RE: Persistent storage / forwarding of events

Hi Jens,

Your investigation sounds like you're looking at mod-pubsub for the
right kinds of things.

Answers to your questions:

> Do topics have the capability to store-and-forward events?

Not really.  It is one of the features we were thinking of adding to
the client libraries so that, for example,

  pubsub client -> pubsub server -> pubsub client -> pubsub server -> pubsub client

multihops could use a store-and-forward paradigm at each of the client hops,
with servers storing as little as possible so they act more like routers and
less like databases.  If this is something you're interested in working on,
I say go for it. 

> If I set up a route from a source topic to my 'journal' topic, and then 
> disconnect, will the journal queue up the incoming events as a POP 
> server would, letting me reconnect later and receive the events I've 
> received in the interim?

No.  There is partial support for this but it doesn't quite work at
the moment.  Want to work on this feature?  I hear there's plenty of
room to run for anyone who wants to work on the server... :)

> Can topics store their events persistently so they can be retrieved 
> later by clients that connect to them?

Yes.

> For example, a topic might implement a newsgroup, and a new subscriber
> would want to be able to download the existing posts, or at least the
> last hundred or so.

Yes, this is a subscribe option using do_max_age or do_max_n; for example,

  kn_subscribe(conf_topic, onConfigure, {do_max_age: "infinity"});

would get all unexpired events coming out of conf_topic and send them to
the onConfigure() callback function.  Another example:

  kn.subscribe(rss_topic, rss_handleEvent, { do_max_n: 10 });

would get the last 10 events coming out of the rss_topic and send them to
the rss_handleEvent() callback function.

> If this isn't built-in, would it be straightforward to build a solution 
> on top of mod-pubsub, or would this be a bad design choice, attempting 
> to force it to do something it just wasn't meant to?

I think it's one of the features we use in apps a lot.  I'm not sure how
good of a design choice it was, though, since it forces the PubSub Server
to have some database-like functionality.  There are some among us who would
advocate using the PubSub Server as just a router and let the database store
any data you might later want to go back and query for.

   Adam


-----Original Message-----
From: Jens Alfke [mailto:[email protected]]
Sent: Thursday, October 16, 2003 9:11 AM
To: [email protected]
Subject: [Mod-pubsub-developer] Persistent storage / forwarding of
events


First questions. I should preface this by saying that I'm mostly 
investigating using mod-pubsub for asynchronous messaging, things like 
email or newsgroups or blogs where a recipient may not be online at the 
time a message is sent. So:

(1) Do topics have the capability to store-and-forward events? If I set 
up a route from a source topic to my 'journal' topic, and then 
disconnect, will the journal queue up the incoming events as a POP 
server would, letting me reconnect later and receive the events I've 
received in the interim?

(2) Can topics store their events persistently so they can be retrieved 
later by clients that connect to them? For example, a topic might 
implement a newsgroup, and a new subscriber would want to be able to 
download the existing posts, or at least the last hundred or so.

If this isn't built-in, would it be straightforward to build a solution 
on top of mod-pubsub, or would this be a bad design choice, attempting 
to force it to do something it just wasn't meant to?

--Jens



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