[erlang-questions] Newbie training project proposal

Joe Armstrong <>
Tue Sep 11 09:41:39 CEST 2007


Exercise 0)

Divide yourself into groups to solve 1 2 3 ...

Exercise 1)

make an Erlang equivalent of all the terms in the STOMP protocol

For example:

SEND
destination: /queue/a
transaction: tx1

hello queue a!
^@


is represented by {send,"/queue/a","tx1","hello queue a!"}.

Write down the Erlang equivalents of all the messages in the protocol.

Write the functions

erl2stop(Term) and stomp2erl(String)

That converts between the two forms of the message.

Exercise 2)

Write a function is_complete_stomp_message(Str) -> Bool
that figures out if we have received an entire STOMP message

Exercise 3)

Write documentation - define the directory structure of your project
the names of the major modules, who writes what.

Exercise 4)

Write the socket stuff to glue 1 and 2 together (hint - read the Erlang book)

Exercise 5)

Think about how to implement transactions
(I won't tell you)

Cheers

/Joe



On 9/11/07, Joe Armstrong <> wrote:
> This project (and actually the way of working) sounds like a
> good way to learn Erlang. If you like I can act as a "mentor" and
> give you some advice with your code now and then.
>
> I'll start with a little rant about the architecture of STOMP :-)
>
> Here beginneth the lesson
>
> One fun thing might be to *extend* the protocol with a layer that
> allows the end-points to
> negociate the transport format.
>
> Here's the idea:
>
> Client -> Server
> HELLO
> ICanSpeak: Stomp,erlangTerms
>
> Server->Client:
> HELLO_RESPONSE
> erlangTerms
>
> Thereafter the two sides could communicate using serialised Erlang terms
> (made by term_to_binary(Term)) and its inverse.
>
> Then you just use the lib_chan in appendix D of the Erlang book (shameless plug)
>
> <flame on>
>
> Do protocol designers turn off their brains when they design protocols?
>
> I see a lot of the following:
>
>    - text protocols with are "easy" to understand and parse
>      (false - most text protocols are inadequately specified)
>   - XML protocols
>     (crazy - did anybody think how long it would take to parse this crap)
>
> The two most obvious methods for building protocols are:
>
>    - define a packed bit-byte-word structure appropriate to you needs
>    - use serialised lisp S expressions, or Erlang terms (whatever)
>
> Are rarely used.
>
> A pure Erlang version of Stomp would look like this:
>
>     Server ! {connect, user, PassWord},
>     receive
>           {Server, {connected, SessionId}} ->
>                 ...
>     end.
>
> Now when you write your code you should use the technique of a "middle man"
> this is a process that converts Erlang terms to the format of data you
> will see "on the wire"
>
> If you send the message {connect, User, Pass} to the middle man
> it will send a
>
>    CONNECT
>    login: <username>
>    passcode: <passcode>
>
> message to the socket
>
> Something like
>
>       middle_man(SourcePid, DestSocket) ->
>               receive
>                       {SourcePid, {connect, User, Pass}} ->
>                               DestSocket ! [<<"CONNECT\nlogin:">>,User,
>
> <<"\npasscode:">>,Pass,["\^@"],
>                               middle_man(SourePid, DestSocket);
>                        ...
>
> This method (by extension) gives you a modular way to swap out the code in the
> back-end (you could use erlang terms, xml, or even STOMP :-)
>
> </flame>
>
> Here endeth the lesson
>
> Secondly documentation - code is NOT documentation - never was never will be.
>
> Code is what the machine does - documentation is what it is *supposed* to do.
>
> In a well written system the code does what the documentation says it
> is supposed to do.
>
> In a badly written system there is no documentation and we have no
> idea what the code is supposed to do since we only know what it
> actually does - then we have to guess
> what it was supposed to do.
>
> Have fun
>
> /Joe Armstrong
>
>
> On 9/11/07, Brian McCallister <> wrote:
> >
> > On Sep 10, 2007, at 2:18 AM, Tomasz Błachowicz wrote:
> >
> > > Hi all,
> > >
> > > I'm looking for wannabe/newbie erlang developers who'd like to learn
> > > that language in practice. I'm going to create new open source project
> > > from scratch that will aim to implement STOMP messaging protocol [1]
> > > in erlang. I've been considering both client and server side
> > > implementation.
> >
> > I'd like to jump in too. Am an okay general programmer, but quite new
> > to erlang.
> >
> > -Brian
> > _______________________________________________
> > erlang-questions mailing list
> > 
> > http://www.erlang.org/mailman/listinfo/erlang-questions
>


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