Go by Example: Mutexes

In the previous example we saw how to manage simple counter state using atomic operations. For more complex state we can use a mutex to safely access data across multiple goroutines.

 
package main
 
import (
    "fmt"
    "sync"
)

Container holds a map of counters; since we want to update it concurrently from multiple goroutines, we add a Mutex to synchronize access. Note that mutexes must not be copied, so if this struct is passed around, it should be done by pointer.

 
type Container struct {
    mu       sync.Mutex
    counters map[string]int
}

Lock the mutex before accessing counters; unlock it at the end of the function using a defer statement.

 
func (c *Container) inc(name string) {
 
    c.mu.Lock()
    defer c.mu.Unlock()
    c.counters[name]++
}

Note that the zero value of a mutex is usable as-is, so no initialization is required here.

 
func main() {
    c := Container{
 
        counters: map[string]int{"a": 0, "b": 0},
    }
 
    var wg sync.WaitGroup

This function increments a named counter in a loop.

 
    doIncrement := func(name string, n int) {
        for i := 0; i < n; i++ {
            c.inc(name)
        }
        wg.Done()
    }

Run several goroutines concurrently; note that they all access the same Container, and two of them access the same counter.

 
    wg.Add(3)
    go doIncrement("a", 10000)
    go doIncrement("a", 10000)
    go doIncrement("b", 10000)

Wait for the goroutines to finish

 
    wg.Wait()
    fmt.Println(c.counters)
}

Running the program shows that the counters updated as expected.

 
$ go run mutexes.go
map[a:20000 b:10000]

Next we’ll look at implementing this same state management task using only goroutines and channels.

Next example: Stateful Goroutines.

by Mark McGranaghan and Eli Bendersky | source | license