I think the #ActivityPub client-to-server API is extremely important and underrated.

@julian@fietkau.social @evan @julian@activitypub.space @smallcircles

actor: alice
type: as:Accept, as:Add, _:ReplyAck
object: <the reply>
target: <the replies collection>
_:postRepliedTo: <op>

for the Accept, you need to understand it as "accepting the object into the target", which is apparently a thing in AS2-Vocab.

for the Add, you need to understand it as Add is defined -- no surprises there, i think?

for the ReplyAck, it can be whatever you define. here i've used an extension postRepliedTo property.

@julian@fietkau.social in a parallel conversation not about interaction controls, @rimu@piefed.social made the case for batching events, which I'm going to repurpose as an argument against sending additional activities for backward compatibility (unless absolutely necessary.)

> As a user can do a great number of notable things (posting content, liking content, following others) each minute and there can be thousands of instances to send to, a great many POST requests can be sent in a short amount of time.
>
> For example if 5 people cast 20 votes and there are 500 instances, the instance hosting the community containing the posts being voted on must send 5 * 20 * 500 = 50,000 HTTP POSTs.

@trwnh The GTS implementation comes with a vocabulary extension, so adding another activity type would be an option.

I guess the reason they didn't do that for this case (I wasn't around for the decision) is that the Accept(Note) thing is itself a backward compatibility hack that they hoped to be able to drop eventually, when more servers would send ReplyRequests (which can be Accept-ed directly), and adding a new type would have felt too much like “enshrining” it.

@evan @julian@activitypub.space @smallcircles

@julian i think mastodon handles multityping in certain code paths but most other projects don't. it could have been a compatibility thing?

@evan @julian @deadsuperhero

Except when they are called other names instead

A timeline is a different thing than a collection imho. And an AS collection has some very particular functionality, which if I model a timeline in my app may not supported (e.g. reverse ordering).

Collection / 'timeline' is one of those words where sometimes they indicate an app domain, and sometimes a core protocol mechanism. Same is true with 'follow' which is sometimes a user action, sometimes indicates low-level publish/subscribe.

For core capabilities that must be part of the specs, in 'protocol space' it may be better to use terminology that is more common in messaging architectures and all the various architecture patterns that are involved. Perhaps idk we deal with a time-ordered event log or something like that.

@smallcircles @evan An AS2 Collection cannot be a timeline (in general). It’s not even ordered. An AS2 OrderedCollection (a subtype of Collection) might be ordered by time or not, so it’s also not a timeline (in general). When they are ordered by some time value (unspecified in AP) they are often called “streams” in the spec. The Mastodon content timelines are not the same as AP activity streams although a filtered AP stream can be transformed to a content timeline.

@steve I think
we need to emphasize that timelines can be built from regular collections, even unordered ones, by using some intermediate representations specific to the type of timeline that a client wants to render.

The fact that the specification does not directly support a mapping between a collection and a responsive timeline, *DOES NOT MEAN* one can't be built from it, only that it requires a little more effort on the client side.

My goto example is how rich mail clients allow responsive mailbox representations on top of a much less expressive collection method that IMAP provides compared to ActivityPub.

@smallcircles @evan

@mariusor @smallcircles @evan I’m not sure I completely follow. A timeline is ordered by time. I agree that an unordered collection could be sorted by time to create a timeline. The AP OrderedCollection “stream” is a kind of rigid presorting that anticipates what an AP client would want. However, I also agree that even those could be reordered (by time or otherwise) and/or filtered in the client to provide custom views of the activity stream.

@steve yes, that's how I meant it. A client fetches as much of the collection as it can, then applies whatever rules it wants to transform the result into a "timeline" when the user asks for it.

This however most likely requires local caching of the collection to have decent latency.

@smallcircles @evan

@mariusor @smallcircles @evan Yes, it can be done in the client or the server, or both. I’d like to see an interoperable way to define custom timelines (a kind of user-defined timeline algo) that the server maintains. A Mastodon account list timeline is a super simple version of it, but AP could provide something much more powerful (advanced filtering, merging, ranking, …). Ideally, these could be shared and customized further on the client side.

@steve frankly I disagree with this point. Servers should be simple. We need to move away from the paradigm of custom purpose ActivityPub servers that Mastodon pushed where the client and server are the same service.

Timelines should be orthogonal to the ActivityPub specification and, in my opinion, kept well away from it.

What's the benefit for my client application to know what your server's preferred timeline representation is?

Let's not go down the path where everything looks like a nail because we really like hammers.

@smallcircles @evan

@mariusor @smallcircles @evan I think you read something other than what I wrote. . I’m describing *user-defined* timelines where the heavy lifting is done in a server. That server would be (or could be) *general purpose* and not specific to an activity domain. I definitely wasn’t suggesting a monolithic, tightly-coupled client/server architecture. I want my timeline definitions to be portable and interoperable.

@steve apologies, I take "server" in the context of ActivityPub discussion to be an "ActivityPub server", not all the other web-servers involved in the process.

And when I say "client", I mean a "consumer of ActivityPub", which as you say, many times is also a web server.

@smallcircles @evan

@mariusor @steve @evan

> And when I say "client", I mean a "consumer of ActivityPub", which as you say, many times is also a web server.

Indeed. Another term that I see people use in different meaning, also when talking about C2S.

In one meaning the user device is referred to, that you might need to hole-punch with to have a full AP server, or which depends on a server relay.

And the other meaning as role. As in client/server roles, pure conceptual, and which might swap too.

@smallcircles @evan An AS2 Collection cannot be a timeline (in general). It’s not even ordered. An AS2 OrderedCollection (a subtype of Collection) might be ordered by time or not, so it’s also not a timeline (in general). When they are ordered by some time value (unspecified in AP) they are often called “streams” in the spec. The Mastodon content timelines are not the same as AP activity streams although a filtered AP stream can be transformed to a content timeline.

@steve @smallcircles The `inbox` and `outbox` are both sequences ordered by time. I think that should meet your requirements for a 'timeline'?

I think it's fair to call the outbox the actor's 'feed'? It is a feed of all their activities.

@steve @smallcircles I also agree that activities are more primary than content objects like notes and images in ActivityPub. That is by design and reflected in the name of the data format, API and federation protocol.

@steve @smallcircles I also agree that activities are more primary than content objects like notes and images in ActivityPub. That is by design and reflected in the name of the data format, API and federation protocol.

@smallcircles @mariusor @evan C2S is described (too loosely, but…) in the ActivityPub spec. There is a client and server aspect to C2S. A C2S client is software that uses that protocol/API to interact with an ActivityPub C2S-capable server (general or domain-specific). When I refer to an ActivityPub Client, I mean software using C2S rather than consumers of ActivityPub-related data in general.

@steve @smallcircles

That said, I think it would be great to have reverse chronological ordered collections of objects created by the actor.

It would be nice to use `streams` like `endpoints`, as an object, and define properties like `notes`, `images`, `places` and so on off of it.

Unfortunately the loose definition and lack of examples for `streams` makes it hard to use. It's probably better just to define them as top-level properties of the actor.