3)
    out := sq(c)

    // Consume output
    for n := range out {
        fmt.Println(n)  // 4 then 9
    }
}

Buffered Generator (No Goroutine Needed):

func gen(nums ...int) <-chan int {
    out := make(chan int, len(nums))
    for _, n := range nums {
        out <- n
    }
    close(out)
    return out
}

2. Fan-Out/Fan-In Pattern

Distribute work across multiple workers and merge results.

Fan-Out: Multiple Workers:

func main() {
    in := gen(2, 3, 4, 5)

    // Fan out: distribute work across two goroutines
    c1 := sq(in)
    c2 := sq(in)

    // Fan in: merge results
    for n := range merge(c1, c2) {
        fmt.Println(n)
    }
}

Merge Function (Fan-In):

func merge(cs ...<-chan int) <-chan int {
    var wg sync.WaitGroup
    out := make(chan int)

    // Start output goroutine for each input channel
    output := func(c <-chan int) {
        for n := range c {
            out <- n
        }
        wg.Done()
    }

    wg.Add(len(cs))
    for _, c := range cs {
        go output(c)
    }

    // Close out once all outputs are done
    go func() {
        wg.Wait()
        close(out)
    }()
    return out
}

3. Explicit Cancellation Pattern

Cancellation with Done Channel:

func sq(done <-chan struct{}, in <-chan int) <-chan int {
    out := make(chan int)
    go func() {
        defer close(out)
        for n := range in {
            select {
            case out <- n * n:
            case <-done:
                return
            }
        }
    }()
    return out
}

Broadcasting Cancellation:

func main() {
    done := make(chan struct{})
    defer close(done)  // Broadcast to all goroutines

    in := gen(done, 2, 3, 4)
    c1 := sq(done, in)
    c2 := sq(done, in)

    // Process subset of results
    out := merge(done, c1, c2)
    fmt.Println(<-out)

    // done closed on return, canceling all pipeline stages
}

Merge with Cancellation:

func merge(done <-chan struct{}, cs ...<-chan int) <-chan int {
    var wg sync.WaitGroup
    out := make(chan int)

    output := func(c <-chan int) {
        defer wg.Done()
        for n := range c {
            select {
            case out <- n:
            case <-done:
                return
            }
        }
    }

    wg.Add(len(cs))
    for _, c := range cs {
        go output(c)
    }

    go func() {
        wg.Wait()
        close(out)
    }()
    return out
}

4. Worker Pool Pattern

Fixed Number of Workers:

func handle(queue chan *Request) {
    for r := range queue {
        process(r)
    }
}

func Serve(clientRequests chan *Request, quit chan bool) {
    // Start handlers
    for i := 0; i < MaxOutstanding; i++ {
        go handle(clientRequests)
    }
    <-quit  // Wait to exit
}

Semaphore Pattern:

var sem = make(chan int, MaxOutstanding)

func handle(r *Request) {
    sem <- 1        // Acquire
    process(r)
    <-sem           // Release
}

func Serve(queue chan *Request) {
    for req := range queue {
        go handle(req)
    }
}

Limiting Goroutine Creation:

func Serve(queue chan *Request) {
    for req := range queue {
        sem <- 1  // Acquire before creating goroutine
        go func() {
            process(req)
            <-sem  // Release
        }()
    }
}

5. Query Racing Pattern

Query multiple sources and return first result:

func Query(conns []Conn, query string) Result {
    ch := make(chan Result)
    for _, conn := range conns {
        go func(c Conn) {
            select {
            case ch <- c.DoQuery(query):
            default:
            }
        }(conn)
    }
    return <-ch
}

6. Parallel Processing Example

Serial MD5 Calculation:

func MD5All(root string) (map[string][md5.Size]byte, error) {
    m := make(map[string][md5.Size]byte)
    err := filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
        if err != nil {
            return err
        }
        if !info.Mode().IsRegular() {
            return nil
        }
        data, err := ioutil.ReadFile(path)
        if err != nil {
            return err
        }
        m[path] = md5.Sum(data)
        return nil
    })
    return m, err
}

Parallel MD5 with Pipeline:

type result struct {
    path string
    sum  [md5.Size]byte
    err  error
}

func sumFiles(done <-chan struct{}, root string) (<-chan result, <-chan error) {
    c := make(chan result)
    errc := make(chan error, 1)

    go func() {
        defer close(c)
        err := filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
            if err != nil {
                return err
            }
            if !info.Mode().IsRegular() {
                return nil
            }

            // Start goroutine for each file
            go func() {
                data, err := ioutil.ReadFile(path)
                select {
                case c <- result{path, md5.Sum(data), err}:
                case <-done:
                }
            }()

            // Check for early cancellation
            select {
            case <-done:
                return errors.New("walk canceled")
            default:
                return nil
            }
        })

        select {
        case errc <- err:
        case <-done:
        }
    }()
    return c, errc
}

func MD5All(root string) (map[string][md5.Size]byte, error) {
    done := make(chan struct{})
    defer close(done)

    c, errc := sumFiles(done, root)

    m := make(map[string][md5.Size]byte)
    for r := range c {
        if r.err != nil {
            return nil, r.err
        }
        m[r.path] = r.sum
    }

    if err := <-errc; err != nil {
        return nil, err
    }
    return m, nil
}

7. Leaky Buffer Pattern

Efficient buffer reuse:

var freeList = make(chan *Buffer, 100)

func server() {
    for {
        b := <-serverChan  // Wait for work
        process(b)

        // Try to reuse buffer
        select {
        case freeList <- b:
            // Buffer on free list
        default:
            // Free list full, GC will reclaim
        }
    }
}

Database Integration

Connection Management

Database Connection Pool:

import "database/sql"

func initDB(dataSourceName string) (*sql.DB, error) {
    db, err := sql.Open("postgres", dataSourceName)
    if err != nil {
        return nil, err
    }

    // Configure connection pool
    db.SetMaxOpenConns(25)
    db.SetMaxIdleConns(5)
    db.SetConnMaxLifetime(5 * time.Minute)
    db.SetConnMaxIdleTime(10 * time.Minute)

    // Verify connection
    if err := db.Ping(); err != nil {
        return nil, err
    }

    return db, nil
}

Query Patterns

Single Row Query:

func getUser(db *sql.DB, userID int) (*User, error) {
    user := &User{}
    err := db.QueryRow(
        "SELECT id, name, email FROM users WHERE id = $1",
        userID,
    ).Scan(&user.ID, &user.Name, &user.Email)

    if err == sql.ErrNoRows {
        return nil, fmt.Errorf("user not found")
    }
    if err != nil {
        return nil, err
    }

    return user, nil
}

Multiple Row Query:

func listUsers(db *sql.DB) ([]*User, error) {
    rows, err := db.Query("SELECT id, name, email FROM users")
    if err != nil {
        return nil, err
    }
    defer rows.Close()

    var users []*User
    for rows.Next() {
        user := &User{}
        if err := rows.Scan(&user.ID, &user.Name, &user.Email); err != nil {
            return nil, err
        }
        users = append(users, user)
    }

    if err := rows.Err(); err != nil {
        return nil, err
    }

    return users, nil
}

Insert/Update with Context:

func createUser(ctx context.Context, db *sql.DB, user *User) error {
    query := "INSERT INTO users (name, email) VALUES ($1, $2) RETURNING id"
    err := db.QueryRowContext(ctx, query, user.Name, user.Email).Scan(&user.ID)
    return err
}

Transaction Handling

func transferFunds(ctx context.Context, db *sql.DB, from, to int, amount decimal.Decimal) error {
    tx, err := db.BeginTx(ctx, nil)
    if err != nil {
        return err
    }
    defer tx.Rollback()  // Rollback if not committed

    // Debit from account
    _, err = tx.ExecContext(ctx,
        "UPDATE accounts SET balance = balance - $1 WHERE id = $2",
        amount, from)
    if err != nil {
        return err
    }

    // Credit to account
    _, err = tx.ExecContext(ctx,
        "UPDATE accounts SET balance = balance + $1 WHERE id = $2",
        amount, to)
    if err != nil {
        return err
    }

    return tx.Commit()
}

Prepared Statements

func insertUsers(db *sql.DB, users []*User) error {
    stmt, err := db.Prepare("INSERT INTO users (name, email) VALUES ($1, $2)")
    if err != nil {
        return err
    }
    defer stmt.Close()

    for _, user := range users {
        _, err := stmt.Exec(user.Name, user.Email)
        if err != nil {
            return err
        }
    }

    return nil
}

Error Handling

Custom Error Types

type ValidationError struct {
    Field string
    Message string
}

func (e *ValidationError) Error() string {
    return fmt.Sprintf("%s: %s", e.Field, e.Message)
}

// Usage
if email == "" {
    return &ValidationError{Field: "email", Message: "required"}
}

Error Wrapping

import "fmt"

func processData(data []byte) error {
    err := validateData(data)
    if err != nil {
        return fmt.Errorf("process data: %w", err)
    }
    return nil
}

// Unwrapping
if errors.Is(err, ErrValidation) {
    // Handle validation error
}

if errors.As(err, &validationErr) {
    // Access ValidationError fields
}

Sentinel Errors

var (
    ErrNotFound = errors.New("not found")
    ErrUnauthorized = errors.New("unauthorized")
    ErrInvalidInput = errors.New("invalid input")
)

// Usage
if errors.Is(err, ErrNotFound) {
    http.Error(w, "Resource not found", http.StatusNotFound)
}

Testing

Unit Tests

func TestGetUser(t *testing.T) {
    db := setupTestDB(t)
    defer db.Close()

    user, err := getUser(db, 1)
    if err != nil {
        t.Fatalf("getUser failed: %v", err)
    }

    if user.Name != "John Doe" {
        t.Errorf("expected name John Doe, got %s", user.Name)
    }
}

Table-Driven Tests

func TestValidateEmail(t *testing.T) {
    tests := []struct {
        name    string
        email   string
        wantErr bool
    }{
        {"valid email", "user@example.com", false},
        {"missing @", "userexample.com", true},
        {"empty string", "", true},
        {"missing domain", "user@", true},
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            err := validateEmail(tt.email)
            if (err != nil) != tt.wantErr {
                t.Errorf("validateEmail(%q) error = %v, wantErr %v",
                    tt.email, err, tt.wantErr)
            }
        })
    }
}

Benchmarks

func BenchmarkConcurrentMap(b *testing.B) {
    m := make(map[string]int)
    var mu sync.Mutex

    b.RunParallel(func(pb *testing.PB) {
        for pb.Next() {
            mu.Lock()
            m["key"]++
            mu.Unlock()
        }
    })
}

HTTP Handler Testing

func TestHandler(t *testing.T) {
    req := httptest.NewRequest("GET", "/api/users", nil)
    w := httptest.NewRecorder()

    handler(w, req)

    resp := w.Result()
    if resp.StatusCode != http.StatusOK {
        t.Errorf("expected status 200, got %d", resp.StatusCode)
    }

    body, _ := ioutil.ReadAll(resp.Body)
    // Assert body content
}

Production Patterns

Graceful Shutdown

func main() {
    srv := &http.Server{
        Addr:    ":8080",
        Handler: router,
    }

    // Start server in goroutine
    go func() {
        if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
            log.Fatalf("listen: %s\n", err)
        }
    }()

    // Wait for interrupt signal
    quit := make(chan os.Signal, 1)
    signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
    <-quit
    log.Println("Shutting down server...")

    // Graceful shutdown with timeout
    ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
    defer cancel()

    if err := srv.Shutdown(ctx); err != nil {
        log.Fatal("Server forced to shutdown:", err)
    }

    log.Println("Server exited")
}

Configuration Management

type Config struct {
    ServerPort  int           `env:"PORT" envDefault:"8080"`
    DBHost      string        `env:"DB_HOST" envDefault:"localhost"`
    DBPort      int           `env:"DB_PORT" envDefault:"5432"`
    LogLevel    string        `env:"LOG_LEVEL" envDefault:"info"`
    Timeout     time.Duration `env:"TIMEOUT" envDefault:"30s"`
}

func loadConfig() (*Config, error) {
    cfg := &Config{}
    if err := env.Parse(cfg); err != nil {
        return nil, err
    }
    return cfg, nil
}

Structured Logging

import "log/slog"

func setupLogger() *slog.Logger {
    return slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
        Level: slog.LevelInfo,
    }))
}

func handler(w http.ResponseWriter, r *http.Request) {
    logger := slog.With(
        "method", r.Method,
        "path", r.URL.Path,
        "remote", r.RemoteAddr,
    )

    logger.Info("handling request")

    // Process request

    logger.Info("request completed", "status", 200)
}

Health Checks

func healthHandler(db *sql.DB) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Check database
        if err := db.Ping(); err != nil {
            w.WriteHeader(http.StatusServiceUnavailable)
            json.NewEncoder(w).Encode(map[string]string{
                "status": "unhealthy",
                "error": err.Error(),
            })
            return
        }

        // Check other dependencies...

        w.WriteHeader(http.StatusOK)
        json.NewEncoder(w).Encode(map[string]string{
            "status": "healthy",
        })
    }
}

Rate Limiting

import "golang.org/x/time/rate"

func rateLimitMiddleware(limiter *rate.Limiter) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            if !limiter.Allow() {
                http.Error(w, "Too Many Requests", http.StatusTooManyRequests)
                return
            }
            next.ServeHTTP(w, r)
        })
    }
}

// Usage
limiter := rate.NewLimiter(rate.Limit(10), 20)  // 10 req/sec, burst 20
handler := rateLimitMiddleware(limiter)(apiHandler)

Panic Recovery

func safelyDo(work *Work) {
    defer func() {
        if err := recover(); err != nil {
            log.Println("work failed:", err)
        }
    }()
    do(work)
}

func server(workChan <-chan *Work) {
    for work := range workChan {
        go safelyDo(work)
    }
}

Microservices Patterns

Service Structure

type UserService struct {
    db     *sql.DB
    cache  *redis.Client
    logger *slog.Logger
}

func NewUserService(db *sql.DB, cache *redis.Client, logger *slog.Logger) *UserService {
    return &UserService{
        db:     db,
        cache:  cache,
        logger: logger,
    }
}

func (s *UserService) GetUser(ctx context.Context, userID string) (*User, error) {
    // Check cache first
    if user, err := s.getFromCache(ctx, userID); err == nil {
        return user, nil
    }

    // Query database
    user, err := s.getFromDB(ctx, userID)
    if err != nil {
        return nil, err
    }

    // Update cache
    go s.updateCache(context.Background(), user)

    return user, nil
}

gRPC Service

type server struct {
    pb.UnimplementedUserServiceServer
    db *sql.DB
}

func (s *server) GetUser(ctx context.Context, req *pb.GetUserRequest) (*pb.User, error) {
    user := &pb.User{}
    err := s.db.QueryRowContext(ctx,
        "SELECT id, name, email FROM users WHERE id = $1",
        req.GetId(),
    ).Scan(&user.Id, &user.Name, &user.Email)

    if err != nil {
        return nil, status.Errorf(codes.NotFound, "user not found")
    }

    return user, nil
}

Service Discovery

type ServiceRegistry struct {
    services map[string][]string
    mu       sync.RWMutex
}

func (r *ServiceRegistry) Register(name, addr string) {
    r.mu.Lock()
    defer r.mu.Unlock()
    r.services[name] = append(r.services[name], addr)
}

func (r *ServiceRegistry) Discover(name string) (string, error) {
    r.mu.RLock()
    defer r.mu.RUnlock()

    addrs := r.services[name]
    if len(addrs) == 0 {
        return "", fmt.Errorf("service %s not found", name)
    }

    // Simple round-robin
    return addrs[rand.Intn(len(addrs))], nil
}

Circuit Breaker

type CircuitBreaker struct {
    maxFailures int
    timeout     time.Duration
    failures    int
    lastFailure time.Time
    state       string  // closed, open, half-open
    mu          sync.Mutex
}

func (cb *CircuitBreaker) Call(fn func() error) error {
    cb.mu.Lock()

    if cb.state == "open" {
        if time.Since(cb.lastFailure) > cb.timeout {
            cb.state = "half-open"
        } else {
            cb.mu.Unlock()
            return errors.New("circuit breaker open")
        }
    }

    cb.mu.Unlock()

    err := fn()

    cb.mu.Lock()
    defer cb.mu.Unlock()

    if err != nil {
        cb.failures++
        cb.lastFailure = time.Now()
        if cb.failures >= cb.maxFailures {
            cb.state = "open"
        }
        return err
    }

    cb.failures = 0
    cb.state = "closed"
    return nil
}

Best Practices

1. Goroutine Management

Always consider goroutine lifecycle and cleanup
Use contexts for cancellation propagation
Avoid goroutine leaks by ensuring all goroutines can exit
Be cautious with closures in loops - pass values explicitly

Anti-pattern:

for _, v := range values {
    go func() {
        fmt.Println(v)  // All goroutines share same v
    }()
}

Correct:

for _, v := range values {
    go func(val string) {
        fmt.Println(val)  // Each goroutine gets its own copy
    }(v)
}

2. Channel Best Practices

Close channels from sender, not receiver
Use buffered channels to prevent goroutine leaks
Consider using select with default for non-blocking operations
Remember: sending on closed channel panics, receiving returns zero value

3. Error Handling

Return errors, don't panic (except for truly exceptional cases)
Wrap errors with context using fmt.Errorf("%w", err)
Use custom error types for programmatic handling
Log errors with sufficient context

4. Performance

Use sync.Pool for frequently allocated objects
Profile before optimizing: go test -bench . -cpuprofile=cpu.prof
Consider sync.Map for concurrent map access patterns
Use buffered channels for known capacity
Avoid unnecessary allocations in hot paths

5. Code Organization

project/
├── cmd/
│   └── server/
│       └── main.go          # Application entry point
├── internal/
│   ├── api/                 # HTTP handlers
│   ├── service/             # Business logic
│   ├── repository/          # Data access
│   └── middleware/          # HTTP middleware
├── pkg/
│   └── utils/               # Public utilities
├── migrations/              # Database migrations
├── config/                  # Configuration files
└── docker/                  # Docker files

6. Security

Validate all inputs
Use prepared statements for SQL queries
Implement rate limiting
Use HTTPS in production
Sanitize error messages sent to clients
Use context timeouts to prevent resource exhaustion
Implement proper authentication and authorization

7. Testing

Write table-driven tests
Use t.Helper() for test helper functions
Mock external dependencies
Use httptest for HTTP handler testing
Write benchmarks for performance-critical code
Aim for >80% test coverage on business logic

Common Pitfalls

1. Race Conditions

Problem:

var service map[string]net.Addr

func RegisterService(name string, addr net.Addr) {
    service[name] = addr  // RACE CONDITION
}

func LookupService(name string) net.Addr {
    return service[name]  // RACE CONDITION
}

Solution:

var (
    service   map[string]net.Addr
    serviceMu sync.Mutex
)

func RegisterService(name string, addr net.Addr) {
    serviceMu.Lock()
    defer serviceMu.Unlock()
    service[name] = addr
}

2. Goroutine Leaks

golang-backend-development

Go Backend Development

When to Use This Skill