PG-MQ?

I'm seeing some applications where it appears that there would be
value in introducing asynchronous messaging, ala "message queueing."
<http://en.wikipedia.org/wiki/Message_queue>

The "granddaddy" of message queuing systems is IBM's MQ-Series, and I
don't see particular value in replicating its functionality.

On the other side, the "big names" these days are:

a) The Java Messaging Service, which seems to implement *way* more
options than I'm even vaguely interested in having (notably, lots
that involve data stores or lack thereof that I do not care to use);

b) AMPQ
<http://en.wikipedia.org/wiki/Advanced_Message_Queuing_Protocol> which
again is a big, complex, protocol-oriented thing that seems mighty big
and complex;

c) There are lesser names, like isectd <http://isectd.sf.net> and the
(infamous?) Spread Toolkit which both implement memory-based messaging
systems.

There's a simple + free system called MQS that is implemented in Perl
and uses XML-RPC; <http://www.foo.be/mqs/> that seems perhaps a little
*too* primitive.

FYI, here's its simple schema (filtered into PG):

create table mqname (
name varchar (255) not null default '',
id serial,
priority integer not null default 10,
primary key (id));

create table message (
id serial,
timestamp timestamptz NOT NULL
cid varchar(255),
queue integer not null references mqname(id),
body text not null,
priority integer not null default 10,
flag integer default 0,
primary key(id)
);

My bias would be to have something that can basically run as a thin
set of stored procedures atop PostgreSQL :-). It would be trivial to
extend that to support SOAP/XML-RPC, if desired.

It would be nice to achieve 'higher availability' by having queues
where you might replicate the contents (probably using the MQ system
itself ;-)) to other servers.

There tend to be varying semantics out there:

- Some queues may represent "subscriptions" where a whole bunch of
listeners want to get all the messages;

- Sometimes you have the semantics where:
- messages need to be delivered at least once
- messages need to be delivered no more than once
- messages need to be delivered exactly once

Is there any existing work out there on this? Or should I maybe be
looking at prototyping something?
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