Looking for advice/opinions

[Eddie Corns <eddie@holyrood.ed.ac.uk>]

Hi all.  For quite some time now I've been wanting to add SNMP functionality to
scheme.  It seems like an ideal time to switch to DrScheme as well.  This may
be over ambitious at this stage as I've only actually written about 4 scheme
programs but I'd  like to give it a bash anyway, this way I can learn scheme
and actually do what I'm paid for for a change - though my employers object to
me using obscure languages :(

When I say SNMP functionality I mean the complete works which means:
a) handling SNMP requests asynchronously, including ...
b) incoming unsolicited notifications (pka traps)
c) conforming to the API described in RFC2271 etc.

The third point really only dictates which C libraries I can use.  The async
handling is a requirement, SNMP applications simply do not scale without it.

I realise there would be a place for threads here but at the same time it seems
like it might be expensive to invoke a thread for each individual request,
especially as it is not uncommon to poll hundreds or even thousands of devices
in one go.  It seems natural to me (because I've used them before probably) to
use callbacks. So (simplifying a bit) I would attach a 2 arg procedure to the
outgoing request (to pass back the return value and success code) to be called
after the response arrives (detected using SIGIO).  Rather than trying to
handle callbacks being invoked at an arbitrary time I think it might be easier
to have a wait procedure that suspends the thread, handles all responses that
have arrived and that continue to arrive until there's none left outstanding
and then return.

Does that make sense?, perhaps a simplified example of what I think it might
look like would help:

(define (get-router-uptimes rtrs password)
  (let* ((res ())
	 (got-resp (lambda (r v) (append! res (list r v))))
	 (reqids (map
		  (lambda (rtr) (snmp-poll rtr "sysUptime.0" password
					   (lambda (v err) (got-resp rtr v))))
		  rtrs)))
    (snmp-wait reqids)
    res))

Where snmp-poll issues a request and snmp-wait invokes the callbacks until all
responses have arrived then returns.

Does this seem a reasonable approach or is there a more schemey one?

Does anyone have pointers to anything similar I can learn from/steal?

Can I achieve this without getting involved in the internals of mzscheme?
Making it an add-on would be better.

Is there a generic way of doing this that I've missed?  I guess the GUI might
be doing something similar but I don't know anything about the GUI yet.

Can I use SIGIO without affecting the normal operation of mzscheme?

(Does SIGIO actually work? nobody seems keen to use it.  Actually I'm still
investigating alternatives here such as aio.)

A pragmatic issue - the leading contender for the actual SNMP library uses glib
(the gtk/gnome C library) is anyone aware of interoperability problems with
mzscheme? I've removed the code which depends on the glib event loop (I
hope).  Initial experiments were encouraging.

Eddie