New blog post: Understanding ActivityPub - Part 4: Threads
A first detailed look into how Threads implements ActivityPub. Learn about the data that is shared (or not), an interesting implementation of HTTP signatures, and Threads' take on quote posts in ActivityPub.
https://seb.jambor.dev/posts/understanding-activitypub-part-4-threads/
Quote posts in Threads now include a link to the ActivityPub data of the quoted posts. This makes it possible for Fediverse services like Firefish and Misskey to fetch the quoted post and show it inline in the quote post. I updated the blog post with some information on this https://seb.jambor.dev/posts/understanding-activitypub-part-4-threads/
Thanks @pcottle for the heads-up, and for some information about design choices that went into this.
@crepels Nice article! In particular, this is the best explanation I've seen yet of the "infinite signature recursion" problem, and the way around it.
I haven't tried to talk to Threads from my own toy implementation yet. I have a suspicion that the "instance actor" requirement is going to give me a headache.
@FenTiger Thanks :)
I can't really make sense of the instance actor requirement. I only tried this from ActivityPub.Academy (i.e., Mastodon); it would be interesting if there are any differences with other services. If you try this out with your implementation at some point, I'd be interested to know what you find.
@crepels @FenTiger I spent *way* too long trying to figure out how to get my library (https://github.com/jahfer/clj-activitypub) to talk to Threads. Thank you! Your post was the thing that helped me to pull an actor document.
Being stubborn, I tried to get to the bottom of why the instance actor stuff was required. Turns out...it's not! Threads just requires that the signing actor has a `url` field, which isn't common except for the instance actor. How anticlimactic 😑 I'm going to flag this to the Threads folks.
@crepels You’re right that Mastodon does specify the `url` field for all of their actors, hmm.
When you say you tried to reproduce this, did Threads reject your request using a normal actor?
I just tried again using an instance actor, plain actor with `url`, and a plain actor missing a `url`, and all of them succeeded. I’m going to guess @csu and co. fixed this already? But if you are still being rejected, I can share my actor document which is pretty minimal but *is* working.
Sebastian Jambor
Mike P
Jahfer
Christopher Su
Cam