fedi vs web

why does fedi use (or not use) activitypub?

first, consider ActivityPub C2S. why is there close to zero usage of this in fediverse software? simple: it doesn't solve any needs for building a "network" "protocol".

now consider S2S. why are there zero compliant impls in fedi? because AP as specified doesn't address the needs of fedi. what does fedi need? well, i find it telling that the "real" reason AP was adopted was... to implement followers-only posts.

which is to say: the primary reason that ActivityPub is used (to the extent you can say it is being used at all) in the fediverse is mostly historical.

fedi grew out of a long line of open protocols, and before AP was adopted, it was at the point where people primarily used "activity streams" as their vocabulary and data model, stuffed into atom feeds. atom feeds don't do private posts unless you make an entirely new access-controlled feed, possibly with a token of some sort. hence, AS2 and AP.

when ActivityPub was being standardized alongside AS2 it basically had two compelling reasons for what would become the fediverse to adopt it:

• it was built on AS2, which was an evolution of AS1, which was already being used. so it wasn't hard to make the jump.
• it made followers-only posts possible, because while atom feeds could do this, it was wildly inconvenient to actually do it that way. posting something private to an inbox is a lot simpler, no juggling access control tokens.

but beyond that, what does ActivityPub actually do for fediverse as a "network" "protocol"? basically nothing. you have a basic mechanism for delivering activities directly to subscribers, but no specified shape or structure for that payload. and you still need a lot of other specs to end up with something that talks to the "network". even with AS2 vocab, you need more vocab extensions to express things you want to.

simply put, AP is not enough for a "protocol" to build a "network".

fedi as a "network"

but before you build a "protocol" for a "network", consider: what even is a "network", in this context? and, here's the hot take: do you even want that kind of "network"? do you want a separate reified fediverse network?

because the answer that ActivityPub gives is actually a different one. There is no "AP network", because AP as a protocol is not enough to build a concrete network. it is intended to provide, and exists in context of, the larger Web.

this is the fundamental divide between fediverse thinking and Web thinking, where ActivityPub straddles the line between both.

what even is context

well, there is the concept of "context" in json-ld, which represents shared understanding.

when i say john knows sally, there are several ambiguities. we can solve ambiguities by disambiguating. one way to disambiguate is to be explicit about what any term or symbol means. one way to be explicit is to use uniform identifiers.

in particular, http/https uris have some convenient properties

• they have authority, so you can qualify an id based on who's assigning it.
• you can use the authority component as a namespace
• you can fetch the uri and it might return something useful

so let's say john is https://example.com/john and sally is example.com/sally. what do we use for knows? well, there are multiple senses of the word "knows":

1. is aware of the existence of
2. is familiar with
3. is having sexual intercourse with

we mean definition 1. so we might use https://example.com/vocab/knows/1. now we have the statement:

<https://example.com/john> <https://example.com/vocab/knows/1> <https://example.com/sally>
						

this is unambiguous, but we can go one step further: we can provide definitions at the uri.

say some random person sees the statement above. they don't know who john or sally are, and they don't know what "knows" means in this context. well, if we do a little work upfront, they actually can know what all of these terms mean, without ever asking us directly

we put a resource on example.com for each of these terms, and each resource describes the subject of that identifier -- it is a "resource descriptor". the resource for knows/1 can define itself explicitly with a schema. so at minimum we have the following schema for knows/1

how to represent it in plain text

"knows"

how to define it

"is aware of the existence of"

the RDF Schema gives us label and comment, as defined by the RDF Schema.

label

"knows"

comment

"is aware of the existence of"

but we need to know what "label" and "comment" mean as well! not to worry, we qualify those terms with the rdfs namespace:

rdfs:label

"knows"

rdfs:comment

"is aware of the existence of"

conventionally, we need a way to unambiguously map rdfs: to its actual prefix, https://www.w3.org/2000/01/rdf-schema#. in Turtle, it would be done like so:

@prefix rdfs: <https://www.w3.org/2000/01/rdf-schema#> .
						

but in json-ld, we would use @context.

ok so how/why do we deal with context

now at this point you're probably wondering what this has to do with social networking. and on a practical level, if you're just interested in building a "social networking protocol", this is mostly all extraneous.

the part that implementers have to deal with is the notion of "context" and, more specifically, how json-ld handles it, and even more specifically, what to do when two shorthand terms conflict.

remember, the open-world solution is namespacing. what does closed-world do?

well, let's look at actor. in AS2 terms it refers to the entity that performed an activity. but in schema.org terms it refers to someone playing a role in a movie or other performance. in an open-world case, you would namespace these as https://www.w3.org/ns/activitystreams#actor and http://schema.org/actor. but in a closed-world sense, you don't want to be aware of context. you don't want to have to deal with it. but even so, you still have an "implicit context" that you are using, based on how you define each term in your own understanding, what you hardcode into your software. in this example, your "implicit context" is that actor always means https://www.w3.org/ns/activitystreams#actor.

what json-ld does, or what it allows you to do, is explicitly declare a @context that is equivalent to your "implicit context".

this works fine if there is only one declaration that is shared exactly between two parties, but it gets complicated when the "implicit context" differs or isn't an exact match.

this means that there cannot ever be a singular fediverse network, because the "implicit context" differs between each software project. the only guaranteed overlap is the AS2 one.

but it's not like AS2 didn't think of this. they wrote in this requirement:

Activity Streams 2.0 implementations that wish to fully support extensions MUST support Compact URI expansion as defined by the JSON-LD specification.

Activity Streams 2.0, §5 "Extensibility"

note, you aren't required to implement all of json-ld. you just need to handle the bit where you can identify the equivalence between a uri and some arbitrary string.

but fediverse mostly decided this is too hard, and so they ignore context.

what happens when you choose to Not

now there's a few thoughts i have here.

first: fediverse culturally seems to ignore a lot of other things as well. they ignore http caching for example. they ignore http status codes like 301 Permanent Redirect. these requirements are arguably more important than context, and they still get ignored.

in fact, most fedi software is mostly just reimplementing Web browsers, but with what they consider to be the "bare minimum" of compliance. and the web they let you browse is smaller than the Web

are these things part of the "protocol"? how far does the "protocol" extend to cover? because, as we established, ActivityPub is not enough to build a fully functional fediverse -- and a lot of extensions and additional specs are things that ought to be included in this "protocol", insofar as this "protocol" is desirable.

second: if you ignore things, that means there are cases you're not handling, losing out on robustness. ignoring context is to ignore shared understanding.

so what do you actually lose out on when you ignore json-ld context? you have to fall back to the "implicit context", where AS2 terms are generally agreed upon, but nothing else is guaranteed.

take something like discoverable from mastodon. what does it mean? well, it means whatever is defined in the mastodon codebase and documentation. so we could represent that as http://joinmastodon.org/ns#discoverable or shorten that with a prefix. but if we do, then most fediverse will choke on that. this is because #fediverse is ignoring context. the implicit context is that discoverable means http://joinmastodon.org/ns#discoverable but they don't know that. so they can't actually handle the extension in its fullest form.

what AS2 calls out as "full support for extensions" requires being able to identify this equivalence and handle it. again, fedi does... let's call it "partial support".

the "implicit context" is now a hardcoded but unstated requirement of this "protocol".

which is to say: fediverse software generally expects LD-aware producers to compact against their own "implicit context", but they don't always define that context. it's left undeclared and undefined. or it actually is declared, but if you give them their own expanded form then they'll not understand it.

if we go back to the "john knows sally" example, then fedi behavior is like this exchange:

1. someone tells you: when i say knows, i mean is familiar with
2. you say: ok. john is familiar with sally.
3. they respond: WTF? what does is familiar with mean? i only understand knows.

it's like... you literally just told me "knows" = "is familiar with", but because of your own ignoring of your own context, you can't handle me saying "is familiar with"?

in this way, as long as the fediverse remains ignorant of context, they will remain fragile and without any sort of robustness in their "protocol".

the alternative they have is to extend the only context they share, which is the AS2 one. but this doesn't solve the problem. it just officially blesses a single term.