This section provides a tutorial example on how write an XML communication server program to accept client connections, send and receive XML messages.
In the following example, I wrote a simple client and
server based game application. It's a simple game, in which the server
accepts new client connections, and holds a number for the client to guess.
The information is exchanged between the client and server in plain
XML format, and delivered with plain TCP communication protocol.
Here is the server program, GameServer.pl:
#- GameServer.pl
#- Copyright (c) 1999 by Dr. Herong Yang, http://www.herongyang.com/
#
use Socket;
use XML::Simple;
$xs = new XML::Simple(keeproot => 1, forcearray => 1);
$gameID = 0;
$number = 0;
$response = "<s><i>0</i><m>hello</m></s>";
&runServer;
exit;
sub runServer {
open(LOG,">> GameServer.log");
select(LOG); $|=1;
open(STDERR, ">&LOG") || die "Die: Setting STDERR to log file";
open(REC,">> GameServer.rec");
select(REC); $|=1;
socket(SOCK,PF_INET,SOCK_STREAM,'tcp');
bind(SOCK, pack_sockaddr_in("8080", INADDR_ANY));
listen(SOCK, 1); # can only take one client at a time
print LOG localtime().": Listening to port 8080\n";
for (;;) {
$cAddress = accept(NEWSOCK,SOCK) || die "Error: Accepting: $!";
($cPort, $cHost) = unpack_sockaddr_in($cAddress);
$cHostName = inet_ntoa($cHost);
print LOG localtime().": Connected with $cHostName at $cPort\n";
open(STDIN, "+<&NEWSOCK") || die "Die: Setting socket STDIN";
open(STDOUT, "+>&NEWSOCK") || die "Die: Setting socket STDOUT";
select(STDOUT); $|=1;
&serve;
close(STDIN);
close(STDOUT);
}
}
sub serve {
my ($tag,$fin,$msg);
while (<STDIN>) {
/<(\w+)>/ && ($tag=$1) unless $tag;
/<\/$tag>/ && ($fin=$tag) if $tag;
$msg = $msg.$_;
last if $fin;
}
print REC "$msg\n";
my $ref = $xs->XMLin($msg);
if (exists($ref->{c}->[0]->{i})) {
my $gid = $ref->{c}->[0]->{i}->[0];
my $num = $ref->{c}->[0]->{n}->[0];
if ($gid == $gameID) {
$msg = &oldGame($gid,$num);
} else {
$msg = &invalidGame($gid,$num);
}
} else {
$msg = &newGame;
}
print STDOUT $msg;
print REC "$msg\n";
}
sub oldGame {
my ($gid,$num) = @_;
my $ref = $xs->XMLin($response);
my $msg;
if ($num == $number) {
$msg = "Congratulations!\n"
."I have another number between 0 and 99 for you to guess.";
$gameID++;
$number = rand(100);
} elsif ($num > $number) {
$msg = "Your guess is too high.\n Please make another quess.";
} else {
$msg = "Your guess is too low.\n Please make another quess.";
}
$ref->{s}->[0]->{i}->[0] = $gameID;
$ref->{s}->[0]->{m}->[0] = $msg;
return $xs->XMLout($ref);
}
sub newGame {
$gameID++;
$number = rand(100);
my $ref = $xs->XMLin($response);
$ref->{s}->[0]->{m}->[0] = "Welcome to Game Server!\n"
."I have a number between 0 and 99 for you to guess.";
$ref->{s}->[0]->{i}->[0] = $gameID;
return $xs->XMLout($ref);
}
sub invalidGame {
my ($sid,$num) = @_;
my $ref = $xs->XMLin($response);
$ref->{s}->[0]->{m}->[0] = "Sorry. Your game ID doesn't exist.";
return $xs->XMLout($ref);
}
The game ID concept is used to identify if an incoming XML message
is for continuing an existing game or starting a new game. When the server
receives a client message for the first time, it will assign a new game ID,
and return the game ID to the client. The client program must use this game
ID in the subsequent messages to continue the same game.
The server is not written to serve multiple clients simultaneously.
Let's run GameServer.pl in command window, and let it listen to port 8080
for any client program connection.
Now in another command window, we can run the telnet command to test the server,
"telnet localhost 8080". Then enter the following XML message to the telnet program.
Remember telnet will not echo your input on the screen, so you have to type
in the message "blindly".
<c><m>Hi there!</m></c>
You will get the following message from the server displayed on the telnet window
<s>
<m>Welcome to Game Server!
I have a number between 0 and 99 for you to guess
.</m>
<i>2</i>
</s>
