I started watching the Salmon Protocol (you guys need a mailing list!) a while back when I noticed a few posts about it on Twitter.
In relation to the issues you raised, subscriptions can be given a limited time to live, but the issue is in who defines the TTL - some Hubs may assume unlimited TTLs for all subscriptions so it can become important to make sure Subscribers are passing a realistic lease_seconds value. I'm not sure that Hubs can manage this any other way, other than to periodically purge inactive feeds through some garbage collection tasks - such a task is undefined in the specification but maybe automatic re-subscriptions will ease in the behaviour most suitable for comment feeds. It still needs Subscriber support for the most part in being intelligent about their lease period. It's really a very messy area. Can Hubs filter subscriptions? Eliminate subscriptions to feeds without updates within X months, etc? As someone on another topic noted, Subscribers do need to pay attention to their subscriptions and manage them. I think this is a good practice so Hubs are not unduly burdened with tracking subscriptions to dead or inactive feeds, or feeds with a limited useful lifetime (in terms of activity not content). On Hubs changing the source feed - most probably won't. It costs little to maintain an entry as-is for distribution. Some Hubs may basically condense the feed changes to a narrow spectrum of elements - removing the rest. It's not really specified so it's up to Publishers to select appropriate Hubs. You also can't rely on Subscribers tracking the different behaviour of individual Hubs. The PHP Hub I'm writing at the moment won't perform any filtering since doing so potentially robs feeds of information that looks unnecessary in one use case, but becomes essential in another (e.g. Salmon). Paddy Pádraic Brady http://blog.astrumfutura.com http://www.survivethedeepend.com OpenID Europe Foundation Irish Representative ________________________________ From: John Panzer <[email protected]> To: [email protected] Sent: Fri, October 23, 2009 7:30:34 PM Subject: [pubsubhubbub] Salmon Protocol The Salmon Protocol (http://salmon-protocol.org) leverages PubSubHubbub and web hooks to build a real time, decentralized commenting and annotation system. The basic idea is that commentary swims upstream to the thing being commented on, which can then redistribute comments back out via PubSubHubbub to interested subscribers. There's a demo available at http://salmon-playground.appspot.com/ros which posts comments back to a Blogger blog (by proxying to an existing API, just for demo purposes). Aside from evangelizing Salmon, I'm also interested in getting feedback on the use of PubSubHubbub. One specific question: At the moment, Salmon specifies that subscribers should follow the rel="comment" link for each entry in an Atom feed to find the comment feed to subscribe to. This could lead to a lot of subscriptions that have initial activity but then die out; not sure if this is a problem for PubSubHubbub or not. It might be better to define a feed for, and there fore a way to subscribe to, "all comments for items in this feed" as a whole -- but there's no standard way to do this at the moment. Also, Salmon definitely needs some extensions to get passed through un-modified (like crosspost:source, thr:in-reply-to, etc.) so the discussion about what hubs may/may not change is very relevant. Comments, critiques, feedback of all kinds welcomed. -- John Panzer / Google [email protected] / abstractioneer.org / @jpanzer
