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In the previous example we looked at setting up a simple [HTTP server](http-servers). HTTP servers are useful for demonstrating the usage of `context.Context` for controlling cancellation. A `Context` carries deadlines, cancellation signals, and other request-scoped values across API boundaries and goroutines. 
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 a few 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():

    






    
    
         The context's `Err()` method returns an error that explains why the `Done()` channel was closed.
    
		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)

    






    
    
}

    






    
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