Go concurrency foundations

Go Concurrency Foundations

Rules derived from "100 Go Mistakes" #55-#60. Apply whenever writing or reviewing concurrent Go code.

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1. Concurrency vs Parallelism (#55)

• Concurrency is about structure -- decomposing a problem into independently executing steps that coordinate.
• Parallelism is about execution -- running the same step on multiple cores simultaneously.
• Concurrency enables parallelism but is not the same thing.
• When restructuring code, ask: "Am I changing the structure (concurrency) or adding more workers to the same step (parallelism)?"

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2. Concurrency Is Not Always Faster (#56)

Go scheduling essentials

• Goroutines are multiplexed onto OS threads (M) by the Go runtime, not the OS.
• GOMAXPROCS limits the number of OS threads executing user-level Go code simultaneously (defaults to logical CPU count since Go 1.5).
• The scheduler uses per-P local queues, a global queue, and work stealing.
• Since Go 1.14 the scheduler is preemptive (10 ms time slice).

Key rule: small workloads kill parallelism

Spinning up a goroutine per tiny unit of work makes things slower -- the goroutine creation and scheduling overhead dominates. Always use a threshold to fall back to sequential execution for small inputs.

const threshold = 2048 // tune via benchmarks on target hardware

func parallelMergesort(s []int) {
    if len(s) <= 1 {
        return
    }
    if len(s) <= threshold {
        sequentialMergesort(s) // fall back to sequential
        return
    }

    middle := len(s) / 2
    var wg sync.WaitGroup
    wg.Add(2)
    go func() {
        defer wg.Done()
        parallelMergesort(s[:middle])
    }()
    go func() {
        defer wg.Done()
        parallelMergesort(s[middle:])
    }()
    wg.Wait()
    merge(s, middle)
}

Checklist before adding concurrency

1. Start with a correct sequential version.
2. Profile and benchmark to confirm the bottleneck.
3. Introduce concurrency with a tunable threshold or pool size.
4. Benchmark the concurrent version -- if it is not measurably faster, keep the sequential one.

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3. Channels vs Mutexes (#57)

Use this decision guide:

Situation	Prefer
Parallel goroutines accessing/mutating a shared resource	sync.Mutex (or sync/atomic)
Concurrent goroutines that need to coordinate, signal, or transfer ownership	Channels
Signaling completion or readiness (with or without data)	Channels (chan struct{} for no data)
Protecting a critical section (read/write to shared state)	sync.Mutex / sync.RWMutex
Transferring ownership of a resource from one stage to the next	Channels

• Do NOT force channels everywhere just because Go says "share memory by communicating." Mutexes and channels are complementary.
• If goroutines are parallel (same step, multiple workers): think mutexes.
• If goroutines are concurrent (different steps in a pipeline): think channels.

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4. Data Races vs Race Conditions (#58)

Definitions

• Data race: two+ goroutines access the same memory location concurrently and at least one writes. Detected by go test -race / go run -race.
• Race condition: behavior depends on uncontrolled timing of events. A data-race-free program can still have race conditions.

Eliminating data races does NOT guarantee deterministic results.

Preventing data races

Choose one of:

1. Atomic operations -- sync/atomic for simple numeric types.
2. Mutex -- sync.Mutex / sync.RWMutex to guard a critical section.
3. Channel communication -- ensure only one goroutine writes to the variable.

// BAD -- data race
i := 0
go func() { i++ }()
go func() { i++ }()

// GOOD -- atomic
var i int64
go func() { atomic.AddInt64(&i, 1) }()
go func() { atomic.AddInt64(&i, 1) }()

// GOOD -- mutex
var mu sync.Mutex
i := 0
go func() { mu.Lock(); i++; mu.Unlock() }()
go func() { mu.Lock(); i++; mu.Unlock() }()

// GOOD -- channel (only parent writes)
ch := make(chan int)
go func() { ch <- 1 }()
go func() { ch <- 1 }()
i := <-ch + <-ch

Race condition (data-race-free but non-deterministic)

// No data race, but i is unpredictably 1 or 2
var mu sync.Mutex
i := 0
go func() { mu.Lock(); i = 1; mu.Unlock() }()
go func() { mu.Lock(); i = 2; mu.Unlock() }()

To enforce ordering, use channels for coordination, not just mutexes.

Go memory model guarantees

Memorize these ordering rules:

1. Goroutine creation happens-before the goroutine starts executing.
2. Goroutine exit is NOT guaranteed to happen before any event -- always synchronize if the parent reads state written by the child.
3. Channel send happens-before the corresponding receive completes.
4. Channel close happens-before a receive observing the closure.
5. Unbuffered channel receive happens-before the send completes.
   • This means with an unbuffered channel, a write before the receive is guaranteed visible after the send returns.
   • This guarantee does NOT hold for buffered channels.

// SAFE -- unbuffered channel guarantees ordering
i := 0
ch := make(chan struct{})
go func() {
    i = 1
    <-ch
}()
ch <- struct{}{}
fmt.Println(i) // guaranteed to print 1

// UNSAFE -- buffered channel, data race on i
ch := make(chan struct{}, 1)
go func() {
    i = 1
    <-ch
}()
ch <- struct{}{}
fmt.Println(i) // data race

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5. Worker Pool Sizing by Workload Type (#59)

Workload	Pool size guideline
CPU-bound	runtime.GOMAXPROCS(0) (number of OS threads, defaults to logical CPUs)
I/O-bound	Depends on the external system's capacity; tune via load testing

• Use runtime.GOMAXPROCS(0) (read-only call) to get the current value.
• Do NOT use runtime.NumCPU() for pool sizing -- GOMAXPROCS may be set lower than the CPU count (e.g., in containers).
• For CPU-bound work, more goroutines than GOMAXPROCS causes unnecessary context switching with no throughput gain.

Worker pool template

func process(r io.Reader) (int, error) {
    var count int64
    n := runtime.GOMAXPROCS(0) // CPU-bound: match available threads

    ch := make(chan []byte, n)
    var wg sync.WaitGroup
    wg.Add(n)
    for i := 0; i < n; i++ {
        go func() {
            defer wg.Done()
            for b := range ch {
                v := task(b)
                atomic.AddInt64(&count, int64(v))
            }
        }()
    }

    for {
        b := make([]byte, 1024)
        _, err := r.Read(b)
        if err != nil {
            if err == io.EOF {
                break
            }
            close(ch)
            return 0, err
        }
        ch <- b
    }
    close(ch)
    wg.Wait()
    return int(count), nil
}

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6. Go Contexts (#60)

When to create which context

Constructor	Use case
context.WithTimeout(parent, d)	Cancel after a duration (e.g., RPC deadline)
context.WithDeadline(parent, t)	Cancel at an absolute time
context.WithCancel(parent)	Manual cancellation signal (e.g., graceful shutdown)
context.WithValue(parent, k, v)	Carry request-scoped metadata (trace IDs, auth)
context.Background()	Top-level / main / test entry point
context.TODO()	Placeholder when the correct context is not yet available

Mandatory rules

1. Always defer cancel() after WithTimeout, WithDeadline, or WithCancel. Forgetting leaks the internal timer goroutine until the timeout fires.

   ctx, cancel := context.WithTimeout(ctx, 4*time.Second)
   defer cancel() // always, even if the function returns early
   

2. Use unexported key types for context values to prevent cross-package collisions.

   type ctxKey string
   const traceIDKey ctxKey = "traceID"
   
   ctx = context.WithValue(ctx, traceIDKey, "abc-123")
   

3. Never block on channel send/receive in a context-aware function without selecting on ctx.Done().

   // BAD -- blocks even if context is canceled
   ch <- msg
   v := <-ch
   
   // GOOD -- respects context cancellation
   select {
   case <-ctx.Done():
       return ctx.Err()
   case ch <- msg:
   }
   
   select {
   case <-ctx.Done():
       return ctx.Err()
   case v := <-ch:
       // use v
   }
   

4. Check ctx.Err() to distinguish cancellation causes:

   • context.Canceled -- explicit cancel.
   • context.DeadlineExceeded -- timeout or deadline passed.

5. Functions that users wait for should accept a context.Context as the first parameter so upstream callers can control cancellation.

6. Prefer context.TODO() over context.Background() when the right context is unclear or not yet propagated -- it signals intent to revisit.