Go by Example: Context

In the previous example we looked at setting up a simple HTTP server. HTTP servers are useful for demonstrating the usage of `context.Context` for controlling cancellation.	package main
	import ( "fmt" "net/http" "time" )
	func hello(w http.ResponseWriter, req *http.Request) {
A `context.Context` is created for each request by the `net/http` machinery, and is available with the `Context()` method.	ctx := req.Context() fmt.Println("server: hello handler started") defer fmt.Println("server: hello handler ended")
Wait for 3 seconds before sending a reply to the client. This could simulate some work the server is doing. While working, keep an eye on the context’s `Done()` channel for a signal that we should cancel the work and return as soon as possible.	select { case <-time.After(10 * time.Second): fmt.Fprintf(w, "hello\n") case <-ctx.Done(): err := ctx.Err() fmt.Println("server:", err) internalError := http.StatusInternalServerError http.Error(w, err.Error(), internalError) } }
	func main() {
As before, we register our handler on the “/hello” route, and start serving.	http.HandleFunc("/hello", hello) http.ListenAndServe(":8090", nil) }

Run the server in the background.	$ go run context-in-http-servers.go &
Simulate a client request to `/hello`, hitting Ctrl+C shortly after starting to signal cancellation.	$ curl localhost:8090/hello server: hello handler started ^C server: context canceled server: hello handler ended