When should you use sync.Map instead of a mutex-protected regular map?

sync.Map (Go 1.9+) is a specialised concurrent map optimised for specific access patterns. It is not a general-purpose replacement for map + sync.Mutex.

// sync.Map — optimised for: once written, many reads
var sm sync.Map

// Store
sm.Store("key", 42)

// Load
val, ok := sm.Load("key")
if ok {
    fmt.Println(val.(int)) // type assertion required
}

// LoadOrStore — atomic get-or-set
actual, loaded := sm.LoadOrStore("key", 99)
fmt.Println(actual, loaded) // 42 true (existing value returned)

// Range — iterate (snapshot semantics during iteration not guaranteed)
sm.Range(func(k, v any) bool {
    fmt.Println(k, v)
    return true // return false to stop iteration
})

// mutex-protected map — preferred for write-heavy or type-safe needs
type SafeMap[K comparable, V any] struct {
    mu sync.RWMutex
    m  map[K]V
}
func (s *SafeMap[K, V]) Get(k K) (V, bool) {
    s.mu.RLock()
    defer s.mu.RUnlock()
    v, ok := s.m[k]
    return v, ok
}

sync.Map uses two internal maps: a read-only map (accessed without locks using atomic loads) and a dirty map (accessed under a mutex) for new and updated keys. A key migrates from dirty to read when enough dirty reads occur. This optimises read performance but makes writes heavier. Use it when: caches with stable keys (add once, read many), or when goroutines read disjoint key sets.

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