`sync.Once` ensures that a function is executed only once, even when called by multiple goroutines concurrently.

• One-time initialization (lazy init), e.g., create a singleton, load config once.
• Safe “initialize-on-first-use” without manual locking.

• You need exactly-once initialization across goroutines.
• The initialization must happen at most once, not zero-or-more.

package main
 
import (
    "fmt"
    "sync"
)
 
var once sync.Once
var value string
 
func initValue() {
    value = "initialized"
}
 
func GetValue() string {
    once.Do(initValue)
    return value
}
 
func main() {
    fmt.Println(GetValue())
}

• If the function passed to `Do()` panics, `Once` is considered “done” (it will not run again).
• Use `Once` for initialization only; do not use it as a general lock.

• sync.Mutex
• sync.WaitGroup

• ensure /ɪnˈʃʊr/: đảm bảo
• executed /ˈeksɪkjuːtɪd/: được thực thi
• initialization /ɪˌnɪʃələˈzeɪʃən/: khởi tạo
• singleton /ˈsɪŋɡəlˌtən/: đối tượng duy nhất (một instance)
• lazy /ˈleɪzi/: trì hoãn (chỉ làm khi cần)
• panic /ˈpænɪk/: lỗi runtime kiểu “panic”