Book Review: ActivityPub by Evan Prodromou

As part of my grant from NLnet to improve my Fediverse bot project, I'm spending time making sure I understand all the fundamentals of ActivityPub. The problem with Internet standards is they're scattered all over the place. Abandoned forums, half-maintained wikis, mailing lists with obscure rules, and monolithic blocks of text.

Sometimes you just want one book which collates all the info and presents it in a consistent format. This is nearly that book.

Evan Prodromou has the unenviable task of making the whole ecosystem easy to understand. Thankfully he does that well. Things are logically laid out, there are comprehensive descriptions of even the most obscure parts of the spec, and it builds up nicely from the fundamentals. Oh, it's also surprisingly light-hearted.

This principle is so important and long-winded that web architects have given it an acronym, HATEOAS. (We often pronounce it “HATE-ee-OH-us,” which sounds like a breakfast cereal nobody wants to eat. This is one reason web architects aren’t allowed to name breakfast cereals.)

There's an excellent checklist for all the steps needed when building a minimal ActivityPub server.

As with all O'Reilly books, it is a beautifully typeset ePub. Even better, it was supplied DRM-free through Kobo.

About the only significant thing missing is details of how to verify RFC 9421 HTTP Signatures. That's understandable as they're pretty new, but a bit annoying as that's what a lot of servers are sending now. Similarly, there's a good write up of how to publish a poll, but not much about voting or publishing the results.

The "Far Horizons" chapter is particularly exciting - giving a speculative overview of what AP could be used for. I, for one, am particularly looking forward to putting my Internet Connected Fridge on social media 😆

Ultimately ActivityPub is a living and evolving set of standards. No book can possibly keep up with all the changes happening to it - but Evan does a brilliant job of bringing together all the moving parts and creating a coherent picture of the standard.

Highly recommended if you're interested in understanding the fundamentals of the Fediverse!