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An even more distributed ActivityPub

By Christopher Allan Webber on Thu 06 October 2016

So ActivityPub is nearing Candidate Recommendation status. If you want to hear a lot more about that whole process of getting there, and my recent trip to TPAC, and more, I wrote a post on the MediaGoblin blog about it.

Last night my brother Stephen came over and he was talking about how he wished ActivityPub was more of a "transactional" system. I've been thinking about this myself. ActivityPub as it is designed is made for the social network of 2014 more or less: trying to reproduce what the silos do, which is mutate a big database for specific objects, but reproduce that in a distributed way. Well, mutating distributed systems is a bit risky. Can we do better, without throwing out the majority of the system? I think it's possible, with a couple of tweaks.

  • The summary is to move to objects and pointers to objects. There's no mutation, only "changing" pointers (and even this is done via appending to a log, mostly).

    If you're familiar with git, you could think of the objects as well, objects, and the pointers as branches.

    Except... the log isn't in the objects pointing at their previous revisions really, the logging is on the pointers:

    [pointer id] => [note content id]
    
  • There's (activitystreams) objects (which may be content addressed, to be more robust), and then "pointers" to those, via signed pointer-logs.

  • The only mutation in the system is that the "pointers", which are signed logs (substitute "logs" for "ledger" and I guess that makes it a "blockchain" loosely), are append-only structures that say where the new content is. If something changes a lot, it can have "checkpoints". So, you can ignore old stuff eventually.

  • Updating content means making a new object, and updating the pointer-log to point to it.

  • This of course leads to a problem: what identifier should objects use to point at each other? The "content" id, or the "pointer-log" id? One route is that when one object links to another object, it could link to both the pointer-log id and the object id, but that hardly seems desirable...

  • Maybe the best route is to have all content ids point back at their official log id... this isn't as crazy as it sounds! Have a three step process for creating a brand new object:

    • Open a new pointer-log, which is empty, and get the identifier
    • Create the new object with all its content, and also add a link back to the pointer-log in the content's body
    • Add the new object as the first item in the pointer-log
  • At this point, I think we can get rid of all side effects in ActivityPub! The only mutation thing is append-only to that pointer-log. As for everything else:

    • Create just means "This is the first time you've seen this object." And in fact, we could probably drop Create in a system like this, because we don't need it.
    • Update is really just informing that there's a new entry on the pointer-log.
    • Delete... well, you can delete your own copy. You're mostly informing other servers to delete their copy, but they have a choice if they really will... though that's always been true! You now can also switch to the nice property that removing old content is now really garbage collection :)
  • Addressing and distribution still happens in the same, previous ways it did, I assume? So, you still won't get access to an object unless you have permissions? Though that gets more confusing if you use the (optional) content addressed storage here.

  • You now get a whole lot of things for free:

    • You have a built in history log of everything
    • Even if someone else's node goes down, you can keep a copy of all their content, and keep around the signatures to show that yeah, that really was the content they put there!
    • You could theoretically distribute storage pretty nicely
    • Updates/deletes are less dangerous

(Thanks to Steve for encouraging me to think this through more clearly, and lending your own thoughts, a lot of which is represented here! Thanks also to Manu Sporny who was the first to get me thinking along these lines with some comments at TPAC. Though, any mistakes in the design are mine...)

Of course, you can hit even more distributed-system-nerd points by tossing in the possibility of encrypting everything in the system, but let's leave that as an exercise for the reader. (It's not too much extra work if you already have public keys on profiles.)

Anyway, is this likely to happen? Well, time is running out in the group, so I'm unlikely to push for it in this iteration. But the good news, as I said, is that I think it can be built on top without too much extra work... The systems might even be straight-up compatible, and eventually the old mutation-heavy-system could be considered the "crufty" way of doing things.

Architectural astronaut'ing? Maybe! Fun to think about! Hopefully fun to explore. Gotta get the 2014-made-distributed version of the social web out first though. :)