GOLANG1.7新增了context,最初这个package是在golang.org/x/net/context
中的,后来证明对不少程序都是必须的,就归入了标准库。golang
对context的介绍是在context,读这个blog以前,要先读pipelines(pipelines提出了使用close(chan)
的方式广播退出事件)。web
通常来讲,context是用在request的处理,例如http请求的处理中,可能会开启多个goroutine,好比:c#
http.HandleFunc(func(w http.ResponseWriter, r *http.Request){
wg := sync.WaitGroup{}
wg.Add(2)
var result0 string
go func(){
defer wg.Done()
res0,err := http.Get("http://server0/api0")
// Parse res0 to result0.
}()
var result1 string
go func(){
defer wg.Done()
res1,err := http.Get("http://server1/api1")
// Parse res1 to result1.
}()
wg.Wait()
w.Write([]byte(result0 + result1)
})
实际上这个程序是不能这么写的,若是这两个goroutine请求的时间比较长,让用户一直等着么?若是用户取消了请求,关闭了链接呢?若是用户指定了超时时间呢?api
另外,考虑如何关闭一个http服务器,好比须要关闭listener后从新侦听一个新的端口:服务器
var server *http.Server
go func() {
server = &http.Server{Addr: addr, Handler: nil}
if err = server.ListenAndServe(); err != nil {
ol.E(nil, "API serve failed, err is", err)
return
}
}()
咱们如何确保goroutine已经退出后,而后才返回从新开启服务器?若是只是server.Close
(或者GO1.8以前用listener.Close
),如何接收外部的事件?若是是goroutine本身的问题,例如端口占用了,如何通知程序退出?直接用os.Exit
明显是太粗鲁的作法。ide
在context中,有一段话很是关键:svg
A Context does not have a Cancel method for the same reason the
Done channel is receive-only: the function receiving a
cancelation signal is usually not the one that sends the signal.
In particular, when a parent operation starts goroutines for sub-
operations, those sub-operations should not be able to cancel the
parent. Instead, the WithCancel function (described below)
provides a way to cancel a new Context value.
也就是说,context没有提供Cancel方法,由于parent goroutine会调用Cancel,在全部sub goroutines中只须要读context.Done()
就能够,也就是只是接收退出信号。ui
还有一处地方,说明了Context应该放在参数中:spa
Server frameworks that want to build on Context should provide
implementations of Context to bridge between their packages and
those that expect a Context parameter. Their client libraries
would then accept a Context from the calling code. By
establishing a common interface for request-scoped data and
cancelation, Context makes it easier for package developers to
share code for creating scalable services.
只读取chan的好处是,可使用close(chan)
方式通知全部goroutine退出。scala
使用context和WaitGroup,同步和取消服务器的例子:
func HTTPAPIServe(ctx context.Context) {
http.HandleFunc("/api", func(w http.ResponseWriter, r *http.Request) {
})
ctx,cancel := context.WithCancel(ctx)
wg := &sync.WaitGroup{}
defer wg.Wait()
wg.Add(1)
var server *http.Server
go func(ctx context.Context) {
defer wg.Done()
defer cancel()
server = &http.Server{Addr: addr, Handler: nil}
_ = server.ListenAndServe()
}(ctx)
select {
case <-ctx.Done():
server.Close()
}
}
wg := sync.WaitGroup{}
defer wg.Wait()
ctx, cancel := context.WithCancel(context.Background())
wg.Add(1)
go func(ctx context.Context) {
defer wg.Done()
defer cancel()
HTTPAPIServe(ctx)
}(ctx)
wg.Add(1)
go func(ctx context.Context) {
defer wg.Done()
defer cancel()
// Other server, such as:
// UDPServer(ctx)
}(ctx)
这个程序实际上包含了几条通道:
特别是对于library,在参数中支持context,是很是重要的一个要素,这样能够收取到user要求退出或cancel的信号。