Add Producer Consumer Part

There are three parts changed:

1. Add producer_consumer.md and update the README.md in root dir.

2. Add producer_consumer package:

  We can use assign tasks to producer after producer starts.

  Producer has limit buffer for task. If the buffer is full and no
  consumer to take, the producer will ignore the task.

  Consumer needs to regiester if it wants to do task for producer.

  All the communication is based on RPC.

3. Add example for producer_consumer package.

  The example will show the case that the buffer of producer is full.

README.md

@@ -68,7 +68,7 @@ A curated collection of idiomatic design & application patterns for Go language.
 | [Generators](/concurrency/generator.md) | Yields a sequence of values one at a time | ✔ |
 | [Reactor](/concurrency/reactor.md) | Demultiplexes service requests delivered concurrently to a service handler and dispatches them syncronously to the associated request handlers | ✘ |
 | [Parallelism](/concurrency/parallelism.md) | Completes large number of independent tasks | ✔ |
-| [Producer Consumer](/concurrency/producer_consumer.md) | Separates tasks from task executions | ✘ |
+| [Producer Consumer](/concurrency/producer_consumer.md) | Separates tasks from task executions | ✔ |
 
 ## Messaging Patterns
 

concurrency/producer_consumer.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	pc "./producer_consumer"
+	"strconv"
+	"time"
+)
+
+func main() {
+	// unix socket address
+	pAddr := "/tmp/producer"
+	cAddr1 := "/tmp/consumer1"
+	cAddr2 := "/tmp/consumer2"
+
+	// producer buffer size
+	bufSize := 2
+
+	// start producer
+	producer := pc.NewProducer(pAddr, bufSize)
+	pc.StartProducer(producer)
+
+	// generate tasks
+	for i := 0; i < 10; i++ {
+		go func(num int) {
+			pc.EnqueueTask(pAddr, pc.Task{strconv.Itoa(num)})
+		}(i)
+	}
+
+	// start three consumers
+	consumer1 := pc.NewConsumer(cAddr1)
+	pc.StartConsumer(consumer1, pAddr)
+	consumer2 := pc.NewConsumer(cAddr2)
+	pc.StartConsumer(consumer2, pAddr)
+
+	time.Sleep(30 * time.Second)
+	pc.Shutdown(pAddr, true)
+	pc.Shutdown(cAddr1, false)
+	pc.Shutdown(cAddr2, false)
+}

concurrency/producer_consumer.md

@@ -0,0 +1,12 @@
+# Producer Consumer
+
+What is the producer-consumer pattern?
+> In computing, the producer–consumer problem (also known as the bounded-buffer problem) is a classic example of a multi-process synchronization problem. The problem describes two processes, the producer and the consumer, who share a common, fixed-size buffer used as a queue. The producer's job is to generate data, put it into the buffer, and start again. At the same time, the consumer is consuming the data (i.e., removing it from the buffer), one piece at a time. The problem is to make sure that the producer won't try to add data into the buffer if it's full and that the consumer won't try to remove data from an empty buffer. --- from [wikipedia](https://en.wikipedia.org/wiki/Producer%E2%80%93consumer_problem)
+
+## Implementaion
+
+More information can be found in [producer_consumer package](producer_consumer).
+
+## Example
+
+An example can be found in [producer_consumer.go](producer_consumer.go).

concurrency/producer_consumer/common.go

@@ -0,0 +1,67 @@
+package producer_consumer
+
+import (
+	"errors"
+	"log"
+	"net"
+	"net/rpc"
+	"os"
+	"time"
+)
+
+const (
+	TimeoutEnqueueTask = 5 * time.Second
+	TimeoutRegister    = 5 * time.Second
+	TimeoutDial        = 5 * time.Second
+	TimeoutShutdown    = 5 * time.Second
+
+	ProcessDuration = 2 * time.Second
+)
+
+var (
+	ErrorTEnqueueTask = errors.New("Timeout for enqueuing task")
+	ErrorTRegister    = errors.New("Timeout for registering")
+	ErrorTDial        = errors.New("Timeout for dailing")
+
+	LogInfo  = log.New(os.Stdout, "[Info]  ", log.Ltime)
+	LogError = log.New(os.Stderr, "[Error] ", log.Ltime)
+)
+
+func Shutdown(address string, isProducer bool) {
+	serviceMethod := "Producer.Shutdown"
+	if !isProducer {
+		serviceMethod = "Consumer.Shutdown"
+	}
+
+	err := rpcCall(address, serviceMethod, struct{}{}, &struct{}{})
+	if err != nil {
+		LogError.Println(err)
+	}
+}
+
+func rpcCall(address string, serviceMethod string, args interface{}, reply *struct{}) error {
+	conn, err := net.DialTimeout("unix", address, TimeoutDial)
+	if err != nil {
+		return err
+	}
+
+	defer conn.Close()
+
+	client := rpc.NewClient(conn)
+	err = client.Call(serviceMethod, args, reply)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func serverAccept(server *rpc.Server, l net.Listener) {
+	for {
+		conn, err := l.Accept()
+		if err != nil {
+			LogError.Println(err)
+			return
+		}
+		go server.ServeConn(conn)
+	}
+}

concurrency/producer_consumer/consumer.go

@@ -0,0 +1,55 @@
+package producer_consumer
+
+import (
+	"net"
+	"net/rpc"
+	"os"
+	"time"
+)
+
+type Consumer struct {
+	address string
+
+	l net.Listener
+}
+
+func NewConsumer(address string) *Consumer {
+	return &Consumer{address: address}
+}
+
+func StartConsumer(c *Consumer, pAddr string) {
+	server := rpc.NewServer()
+	server.Register(c)
+
+	os.Remove(c.address)
+	l, err := net.Listen("unix", c.address)
+	if err != nil {
+		LogError.Println(err)
+	}
+
+	c.l = l
+	go serverAccept(server, c.l)
+
+	err = rpcCall(pAddr, "Producer.Register", c.address, &struct{}{})
+	if err != nil {
+		LogError.Println(err)
+		c.l.Close()
+	}
+}
+
+func (c *Consumer) DoTask(task Task, _ *struct{}) error {
+	// takes time to do the task
+	time.Sleep(ProcessDuration)
+
+	LogInfo.Printf("[%s] finish task[%s] successfully!", c.address, task.String())
+	return nil
+}
+
+func (c *Consumer) Shutdown(_ struct{}, _ *struct{}) error {
+	if err := c.l.Close(); err != nil {
+		return err
+	}
+
+	LogInfo.Printf("[%s] shutdown successfully!", c.address)
+	return nil
+}

concurrency/producer_consumer/producer.go

@@ -0,0 +1,106 @@
+package producer_consumer
+
+import (
+	"net"
+	"net/rpc"
+	"os"
+	"time"
+)
+
+type Producer struct {
+	address string
+	tasks   chan Task
+	done    chan struct{}
+
+	consumers chan string
+	l         net.Listener
+}
+
+func NewProducer(address string, capacity int) *Producer {
+	return &Producer{
+		address:   address,
+		tasks:     make(chan Task, capacity),
+		consumers: make(chan string, capacity),
+		done:      make(chan struct{}, 0),
+	}
+}
+
+func StartProducer(p *Producer) {
+	server := rpc.NewServer()
+	server.Register(p)
+
+	os.Remove(p.address)
+	l, err := net.Listen("unix", p.address)
+	if err != nil {
+		LogError.Println(err)
+	}
+
+	p.l = l
+	go serverAccept(server, p.l)
+	go p.schedule()
+}
+
+func EnqueueTask(pAddr string, task Task) {
+	err := rpcCall(pAddr, "Producer.Enqueue", task, &struct{}{})
+	if err != nil {
+		LogError.Println(err)
+	}
+}
+
+func (p *Producer) schedule() {
+	for {
+		select {
+		case task := <-p.tasks:
+			// one consumer consumes one job at one time
+			consumer := <-p.consumers
+
+			// if rpcCall fails, this consumer will be regarded as unavailable.
+			err := rpcCall(consumer, "Consumer.DoTask", task, &struct{}{})
+			if err != nil {
+				LogError.Printf("[%s] %s", p.address, err.Error())
+			} else { // re-register consumer
+				go func(consumer string) {
+					select {
+					case p.consumers <- consumer:
+					case <-time.After(5 * time.Second):
+						LogError.Printf("[%s] %s", p.address, ErrorTRegister)
+					}
+				}(consumer)
+			}
+		case <-p.done:
+			break
+		}
+	}
+}
+
+func (p *Producer) Enqueue(task Task, _ *struct{}) error {
+	select {
+	case p.tasks <- task:
+		LogInfo.Printf("[%s] enqueue task[%s] successfully!", p.address, task.String())
+	case <-time.After(TimeoutEnqueueTask):
+		return ErrorTEnqueueTask
+	}
+
+	return nil
+}
+
+func (p *Producer) Register(address string, _ *struct{}) error {
+	select {
+	case p.consumers <- address:
+		LogInfo.Printf("[%s] register consumer[%s] successfully!", p.address, address)
+	case <-time.After(TimeoutRegister):
+		return ErrorTRegister
+	}
+
+	return nil
+}
+
+func (p *Producer) Shutdown(_ struct{}, _ *struct{}) error {
+	p.done <- struct{}{}
+	if err := p.l.Close(); err != nil {
+		return err
+	}
+
+	LogInfo.Printf("[%s] shutdown successfully!\n", p.address)
+	return nil
+}

concurrency/producer_consumer/task.go

@@ -0,0 +1,10 @@
+package producer_consumer
+
+// Add whatever you want
+type Task struct {
+	Name string
+}
+
+func (t *Task) String() string {
+	return t.Name
+}