golang 的线程池实现

原文链接： Handling 1 Million Requests per Minute with Go

注： 这里说的“线程”并不是真正的线程，而是 golang 的协程。

假如要完成的工作是接受一个 POST 请求，并将请求中的 payload 数组部分依次进行处理。

在本文的程序中，只需将 payload 依次取出，将其包装后发送到 一个名为 JobQueue 的 chan 中，线程池会处理提交的 Job。 

func payloadHandler(w http.ResponseWriter, r *http.Request) {

  if r.Method != "POST" {
    w.WriteHeader(http.StatusMethodNotAllowed)
    return
  }

  // Read the body into a string for json decoding
  var content = &PayloadCollection{}
  err := json.NewDecoder(io.LimitReader(r.Body, MaxLength)).Decode(&content)
    if err != nil {
    w.Header().Set("Content-Type", "application/json; charset=UTF-8")
    w.WriteHeader(http.StatusBadRequest)
    return
  }

  // Go through each payload and queue items individually to be posted to Worker
  for _, payload := range content.Payloads {

    // let's create a job with the payload
    work := Job{Payload: payload}

    // Push the work onto the queue.
    JobQueue <- work
  }

  w.WriteHeader(http.StatusOK)
}

线程池的实现如下。 

// 按照 12-Factor，使用环境变量作为配置
var (
        MaxWorker = os.Getenv("MAX_WORKERS")
        MaxQueue  = os.Getenv("MAX_QUEUE")
)

// Job represents the job to be run
type Job struct {
        Payload Payload
}

// A buffered channel that we can send work requests on.
var JobQueue chan Job

// Worker represents the worker that executes the job
type Worker struct {
  // WorkerPool 是个指向全局唯一的 chan 的引用, 
  // 负责传递 Worker 接收 Job 的 chan。
  // Worker 空闲时，将自己的 JobChannel 放入 WorkerPool 中。
  // Dispatcher 收到新的 Job 时，从 JobChannel 中取出一个 chan， 并将 Job
  // 放入其中，此时 Worker 将从 Chan 中接收到 Job，并进行处理
        WorkerPool  chan chan Job
  // Worker 用于接收 Job 的 chan
        JobChannel  chan Job
  // 用于给 Worker 发送控制命令的 chan，用于停止 chan
        quit            chan bool
}

func NewWorker(workerPool chan chan Job) Worker {
        return Worker{
                WorkerPool: workerPool,
                JobChannel: make(chan Job),
                quit:       make(chan bool)}
}

// Start method starts the run loop for the worker, listening for a quit channel in
// case we need to stop it
func (w Worker) Start() {
        go func() {
                for {
                        // register the current worker into the worker queue.
                        w.WorkerPool <- w.JobChannel

                        select {
                        case job := <-w.JobChannel:
                                // we have received a work request.
        // DO THE JOB

                        case <-w.quit:
                                // we have received a signal to stop
                                return
                        }
                }
        }()
}

// Stop signals the worker to stop listening for work requests.
func (w Worker) Stop() {
        go func() {
                w.quit <- true
        }()
}

有了 Worker 还需要一个用于启动，分发 Job 的角色: 

type Dispatcher struct {
        // A pool of workers channels that are registered with the dispatcher
        WorkerPool chan chan Job
}

func NewDispatcher(maxWorkers int) *Dispatcher {
        pool := make(chan chan Job, maxWorkers)
        return &Dispatcher{WorkerPool: pool}
}

func (d *Dispatcher) Run() {
  // starting n number of workers
        for i := 0; i < d.maxWorkers; i++ {
                worker := NewWorker(d.pool)
                worker.Start()
        }

        go d.dispatch()
}

func (d *Dispatcher) dispatch() {
        for {
                select {
                case job := <-JobQueue:
                        // a job request has been received
                        go func(job Job) {
                                // try to obtain a worker job channel that is available.
                                // this will block until a worker is idle
                                jobChannel := <-d.WorkerPool

                                // dispatch the job to the worker job channel
                                jobChannel <- job
                        }(job)
                }
        }
}

初始化时只需执行下面两句即可： 

dispatcher := NewDispatcher(MaxWorker)
dispatcher.Run()

此时，就可以像开头第一个程序那样只需向 JobQueue 中发送 Job 即可将任务交由线程池处理。

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