Joe’s Blog!

In an attempt to do something other than XML parsing (here and here) in Erlang this week I started playing around with Erlang processes and message passing. It’s pretty cool and fairly intuitive. I wrote two small programs one a receiver and the other a sender. The receive runs as a registered PID and will recieve a message and depending on what it gets will perform an operation. The sender is a basic API that sends messages to a registered PID on a host you specify. Pretty basic but a good practice. Here’s my code:

message_receiver.erl

-module(message_receiver).
-export([run/0, start/0]).

start() ->
P = spawn(message_receiver, run, []),
register(msgpr, P).

run() ->
receive
{mesg, Content} ->
io:format(“Message Received ~p~n”, [Content]),
run();
{add, Value1, Value2} ->
Total = Value1 + Value2,
io:format(“Total: ~p~n”, [Total]),
run();
{file, Data} ->
io:format(“Output: ~p~n”, [Data]),
run();
stop ->
io:format(“Stopped ~n”);
Error ->
Error
end.

So when message_passer:start(). is run this code will spawn and register a PID and wait for connections. Within the receive block it will perform the different operations, such as print the message, add two values and print the result or etc.

message_sender.erl

-module(message_sender).
-export([message/2, add/3, stop/1, file/2]).

message(Host, Content) ->
{msgpr, Host} ! {mesg, Content}.

add(Host, Value1, Value2) ->
{msgpr, Host} ! {add, Value1, Value2}.

file(Host, Filename) ->
{ok, Data} = file:read_file(Filename),
file:close(Filename),
{msgpr, Host} ! {file, Data}.

stop(Host) ->
{msgpr, Host} ! stop.

The sender code will send messages to the PID, for instance running message_sender:file(HOSTNAME, FILENAME). will read in the specified file and send the message containing what is in the file. message_sender:file(HOSTNAME, NUMBER, NUMBER1) will send both numbers to the receiver and the receiver will add them together.

Pretty fun stuff, it’s obvious how Erlang could be used as a powerful tool in distributed computing. More to come at the next (and possibly final) ErloungeSTL.

There has been too much to do lately to blog. I have been getting the wedding stuff together, moving and searching for a job in Seattle on top of work and the usual. I can usually find time to throw up some tweets, so hit twitter for the play by play.

The last couple days I have been a bit distracted from the Erlang stuff I have been doing lately and ended up some how playing with Ruby and the SSH library. For running commands on a bunch of machines at once it would work really well. Here’s some code I wrote and paraphrased from various sources.

require ‘rubygems’
require ‘net/ssh’

username=”yourusername”
hostnames=["node01","node02"]
script=”date;uptime;”

hostnames.each {|hostname|
Net::SSH.start( hostname, username ) do |session|
session.open_channel do |channel|
channel.on_data { |chan,output| puts “#{output.inspect}” }
channel.on_extended_data { |chan,type,output| print output }
channel.exec script
end
session.loop
end
}

This will run the commands contained in the script variable on the hosts in the hostnames array as the specified user. As it is currently it does not supply a password, so you’ll need keys setup. Adding your password is pretty simple, just check out the API here.

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