go/by-example/StatefulGoroutines/stateful_go.go

package main

// Built-in synchronization features of goroutines and channels
// to achieve the same results as with Mutexes. Tis channel-based
// approach aligns with Go's idea of sharing memory by
// communicating and having each piece of data owned by exactly 1
// goroutine.

import (
	"fmt"
	"math/rand"
	"sync/atomic"
	"time"
)

// Our state will be owned by a single goroutine. This will
// guarantee that the data is never corrupted with concurrent
// access. In order to read or write that state, other goroutines
// will send messages to the owning goroutine and receive
// corresponding replies. These readOp and writeOp structs
// encapsulate those requests and a way for the owning goroutine
// to respond.
type readOp struct {
	key  int
	resp chan int
}
type writeOp struct {
	key  int
	val  int
	resp chan bool
}

func main() {
	var readOps uint64
	var writeOps uint64

	// The read and writes channels will be used by other
	// goroutines to issue read and write requests, respectively.
	reads := make(chan *readOp)
	writes := make(chan *writeOp)

	// Here is the goroutine that owns the state, which is a map
	// as in the previous example but now private to the stateful
	// goroutine. This goroutine repeatedly selects on the reads
	// and writes channels, responding to requests as they arrive
	// A response is executed by first performing the requested
	// operation and then sending a value on the response channel
	// resp to indicate success (and the desired value in the case
	// of reads)>
	go func() {
		var state = make(map[int]int)
		for {
			select {
			case read := <-reads:
				read.resp <- state[read.key]
			case write := <-writes:
				state[write.key] = write.val
				write.resp <- true
			}
		}
	}()

	// This starts 100 goroutines to issue reads to the
	// state-owning goroutine via the reads channel. Each read
	// requires constructing a readOp, sending it over the reads
	// channel, and the receiving the result over the provided
	// resp channel.
	for r := 0; r < 100; r++ {
		go func() {
			for {
				read := &readOp{
					key:  rand.Intn(5),
					resp: make(chan int)}
				reads <- read
				<-read.resp
				atomic.AddUint64(&readOps, 1)
				time.Sleep(time.Millisecond)
			}
		}()
	}

	// We start 10 writes as well, using a similar approach.
	for w := 0; w < 10; w++ {
		go func() {
			for {
				write := &writeOp{
					key:  rand.Intn(5),
					val:  rand.Intn(100),
					resp: make(chan bool)}
				writes <- write
				<-write.resp
				atomic.AddUint64(&writeOps, 1)
				time.Sleep(time.Millisecond)
			}
		}()
	}

	time.Sleep(time.Second)

	// Capture and report the op counts.
	readOpsFinal := atomic.LoadUint64(&readOps)
	fmt.Println("readOps:", readOpsFinal)
	writeOpsFinal := atomic.LoadUint64(&writeOps)
	fmt.Println("writeOps:", writeOpsFinal)
}

// The output displays the total number of operations completed
// by this state management example.