Federated access control with Atom and WebSub

I’ve been ranting about ActivityPub vs. RSS/Atom a lot lately, and I think I’ve proven to myself (and maybe a few others) that for fully-public content feeds, Atom (combined with WebSub and WebMention) is superior to ActivityPub; it’s simpler to implement, works with many more hosting environments and configurations, it generally scales better (and handles scaling failures better), and it’s modular and allows for much eaiser migrations between hosting setups and so on.

But one thing ActivityPub supports which Atom does not is the notion of private content. The way it does support this is a bit hamfisted (in that ActivityPub publishers choose to only push content to endpoints which have a trusted user, and endpoints only forward that content over to the trusted users, albeit in a not-very-trustable way). It doesn’t inherently support the ability to backfill older content (or make it otherwise browseable) to someone who is granted friends-only access after-the-fact, though, and it has many scaling and security implications in how this works (since it requires push to be reliable and requires the recipient’s storage of said push notifications to also be reliable).

I’ve put a lot of thought into how to add friends-only stuff to Atom on and off over the years; my previous blog (which used Movable Type for publishing and phpBB for comments) actually had an ad-hoc implementation which worked sort of okay; people could authenticate with my site’s forum, and people in a trusted friend group would see private content. On the public feed, if their reader were logged into the forum (via cookie sharing etc.) it would see the private content in the feed, otherwise it would see placeholders saying “THis is a friends-only entry, please visit the site to read it.” It worked okay but it was never great.

Anyway, I think I have finally come up with an auth approach that works with Atom and offers a… well, least-bad solution all around, which scales better and more reliably than ActivityPub while working with WebSub and existing/legacy feed readers.

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Some WebSub-Atom observations

As part of testing my WebSub changes for FoF, I decided to switch to a WebSub hub for myself that provides some subscriber analytics and so on. One neat thing about how WebSub works is that the “hub” layer is completely modular and it really doesn’t matter at all which one you use, and if the one you use has problems you can switch to another one just by changing the URL in your feed and all subscribers will eventually seamlessly migrate (at their next normal polling interval); if anyone even notices a problem it will just be that they don’t receive a push update during that polling interval. (Which, let’s be honest, is incredibly unlikely for most RSS feeds.)

Anyway, because of these new analytics, as well as information I gathered from my new WebSub-supporting reader, I now know a bit more about the state of WebSub.

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