go-concurrency-patterns

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# 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

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