在sync.Map中，必须对复杂值使用Load和LoadOrStore

Z时代
2024-01-10
分类：问答

在代码中，可以通过多个并发线程修改具有昂贵的生成价值结构的全局映射的哪个模式是正确的？

// equivalent to map[string]*activity where activity is a
// fairly heavyweight structure
var ipActivity sync.Map
// version 1: not safe with multiple threads, I think
func incrementIP(ip string) {
  val, ok := ipActivity.Load(ip)
  if !ok {
    val = buildComplexActivityObject()
    ipActivity.Store(ip, val)
  }
  updateTheActivityObject(val.(*activity), ip)
}
// version 2: inefficient, I think, because a complex object is built 
// every time even through it's only needed the first time
func incrementIP(ip string) {
  tmp := buildComplexActivityObject()
  val, _ := ipActivity.LoadOrStore(ip, tmp)
  updateTheActivity(val.(*activity), ip)
}
// version 3: more complex but technically correct?
func incrementIP(ip string) {
  val, found := ipActivity.Load(ip)
  if !found {
     tmp := buildComplexActivityObject()
     // using load or store incase the mapping was already made in 
     // another store
     val, _ = ipActivity.LoadOrStore(ip, tmp)
  }
  updateTheActivity(val.(*activity), ip)
}