go-concurrency-patterns

What this skill does

# Go Concurrency Patterns

Production patterns for Go concurrency including goroutines, channels, synchronization primitives, and context management.

## When to Use This Skill

- Building concurrent Go applications
- Implementing worker pools and pipelines
- Managing goroutine lifecycles
- Using channels for communication
- Debugging race conditions
- Implementing graceful shutdown

## Core Concepts

### 1. Go Concurrency Primitives

| Primitive | Purpose |
|-----------|---------|
| `goroutine` | Lightweight concurrent execution |
| `channel` | Communication between goroutines |
| `select` | Multiplex channel operations |
| `sync.Mutex` | Mutual exclusion |
| `sync.WaitGroup` | Wait for goroutines to complete |
| `context.Context` | Cancellation and deadlines |

### 2. Go Concurrency Mantra

```
Don't communicate by sharing memory;
share memory by communicating.
```

## Quick Start

```go
package main

import (
    "context"
    "fmt"
    "sync"
    "time"
)

func main() {
    ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
    defer cancel()

    results := make(chan string, 10)
    var wg sync.WaitGroup

    // Spawn workers
    for i := 0; i < 3; i++ {
        wg.Add(1)
        go worker(ctx, i, results, &wg)
    }

    // Close results when done
    go func() {
        wg.Wait()
        close(results)
    }()

    // Collect results
    for result := range results {
        fmt.Println(result)
    }
}

func worker(ctx context.Context, id int, results chan<- string, wg *sync.WaitGroup) {
    defer wg.Done()

    select {
    case <-ctx.Done():
        return
    case results <- fmt.Sprintf("Worker %d done", id):
    }
}
```

## Patterns

### Pattern 1: Worker Pool

```go
package main

import (
    "context"
    "fmt"
    "sync"
)

type Job struct {
    ID   int
    Data string
}

type Result struct {
    JobID int
    Output string
    Err   error
}

func WorkerPool(ctx context.Context, numWorkers int, jobs <-chan Job) <-chan Result {
    results := make(chan Result, len(jobs))

    var wg sync.WaitGroup
    for i := 0; i < numWorkers; i++ {
        wg.Add(1)
        go func(workerID int) {
            defer wg.Done()
            for job := range jobs {
                select {
                case <-ctx.Done():
                    return
                default:
                    result := processJob(job)
                    results <- result
                }
            }
        }(i)
    }

    go func() {
        wg.Wait()
        close(results)
    }()

    return results
}

func processJob(job Job) Result {
    // Simulate work
    return Result{
        JobID:  job.ID,
        Output: fmt.Sprintf("Processed: %s", job.Data),
    }
}

// Usage
func main() {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    jobs := make(chan Job, 100)

    // Send jobs
    go func() {
        for i := 0; i < 50; i++ {
            jobs <- Job{ID: i, Data: fmt.Sprintf("job-%d", i)}
        }
        close(jobs)
    }()

    // Process with 5 workers
    results := WorkerPool(ctx, 5, jobs)

    for result := range results {
        fmt.Printf("Result: %+v\n", result)
    }
}
```

### Pattern 2: Fan-Out/Fan-In Pipeline

```go
package main

import (
    "context"
    "sync"
)

// Stage 1: Generate numbers
func generate(ctx context.Context, nums ...int) <-chan int {
    out := make(chan int)
    go func() {
        defer close(out)
        for _, n := range nums {
            select {
            case <-ctx.Done():
                return
            case out <- n:
            }
        }
    }()
    return out
}

// Stage 2: Square numbers (can run multiple instances)
func square(ctx context.Context, in <-chan int) <-chan int {
    out := make(chan int)
    go func() {
        defer close(out)
        for n := range in {
            select {
            case <-ctx.Done():
                return
            case out <- n * n:
            }
        }
    }()
    return out
}

// Fan-in: Merge multiple channels into one
func merge(ctx context.Context, 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) {
        defer wg.Done()
        for n := range c {
            select {
            case <-ctx.Done():
                return
            case out <- n:
            }
        }
    }

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

    // Close out after all inputs are done
    go func() {
        wg.Wait()
        close(out)
    }()

    return out
}

func main() {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    // Generate input
    in := generate(ctx, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

    // Fan out to multiple squarers
    c1 := square(ctx, in)
    c2 := square(ctx, in)
    c3 := square(ctx, in)

    // Fan in results
    for result := range merge(ctx, c1, c2, c3) {
        fmt.Println(result)
    }
}
```

### Pattern 3: Bounded Concurrency with Semaphore

```go
package main

import (
    "context"
    "fmt"
    "golang.org/x/sync/semaphore"
    "sync"
)

type RateLimitedWorker struct {
    sem *semaphore.Weighted
}

func NewRateLimitedWorker(maxConcurrent int64) *RateLimitedWorker {
    return &RateLimitedWorker{
        sem: semaphore.NewWeighted(maxConcurrent),
    }
}

func (w *RateLimitedWorker) Do(ctx context.Context, tasks []func() error) []error {
    var (
        wg     sync.WaitGroup
        mu     sync.Mutex
        errors []error
    )

    for _, task := range tasks {
        // Acquire semaphore (blocks if at limit)
        if err := w.sem.Acquire(ctx, 1); err != nil {
            return []error{err}
        }

        wg.Add(1)
        go func(t func() error) {
            defer wg.Done()
            defer w.sem.Release(1)

            if err := t(); err != nil {
                mu.Lock()
                errors = append(errors, err)
                mu.Unlock()
            }
        }(task)
    }

    wg.Wait()
    return errors
}

// Alternative: Channel-based semaphore
type Semaphore chan struct{}

func NewSemaphore(n int) Semaphore {
    return make(chan struct{}, n)
}

func (s Semaphore) Acquire() {
    s <- struct{}{}
}

func (s Semaphore) Release() {
    <-s
}
```

### Pattern 4: Graceful Shutdown

```go
package main

import (
    "context"
    "fmt"
    "os"
    "os/signal"
    "sync"
    "syscall"
    "time"
)

type Server struct {
    shutdown chan struct{}
    wg       sync.WaitGroup
}

func NewServer() *Server {
    return &Server{
        shutdown: make(chan struct{}),
    }
}

func (s *Server) Start(ctx context.Context) {
    // Start workers
    for i := 0; i < 5; i++ {
        s.wg.Add(1)
        go s.worker(ctx, i)
    }
}

func (s *Server) worker(ctx context.Context, id int) {
    defer s.wg.Done()
    defer fmt.Printf("Worker %d stopped\n", id)

    ticker := time.NewTicker(time.Second)
    defer ticker.Stop()

    for {
        select {
        case <-ctx.Done():
            // Cleanup
            fmt.Printf("Worker %d cleaning up...\n", id)
            time.Sleep(500 * time.Millisecond) // Simulated cleanup
            return
        case <-ticker.C:
            fmt.Printf("Worker %d working...\n", id)
        }
    }
}

func (s *Server) Shutdown(timeout time.Duration) {
    // Signal shutdown
    close(s.shutdown)

    // Wait with timeout
    done := make(chan struct{})
    go func() {
        s.wg.Wait()
        close(done)
    }()

    select {
    case <-done:
        fmt.Println("Clean shutdown completed")
    case <-time.After(timeout):
        fmt.Println("Shutdown timed out, forcing exit")
    }
}

func main() {
    // Setup signal handling
    ctx, cancel := context.WithCancel(context.Background())

    sigCh := make(chan os.Signal, 1)
    signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)

    server := NewServer()
    server.Start(ctx)

    // Wait for signal
    sig := <-sigC