8.2 Example: Concurrent Clock Server
Networking is a natural domain in which to use concurrency since
servers typically handle many connections from their clients at once,
each client being essentially independent of the others.
In this section, we’ll introduce the net package, which
provides the components for building networked client and server
programs that communicate over TCP, UDP, or Unix domain sockets.
The net/http package we’ve been using since Chapter 1 is built on top of functions from the
net package.
Our first example is a sequential clock server that writes the current time to the client once per second:
// Clock1 is a TCP server that periodically writes the time.
package main
import (
"io"
"log"
"net"
"time"
)
func main() {
listener, err := net.Listen("tcp", "localhost:8000")
if err != nil {
log.Fatal(err)
}
for {
conn, err := listener.Accept()
if err != nil {
log.Print(err) // e.g., connection aborted
continue
}
handleConn(conn) // handle one connection at a time
}
}
func handleConn(c net.Conn) {
defer c.Close()
for {
_, err := io.WriteString(c, time.Now().Format("15:04:05
"))
if err != nil {
return // e.g., client disconnected
}
time.Sleep(1 * time.Second)
}
}
The Listen function creates a net.Listener, an object
that listens for incoming connections on a network port,
in this case TCP port localhost:8000.
The listener’s Accept method blocks until an incoming connection
request is made, then returns a net.Conn object representing the connection.
The handleConn function handles one complete client connection.
In a loop, it writes the current time, time.Now(), to the client.
Since net.Conn satisfies the io.Writer interface, we can
write directly to it.
The loop ends when the write fails, most likely because the client has
disconnected, at which point handleConn closes its side of the
connection using a deferred call to Close and goes back to
waiting for another connection request.
The time.Time.Format method provides a way to format
date and time information by example.
Its argument is a template indicating how to format a reference time,
specifically Mon Jan 2 03:04:05PM 2006 UTC-0700.
The reference time has eight components (day of the week, month, day of
the month, and so on). Any collection of them can appear in the
Format string in any order and in a number of formats; the
selected components of the
date and time will be displayed in the selected formats.
Here we are just using the hour, minute, and second of the time.
The time package defines templates for many standard
time formats, such as time.RFC1123.
The same mechanism is used in reverse when parsing a time using
time.Parse.
To connect to the server, we’ll need a client program such as nc (“netcat”),
a standard utility program for manipulating network connections:
$ go build gopl.io/ch8/clock1 $ ./clock1 & $ nc localhost 8000 13:58:54 13:58:55 13:58:56 13:58:57 ^C
The client displays the time sent by the server each second
until we interrupt the client with Control-C,
which on Unix systems is echoed as ^C
by the shell.
If nc or netcat is not installed on your system,
you can use telnet or this simple Go version of netcat that
uses net.Dial to connect to a TCP server:
// Netcat1 is a read-only TCP client.
package main
import (
"io"
"log"
"net"
"os"
)
func main() {
conn, err := net.Dial("tcp", "localhost:8000")
if err != nil {
log.Fatal(err)
}
defer conn.Close()
mustCopy(os.Stdout, conn)
}
func mustCopy(dst io.Writer, src io.Reader) {
if _, err := io.Copy(dst, src); err != nil {
log.Fatal(err)
}
}
This program reads data from the connection and writes it to the
standard output until an end-of-file condition or an error occurs.
The mustCopy function is a utility used in several examples in
this section.
Let’s run two clients at the same time on different terminals,
one shown to the left and one to the right:
$ go build gopl.io/ch8/netcat1
$ ./netcat1
13:58:54 $ ./netcat1
13:58:55
13:58:56
^C
13:58:57
13:58:58
13:58:59
^C
$ killall clock1
The killall command is a Unix utility that
kills all processes with the given name.
The second client must wait until the first client is finished because
the server is sequential; it deals with only one client at a time.
Just one small change is needed to make the server concurrent: adding
the go keyword to the call to handleConn causes each
call to run in its own goroutine.
for {
conn, err := listener.Accept()
if err != nil {
log.Print(err) // e.g., connection aborted
continue
}
go handleConn(conn) // handle connections concurrently
}
Now, multiple clients can receive the time at once:
$ go build gopl.io/ch8/clock2
$ ./clock2 &
$ go build gopl.io/ch8/netcat1
$ ./netcat1
14:02:54 $ ./netcat1
14:02:55 14:02:55
14:02:56 14:02:56
14:02:57 ^C
14:02:58
14:02:59 $ ./netcat1
14:03:00 14:03:00
14:03:01 14:03:01
^C 14:03:02
^C
$ killall clock2
Exercise 8.1:
Modify clock2 to accept a port number, and
write a program, clockwall, that acts as a client of several
clock servers at once, reading the times from each one and displaying
the results in a table, akin to the wall of clocks seen in some
business offices.
If you have access to geographically distributed computers, run
instances remotely; otherwise run local instances on different ports
with fake time zones.
$ TZ=US/Eastern ./clock2 -port 8010 & $ TZ=Asia/Tokyo ./clock2 -port 8020 & $ TZ=Europe/London ./clock2 -port 8030 & $ clockwall NewYork=localhost:8010 Tokyo=localhost:8020 London=localhost:8030
Exercise 8.2:
Implement a concurrent File Transfer Protocol (FTP) server.
The server should interpret commands from each client such as
cd to change directory, ls to list a directory,
get to send the contents of a file, and close to close
the connection.
You can use the standard ftp command as the client, or write
your own.