package main

import (
    "context"
    "fmt"
    "log"
    "net/http"
    "os"
    "os/signal"
    "syscall"
    "time"
)

func main() {
    // Hello world, the web server
    helloHandler := func(w http.ResponseWriter, req *http.Request) {
        fmt.Fprintf(w, "You requessted %s - %s", req.URL, req.Method)
        fmt.Println("Serving requests from hey...")
        time.Sleep(time.Second * 2)
        defer fmt.Println("remaining")  // acts as remaining requests 
    }
    api := &http.Server{
        Addr:           ":8080",
        Handler:        http.HandlerFunc(helloHandler),
        ReadTimeout:    10 * time.Second,
        WriteTimeout:   10 * time.Second,
        MaxHeaderBytes: 1 << 20,
    }

    serverErrors := make(chan error, 1)
    shutdown := make(chan os.Signal, 1)
    go func() {
        log.Printf("main api listening on %s", api.Addr)
        serverErrors <- api.ListenAndServe()
    }()

    signal.Notify(shutdown, os.Interrupt, syscall.SIGTERM)
    //above is not blocking operation although waiting on shutdown channel
    select {
    case err := <-serverErrors:
        log.Fatalf("Error while listening and starting http server: %v", err)

    case <-shutdown:
        log.Println("main: Starting shutdown")
        const timeout = 5 * time.Second
        // Context - it is used for Cancellation and propagation, the context.Background() gives empty context
        ctx, cancel := context.WithTimeout(context.Background(), timeout)
        defer cancel()
        err := api.Shutdown(ctx)
        /*Shutdown gracefully shuts down the server without interrupting any active connections. Shutdown works by first closing all open listeners, then closing all idle connections, and then waiting indefinitely for connections to return to idle and then shut down.If the provided context expires before the shutdown is complete, Shutdown returns the context's error, otherwise it returns any error returned from closing the Server's underlying Listener(s).*/
        if err != nil {
            log.Printf("main: Graceful shutdown didnot complete in %v:%v", timeout, err)
            err = api.Close()
            //Close() immediately closes all active net.Listeners and any connections in state StateNew, StateActive, or StateIdle. For a graceful shutdown, use Shutdown.
        }
        if err != nil {
            log.Fatalf("main: could not stop server gracefully Error: %v", err)
        }
    }
}