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261 lines
7.1 KiB
Markdown
261 lines
7.1 KiB
Markdown
# 14.10 复用
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## 14.10.1 典型的客户端/服务器（C/S）模式
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客户端-服务器应用正是 goroutines 和 channels 的亮点所在。
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客户端(Client)可以是运行在任意设备上的任意程序，它会按需发送请求(request)至服务器。服务器(Server)接收到这个请求后开始相应的工作，然后再将响应(response)返回给客户端。典型情况下一般是多个客户端（即多个请求）对应一个（或少量）服务器。例如我们日常使用的浏览器客户端，其功能就是向服务器请求网页。而Web服务器则会向浏览器响应网页数据。
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使用Go的服务器通常会在协程中执行向客户端的响应，故而会对每一个客户端请求启动一个协程。一个常用的操作方法是客户端请求自身中包含一个通道，而服务器则向这个通道发送响应。
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例如下面这个`Request`结构，其中内嵌了一个`replyc`通道。
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```go
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type Request struct {
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a, b int
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replyc chan int // reply channel inside the Request
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}
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```
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或者更通俗的：
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```go
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type Reply struct{...}
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type Request struct{
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arg1, arg2, arg3 some_type
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replyc chan *Reply
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}
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```
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接下来先使用简单的形式,服务器会为每一个请求启动一个协程并在其中执行`run()`函数，此举会将类型为`binOp`的`op`操作返回的int值发送到`replyc`通道。
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```go
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type binOp func(a, b int) int
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func run(op binOp, req *Request) {
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req.replyc <- op(req.a, req.b)
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}
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```
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`server`协程会无限循环以从`chan *Request`接收请求，并且为了避免被长时间操作所堵塞，它将为每一个请求启动一个协程来做具体的工作：
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```go
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func server(op binOp, service chan *Request) {
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for {
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req := <-service; // requests arrive here
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// start goroutine for request:
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go run(op, req); // don’t wait for op to complete
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}
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}
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```
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`server`本身则是以协程的方式在`startServer`函数中启动：
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```go
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func startServer(op binOp) chan *Request {
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reqChan := make(chan *Request);
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go server(op, reqChan);
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return reqChan;
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}
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```
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`startServer`则会在`main`协程中被调用。
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在以下测试例子中，100个请求会被发送到服务器，只有它们全部被送达后我们才会按相反的顺序检查响应：
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```go
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func main() {
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adder := startServer(func(a, b int) int { return a + b })
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const N = 100
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var reqs [N]Request
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for i := 0; i < N; i++ {
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req := &reqs[i]
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req.a = i
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req.b = i + N
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req.replyc = make(chan int)
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adder <- req // adder is a channel of requests
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}
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// checks:
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for i := N - 1; i >= 0; i-- {
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// doesn’t matter what order
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if <-reqs[i].replyc != N+2*i {
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fmt.Println(“fail at”, i)
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} else {
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fmt.Println(“Request “, i, “is ok!”)
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}
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}
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fmt.Println(“done”)
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}
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```
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这些代码可以在[multiplex_server.go](examples/chapter_14/multiplex_server.go)找到
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输出：
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Request 99 is ok!
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Request 98 is ok!
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...
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Request 1 is ok!
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Request 0 is ok!
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done
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这个程序仅启动了100个协程。然而即使执行100,000个协程我们也能在数秒内看到它完成。这说明了Go的协程是如何的轻量：如果我们启动相同数量的真实的线程，程序早就崩溃了。
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示例： [14.14-multiplex_server.go](examples/chapter_14/multiplex_server.go)
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```go
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package main
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import "fmt"
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type Request struct {
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a, b int
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replyc chan int // reply channel inside the Request
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}
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type binOp func(a, b int) int
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func run(op binOp, req *Request) {
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req.replyc <- op(req.a, req.b)
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}
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func server(op binOp, service chan *Request) {
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for {
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req := <-service // requests arrive here
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// start goroutine for request:
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go run(op, req) // don't wait for op
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}
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}
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func startServer(op binOp) chan *Request {
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reqChan := make(chan *Request)
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go server(op, reqChan)
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return reqChan
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}
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func main() {
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adder := startServer(func(a, b int) int { return a + b })
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const N = 100
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var reqs [N]Request
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for i := 0; i < N; i++ {
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req := &reqs[i]
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req.a = i
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req.b = i + N
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req.replyc = make(chan int)
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adder <- req
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}
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// checks:
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for i := N - 1; i >= 0; i-- { // doesn't matter what order
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if <-reqs[i].replyc != N+2*i {
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fmt.Println("fail at", i)
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} else {
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fmt.Println("Request ", i, " is ok!")
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}
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}
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fmt.Println("done")
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}
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```
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## 14.10.2 卸载（Teardown）：通过信号通道关闭服务器
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在上一个版本中`server`在`main`函数返回后并没有完全关闭，而被强制结束了。为了改进这一点，我们可以提供一个退出通道给`server`：
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```go
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func startServer(op binOp) (service chan *Request, quit chan bool) {
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service = make(chan *Request)
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quit = make(chan bool)
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go server(op, service, quit)
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return service, quit
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}
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```
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`server`函数现在则使用`select`在`service`通道和`quit`通道之间做出选择：
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```go
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func server(op binOp, service chan *request, quit chan bool) {
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for {
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select {
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case req := <-service:
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go run(op, req)
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case <-quit:
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return
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}
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}
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}
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```
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当`quit`通道接收到一个`true`值时，`server`就会返回并结束。
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在`main`函数中我们做出如下更改：
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adder, quit := startServer(func(a, b int) int { return a + b })
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在`main`函数的结尾处我们放入这一行：`quit <- true`
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完整的代码在 multiplex_server2.go,输出和上一个版本是一样的。
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示例： [14.15-multiplex_server2.go](examples/chapter_14/multiplex_server2.go)
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```go
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package main
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import "fmt"
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type Request struct {
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a, b int
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replyc chan int // reply channel inside the Request
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}
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type binOp func(a, b int) int
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func run(op binOp, req *Request) {
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req.replyc <- op(req.a, req.b)
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}
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func server(op binOp, service chan *Request, quit chan bool) {
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for {
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select {
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case req := <-service:
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go run(op, req)
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case <-quit:
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return
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}
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}
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}
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func startServer(op binOp) (service chan *Request, quit chan bool) {
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service = make(chan *Request)
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quit = make(chan bool)
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go server(op, service, quit)
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return service, quit
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}
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func main() {
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adder, quit := startServer(func(a, b int) int { return a + b })
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const N = 100
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var reqs [N]Request
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for i := 0; i < N; i++ {
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req := &reqs[i]
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req.a = i
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req.b = i + N
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req.replyc = make(chan int)
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adder <- req
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}
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// checks:
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for i := N - 1; i >= 0; i-- { // doesn't matter what order
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if <-reqs[i].replyc != N+2*i {
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fmt.Println("fail at", i)
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} else {
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fmt.Println("Request ", i, " is ok!")
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}
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}
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quit <- true
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fmt.Println("done")
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}
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```
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练习 [14.13 multiplex_server3.go](exercises/chapter_14/multiplex_server3.go):使用之前的例子，编写一个在`Request`结构上带有`String()`方法的版本，它能决定服务器如何输出；并使用以下两个请求来测试这个程序：
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```go
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req1 := &Request{3, 4, make(chan int)}
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req2 := &Request{150, 250, make(chan int)}
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...
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// show the output
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fmt.Println(req1,"\n",req2)
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```
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## 链接
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- [目录](directory.md)
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- 上一节：[实现 Futures 模式](14.9.md)
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- 下一节：[限制同时处理的请求数](14.11.md)
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