add lock demo

ch6-cloud/ch6-08-lock.md

@@ -1,11 +1,232 @@
 # 6.8. 分布式锁
 
+在单机程序并发或并行修改全局变量时，需要对修改行为加锁以创造临界区。为什么需要加锁呢？可以看看下段代码：
+
+```go
+package main
+
+import (
+    "sync"
+)
+
+// 全局变量
+var counter int
+
+func main() {
+    var wg sync.WaitGroup
+    for i := 0; i < 1000; i++ {
+        wg.Add(1)
+        go func() {
+            defer wg.Done()
+            counter++
+        }()
+    }
+
+    wg.Wait()
+    println(counter)
+}
+```
+
+多次运行会得到不同的结果：
+
+```shell
+❯❯❯ go run local_lock.go                                ✭
+945
+❯❯❯ go run local_lock.go                                ✭
+937
+❯❯❯ go run local_lock.go                                ✭
+959
+```
+
+## 进程内加锁
+
+想要得到正确的结果的话，把对 counter 的操作代码部分加上锁：
+
+```go
+// ... 省略之前部分
+var wg sync.WaitGroup
+var l sync.Mutex
+for i := 0; i < 1000; i++ {
+    wg.Add(1)
+    go func() {
+        defer wg.Done()
+        l.Lock()
+        counter++
+        l.Unlock()
+    }()
+}
+
+wg.Wait()
+println(counter)
+// ... 省略之后部分
+```
+
+这样就可以稳定地得到计算结果了：
+
+```shell
+❯❯❯ go run local_lock.go                              ✭ ✱
+1000
+```
+
+## trylock
+
+```go
+package main
+
+import (
+	"sync"
+)
+
+// Lock try lock
+type Lock struct {
+	c chan struct{}
+}
+
+// NewLock generate a try lock
+func NewLock() Lock {
+	var l Lock
+	l.c = make(chan struct{}, 1)
+	l.c <- struct{}{}
+	return l
+}
+
+// Lock try lock, return lock result
+func (l Lock) Lock() bool {
+	lockResult := false
+	select {
+	case <-l.c:
+		lockResult = true
+	default:
+	}
+	return lockResult
+}
+
+// Unlock , Unlock the try lock
+func (l Lock) Unlock() bool {
+	unlockResult := false
+	select {
+	case l.c <- struct{}{}:
+		unlockResult = true
+	default:
+	}
+	return unlockResult
+}
+
+var counter int
+
+func main() {
+	var l = NewLock()
+	var wg sync.WaitGroup
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if !l.Lock() {
+				// log error
+				println("lock failed")
+				return
+			}
+			counter++
+			println("current counter", counter)
+			l.Unlock()
+		}()
+	}
+	wg.Wait()
+}
+
+```
+
 ## 基于 redis 的 setnx
 
+```go
+package main
+
+import (
+    "fmt"
+    "sync"
+    "time"
+
+    "github.com/go-redis/redis"
+)
+
+func incr() {
+    client := redis.NewClient(&redis.Options{
+        Addr:     "localhost:6379",
+        Password: "", // no password set
+        DB:       0,  // use default DB
+    })
+
+    var lockKey = "counter_lock"
+    var counterKey = "counter"
+
+    // lock
+    resp := client.SetNX(lockKey, 1, time.Second*5)
+    lockSuccess, err := resp.Result()
+
+    if err != nil || !lockSuccess {
+        fmt.Println(err, "lock result: ", lockSuccess)
+        return
+    }
+
+    // counter ++
+    getResp := client.Get(counterKey)
+    cntValue, err := getResp.Int64()
+    if err == nil {
+        cntValue++
+        resp := client.Set(counterKey, cntValue, 0)
+        _, err := resp.Result()
+        if err != nil {
+            // log err
+            println("set value error!")
+        }
+    }
+    println("current counter is ", cntValue)
+
+    delResp := client.Del(lockKey)
+    unlockSuccess, err := delResp.Result()
+    if err == nil && unlockSuccess > 0 {
+        println("unlock success!")
+    } else {
+        println("unlock failed", err)
+    }
+}
+
+func main() {
+    var wg sync.WaitGroup
+    for i := 0; i < 10; i++ {
+        wg.Add(1)
+        go func() {
+            defer wg.Done()
+            incr()
+        }()
+    }
+    wg.Wait()
+}
+
+```
+
+```shell
+❯❯❯ go run redis_setnx.go
+<nil> lock result:  false
+<nil> lock result:  false
+<nil> lock result:  false
+<nil> lock result:  false
+<nil> lock result:  false
+<nil> lock result:  false
+<nil> lock result:  false
+<nil> lock result:  false
+<nil> lock result:  false
+current counter is  2028
+unlock success!
+```
+
 ## 基于 zk
 
 ## 基于 etcd
 
 ## redlock
 
+```go
+```
+
 ## how to choose