Suggested Example

Steve Elkins <>
Tue Oct 10 05:15:32 CEST 2000

Ulf Wiger wrote:
> On Fri, 6 Oct 2000, Jeffrey Straszhiem wrote:
> >difficulty figuring out the OTP framework for applications,
> >supervisors, gen_server's and such.
> I agree that this is a problem.
> >I understand the difficulty of putting a tutorial like this
> >together, and don't expect to see anything on that order soon.
> >However, it would be nice to have a simple, but fairly complete,
> >Erlang application to look at -- and by that I mean a skeleton of
> >one.  I know I could look at Mnesia or something, but I'd rather
> >focus all of my meager understanding on figuring out the OTP
> >structures instead of the specifics of the database.

Ok, I've figured out most of these issues and have attached a tgz
containing 2 embarrassingly simple apps, as well as a perl script
from the Cookbook that exercises one of them.

One app, dshb, does nothing at all interesting.  Skeletal. ;-)
The other, cosb, is slightly more complicated.  It has a special
process that listens on a UDP port and notifies a gen_event example
when it receives something.  The event handler replies.  Simple.

To play with it...

1.  tar zxvf ctii.tgz
2.  cd ctii
3.  erl -sname xyz -boot start_sasl -pa ./cosb/ebin -pa ./dshb/ebin
4.  application:start(cosb).
5.  udpmsg localhost hi
6.  if all's well, udpmsg should say it received "you said hi"
7.  udp_spc.erl is a standalone echoer, straight from the Special
    Processes chapter of Design Principles.

...assuming everyone can infer each step's context.

Summing up, dshb is a very skeletal app, cosb has a little meat, and 
udp_spc is the forerunner of cosb.  Supplemental reading would be
chapters 5, 7, and 8 of Design Principles, as well as the relevant
man pages.  And sys:trace/2 is wonderful.

Great, so I can build *local* apps.

What's busting *my* chops is getting these apps to run distributed.  I
could really use something as simple as what I've provided here, but
distributed.  The particular place I'm stuck right now is getting a
proper .boot together, since various messages in the archives have led
me to think I can't just do something like step 4 above and hope for
distribution, takeover, etc.  systools:script2boot/1 says ok, but when
I try...

sge:219$ erl -boot start_cosb
{'init terminating in do_boot',{'can not load',erlang,get_file}}
init terminating in do_boot ()

I even ripped out all my stuff so my .script looks like the start_sasl
one, but still the error.  Is it where I'm doing script2boot?

<sigh>  I realize this is only the 1st hurdle.  Any advice at all on 
the general subject of building distributed apps would be greatly 

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#!/usr/bin/perl -w
# udpmsg - send a message to the udpquotd server

use IO::Socket;
use strict;

my($sock, $server_host, $msg, $port, $ipaddr, $hishost, 

$MAXLEN  = 1024;
$PORTNO  = 5151;

$server_host = shift;
$msg         = "@ARGV";
$sock = IO::Socket::INET->new(Proto     => 'udp',
                              PeerPort  => $PORTNO,
                              PeerAddr  => $server_host)
    or die "Creating socket: $!\n";
$sock->send($msg) or die "send: $!";

eval {
    local $SIG{ALRM} = sub { die "alarm time out" };
    alarm $TIMEOUT;
    $sock->recv($msg, $MAXLEN)      or die "recv: $!";
    alarm 0;
    1;  # return value from eval on normalcy
} or die "recv from $server_host timed out after $TIMEOUT seconds.\n";

($port, $ipaddr) = sockaddr_in($sock->peername);
$hishost = gethostbyaddr($ipaddr, AF_INET);
print "Server $hishost responded ``$msg''\n";

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