One of the most interesting areas of exploration in the ActivityPub community right now is the ActivityPub API. Most people who know ActivityPub are familiar with its federation protocol, which lets social networking servers like Mastodon and Pixelfed share data between them. But there is another, closely-related feature in the same specification, called the "social API".

ActivityPub has five normative sections: 3 Objects, 4 Actors, 5 Collections, 6 Client-to-Server Interactions, and 7 Server-to-Server Interactions. 3, 4, and 5 provide a read-only interface to social data that is useful for both the federation protocol and the social API. It lets both clients and servers read information about users on the network, their feeds, and the things they make and share.

Section 6 is focused on the mechanism clients can use to create new activities, and the side-effects of those activities. "Activities" are the most important data structure in ActivityPub (which is why they're featured so prominently in the name!). They represent sentences or statements about things that happen on a social network, like "Christine created the image img123.jpg" or "Evan liked Christine's image img123.jpg" and "Amy shared Christine's image img123.jpg". Creating these statements is how clients can make things happen with ActivityPub.

Section 7 is focused on how and when servers can send these activities across the network to other servers. There are some side-effects that are laid out, but mostly they involve cache management.

So, here's the important point I want to make: the federation protocol which connects ActivityPub servers is defined in sections 3, 4, 5, and 7. The social API is defined in sections 3, 4, 5, and 6. But some people use "server-to-server" or "s2s" as a synonym for the federation protocol, even though "server to server interactions" only covers one section. Similarly, some people use "c2s" or "client-to-server" as a synonym for the social API, even if "client-to-server interactions" is only one section.

I prefer to use "social API" or "ActivityPub API" to refer to the entire part of ActivityPub that lets client apps talk to social servers. Here are some rough reasons why.

• It's what the spec calls it. In the conformance section, we said that 'this specification defines two closely related and interacting protocols: A client to server protocol, or "Social API" [...] A server to server protocol, or "Federation Protocol"'. This was intentional; the names come from the Social Web Working Group charter deliverables. Making a Social API was a key goal of the group. The ActivityPub API satisfies that goal.
• It's a term that all developers and many users are familiar with. People who've used mobile apps or third-party apps know what an API is and what it's for. If you use "c2s", it's not clear to most developers what you're even talking about.
• It's accurate. The social API is a RESTful API that uses JSON, HTTP, and all the fun stuff that app developers are already familiar with.
• It emphasizes what's already available. People often say that Mastodon does not implement the ActivityPub API. But it implements sections 3, 4, and 5 of the specification – it has to, in order to support the federation protocol. That's the entire read-only side of the API. All (almost all?) Fediverse servers support these parts of the spec, too. You can build a pretty OK read-only API client using what's already available from Mastodon and others; see https://acct.swf.pub/ for an example.
• "c2s" doesn't cover the whole API. As I said above, only one section of the doc is called "Client-to-Server Interactions", and it doesn't cover the read-only side of the API.
• "c2s" is insider jargon. c2s isn't a common term for RESTful APIs. It really isn't a common term at all; there are only a few protocol suites, like XMPP, that refer to the "c2s" and "s2s" part.

I think it's fine if others use "c2s" when talking about the API, or especially about section 6 of the ActivityPub spec. It's not going to cause any harm. But the Social Web Community Group task force on implementing the API is called the "ActivityPub API" task force. I think that's a good idea – it emphasizes the API. I intent to use this name and framing for the foreseeable future.

One of the most interesting areas of exploration in the ActivityPub community right now is the ActivityPub API. Most people who know ActivityPub are familiar with its federation protocol, which lets social networking servers like Mastodon and Pixelfed share data between them. But there is another, closely-related feature in the same specification, called the "social API".

ActivityPub has five normative sections: 3 Objects, 4 Actors, 5 Collections, 6 Client-to-Server Interactions, and 7 Server-to-Server Interactions. 3, 4, and 5 provide a read-only interface to social data that is useful for both the federation protocol and the social API. It lets both clients and servers read information about users on the network, their feeds, and the things they make and share.

Section 6 is focused on the mechanism clients can use to create new activities, and the side-effects of those activities. "Activities" are the most important data structure in ActivityPub (which is why they're featured so prominently in the name!). They represent sentences or statements about things that happen on a social network, like "Christine created the image img123.jpg" or "Evan liked Christine's image img123.jpg" and "Amy shared Christine's image img123.jpg". Creating these statements is how clients can make things happen with ActivityPub.

Section 7 is focused on how and when servers can send these activities across the network to other servers. There are some side-effects that are laid out, but mostly they involve cache management.

So, here's the important point I want to make: the federation protocol which connects ActivityPub servers is defined in sections 3, 4, 5, and 7. The social API is defined in sections 3, 4, 5, and 6. But some people use "server-to-server" or "s2s" as a synonym for the federation protocol, even though "server to server interactions" only covers one section. Similarly, some people use "c2s" or "client-to-server" as a synonym for the social API, even if "client-to-server interactions" is only one section.

I prefer to use "social API" or "ActivityPub API" to refer to the entire part of ActivityPub that lets client apps talk to social servers. Here are some rough reasons why.

• It's what the spec calls it. In the conformance section, we said that 'this specification defines two closely related and interacting protocols: A client to server protocol, or "Social API" [...] A server to server protocol, or "Federation Protocol"'. This was intentional; the names come from the Social Web Working Group charter deliverables. Making a Social API was a key goal of the group. The ActivityPub API satisfies that goal.
• It's a term that all developers and many users are familiar with. People who've used mobile apps or third-party apps know what an API is and what it's for. If you use "c2s", it's not clear to most developers what you're even talking about.
• It's accurate. The social API is a RESTful API that uses JSON, HTTP, and all the fun stuff that app developers are already familiar with.
• It emphasizes what's already available. People often say that Mastodon does not implement the ActivityPub API. But it implements sections 3, 4, and 5 of the specification – it has to, in order to support the federation protocol. That's the entire read-only side of the API. All (almost all?) Fediverse servers support these parts of the spec, too. You can build a pretty OK read-only API client using what's already available from Mastodon and others; see https://acct.swf.pub/ for an example.
• "c2s" doesn't cover the whole API. As I said above, only one section of the doc is called "Client-to-Server Interactions", and it doesn't cover the read-only side of the API.
• "c2s" is insider jargon. c2s isn't a common term for RESTful APIs. It really isn't a common term at all; there are only a few protocol suites, like XMPP, that refer to the "c2s" and "s2s" part.

I think it's fine if others use "c2s" when talking about the API, or especially about section 6 of the ActivityPub spec. It's not going to cause any harm. But the Social Web Community Group task force on implementing the API is called the "ActivityPub API" task force. I think that's a good idea – it emphasizes the API. I intent to use this name and framing for the foreseeable future.