38 Go interview questions on concurrency patterns & sync, with detailed answers and code.
RWMutex allows many concurrent readers or one writer. It pays off only when reads heavily outnumber writes and the critical section is long enough that letting readers…
Normal mode: waiters queue in FIFO order, but a woken waiter does not get the lock handed to it. It has to compete with goroutines that are arriving right now, and those…
No. Go mutexes are not reentrant and are not tied to a goroutine. If the same goroutine calls Lock twice, it deadlocks. One goroutine may also Unlock a mutex that a…
type Counter struct { mu sync.Mutex n int } func (c Counter) Inc() { // value receiver c.mu.Lock() defer c.mu.Unlock() c.n++ } func main() { var c Counter var wg…
The classic bug is calling Add inside the goroutine: // BUG: Wait may run before any Add, and return immediately for _, j := range jobs { go func() { wg.Add(1) defer…
once.Do(f) runs f exactly once, even with concurrent callers. All callers block until f returns, and the completion of f happens-before any Do returns, so callers see…
sync.Cond lets goroutines wait for a condition on shared state that a mutex protects. Wait does three things: it atomically unlocks c.L , suspends the goroutine, and…
sync.Pool is a cache of temporary objects, sharded per P (processor), that reduces allocation pressure. Get tries, in order: the local P's private slot, the local P's…
The docs name two cases where sync.Map is better: A key is written once and then read many times, as in a grow-only cache. Multiple goroutines read, write and overwrite…
Go 1.19 added atomic.Int32 , Int64 , Uint32 , Uint64 , Uintptr , Bool and Pointer[T] . They are better than the function-based API for three reasons: No accidental…
func StoreMax(a *atomic.Int64, v int64) { for { cur := a.Load() if v <= cur { return } if a.CompareAndSwap(cur, v) { return } // another goroutine changed it: reload and…
The memory model says when a read in one goroutine is guaranteed to see a write made in another. If a write happens-before a read and no other write intervenes, the read…
var done bool var msg string func setup() { msg = "hello" done = true } func main() { go setup() for !done { } fmt.Println(msg) } No output is guaranteed. This is a data…
-race (for go test , go run and go build ) instruments every memory access and synchronization operation. It uses ThreadSanitizer's happens-before algorithm, based on…
func main() { runtime.GOMAXPROCS(1) var n int var wg sync.WaitGroup for range 1000 { wg.Go(func() { n++ }) } wg.Wait() fmt.Println(n) } It will often print 1000 with one…
func main() { var wg sync.WaitGroup for i := 0; i < 3; i++ { wg.Add(1) go func() { defer wg.Done() fmt.Print(i, " ") }() } wg.Wait() } Go 1.22 and later (when go.mod…
func WorkerPool[In, Out any]( ctx context.Context, workers int, in <-chan In, fn func(context.Context, In) Out, ) <-chan Out { out := make(chan Out) var wg…
Fan-out means several goroutines read from the same input channel to spread the work. Fan-in means merging several channels into one. func Merge[T any](ctx…
Sending to or receiving from a nil channel blocks forever. Inside a select , a case on a nil channel is therefore never chosen. Setting a channel variable to nil is the…
func gen(ctx context.Context, nums ...int) <-chan int { out := make(chan int) go func() { defer close(out) for _, n := range nums { select { case out <- n: case…
func fastest(ctx context.Context, mirrors []string) string { ch := make(chan string) for _, m := range mirrors { go func() { ch <- fetch(ctx, m) }() } return <-ch } Only…
The rule is that only a sender closes a channel, and only when no other sender can still send . Sending on a closed channel panics, closing twice panics, and a receiver…
Buffered channel as a counting semaphore , which is idiomatic and has no dependencies: sem := make(chan struct{}, 10) // at most 10 in flight var wg sync.WaitGroup for…
golang.org/x/sync/errgroup is a WaitGroup plus error propagation plus cancellation. g.Go(f) runs f in a goroutine. g.Wait() blocks until all functions return, then…
The core ideas: stream the input instead of loading it, bound the number of goroutines, and apply backpressure through channels. func Process(ctx context.Context, r…
Ticker: the simplest option is a fixed interval with no bursts. t := time.NewTicker(100 * time.Millisecond) // 10 rps defer t.Stop() for _, req := range reqs { select {…
golang.org/x/sync/singleflight deduplicates concurrent calls for the same key. While one call for a key is in flight, later callers wait for it and share its result.…
func main() { ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM) defer stop() var workers sync.WaitGroup jobs := make(chan Job, 100)…
WithCancelCause (1.20): cancel(err) records why the context was cancelled, and context.Cause(ctx) returns that error while ctx.Err() is still Canceled . It makes logs…
for { select { case msg := <-msgs: handle(msg) case <-time.After(time.Minute): // new timer every iteration log.Println("idle") } } Before Go 1.23: every iteration…
func main() { ch := make(chan int) ch <- 1 // no receiver fmt.Println(<-ch) } It prints fatal error: all goroutines are asleep - deadlock! . The send on an unbuffered…
Naive double-checked locking reads a shared flag or pointer without synchronization: if !initialized { // racy read: may see true before the data writes are visible…
The proverb is guidance, not a rule. Choose by the shape of the problem . Use channels when you are: transferring ownership of data (a producer hands work to a…
Tests built on time.Sleep are slow and flaky. testing/synctest became generally available in Go 1.25. synctest.Test(t, f) runs f in an isolated "bubble": Time is…
func main() { defer func() { if r := recover(); r != nil { fmt.Println("recovered:", r) } }() go func() { panic("boom") }() time.Sleep(time.Second) fmt.Println("done") }…
CPUs keep memory coherent per cache line : 64 bytes on most x86-64 CPUs, 128 bytes on some arm64 and ppc64 chips. When two cores write different variables that sit on…
GOMAXPROCS is the number of Ps, which is the maximum number of goroutines executing Go code at the same moment. Goroutines blocked in syscalls do not count against it.…
A channel delivers each value to one receiver, so broadcasting needs a channel per subscriber. The hard parts are slow consumers, unsubscribing without a…