publish atomic counters

examples.txt

@@ -38,8 +38,9 @@ Timers
 Tickers
 Worker Pools
 Rate Limiting
-# State Goroutine
+Atomic Counters
-# State Mutex
+# Mutexs
+# Stateful Goroutines
 Sorting
 Sorting by Functions
 # Collection Functions

examples/atomic-counters/atomic-counters.go

@@ -0,0 +1,48 @@
+// The primary mechanism for managing state in Go is
+// communication over channels. We saw this for example
+// with [worker pools](worker-pool). There are a few other
+// options for managing state though. Here we'll
+// look at using the `sync/atomic` package for simple
+// counters accessed by multiple goroutines.
+
+package main
+
+import "fmt"
+import "time"
+import "sync/atomic"
+
+func main() {
+
+    // We'll use an unsigned integer to represent our
+    // (always-positive) counter.
+    var ops uint64 = 0
+
+    // To simulate concurrent updates, we'll start 50
+    // goroutines that each increment the counter about
+    // once a millisecond.
+    for i := 0; i < 50; i++ {
+        go func() {
+            for {
+                time.Sleep(time.Millisecond)
+
+                // To atomically increment the counter we
+                // use `AddUint64`, giving it the memory
+                // address of our `ops` counter with the
+                // `&` syntax.
+                atomic.AddUint64(&ops, 1)
+            }
+        }()
+    }
+
+    // Wait a second to allow some ops to accumulate.
+    time.Sleep(time.Second)
+
+    // In order to safely use the counter while it's still
+    // being updated by other goroutines, we extract a
+    // copy of the current value into `opsFinal` via
+    // `LoadUint64`. As above we need to give this
+    // function the memory address `&ops` from which to
+    // fetch the value.
+    opsFinal := atomic.LoadUint64(&ops)
+    fmt.Println("ops:", opsFinal)
+}

examples/atomic-counters/atomic-counters.sh

@@ -0,0 +1,4 @@
+# Running the program shows that we executed about
+# 40,000 operations.
+$ go run atomic-counters.go
+ops: 40200

examples/state-mutex/state-mutex.go

@@ -1,3 +1,5 @@
+// State
+
 package main
 
 import "fmt"
@@ -7,14 +9,6 @@ import "sync"
 import "sync/atomic"
 import "runtime"
 
-func randKey() int {
-    return rand.Intn(10)
-}
-
-func randVal() int {
-    return rand.Intn(100)
-}
-
 // Globally-accessible state.
 var data = make(map[int]int)
 
@@ -27,7 +21,7 @@ var opCount int64 = 0
 func generateReads() {
     total := 0
     for {
-        key := randKey()
+        key := rand.Intn(10)
         dataMutex.Lock()
         total += data[key]
         dataMutex.Unlock()
@@ -39,8 +33,8 @@ func generateReads() {
 // Generate random writes.
 func generateWrites() {
     for {
-        key := randKey()
+        key := rand.Intn(10)
-        val := randVal()
+        val := rand.Intn(100)
         dataMutex.Lock()
         data[key] = val
         dataMutex.Unlock()
@@ -62,5 +56,3 @@ func main() {
     finalOpCount := atomic.LoadInt64(&opCount)
     fmt.Println(finalOpCount)
 }
-
-// todo: "State with Mutexes?"