This isn’t really going to be a blog most, but more of a collection of tidbits and resources I found helpful in implenting the ActivityPub integration for the new version of my blog.

This post was last updated on Mar 7, 2023.

### Specs

• The ActivityStreams 2.0 spec is important, as it’s what ActivityPub is built on top of.
• Similarly, the AS 2.0 Vocabulary defines all the objects and activities that AP actually uses in practice (and many more that it doesn’t).
• The ActivityPub spec itself is quite useful, despite its many omissions.
• There’s also LitePub, which has some extensions to AP.
• The least useful by far spec is JSON-LD which defines how to use JSON to represent linked data and object graphs. AS2 and AP are both built on this, but if you’re going for a simple implementation (or even a complex one), you can entirely ignore this and treat JSON-LD as plain old JSON objects.

This is also a helpful resource about how to go about reading the AP specification.

### Actually Federating

• Gargron’s blog posts on implementing a basic AP server and implementing HTTP signatures are good guides for how to actually get federating with other servers in the wild.
• Lain’s blog post on some of the weird quirks of how ActivityPub actually gets used.
• Kaniini’s blog post about how data actually moves through the fediverse.

### Reference Material

• Darius Kazemi has a simple reference implementation of an ActivityPub server written using Node.js.
• I used the Pleroma source code a great deal when working on my implementation, mainly just because I’m familiar with Elixir.
• I’d also like to think my own implementation is fairly approachable (it’s about 700 lines of not-too-complicated TypeScript).
• Ted Unangst has a collection of sample data which is useful for comparing how different implementations represent things in AP.

### Libraries

• activitystreams for serializing/deserializing AS2 types in Rust
• http-signature-normalization for dealing with part of HTTP signatures in Rust

### Other

• For actually testing federation, ngrok is very useful for testing your implementations against others. It creates a tunnel from your local machine to a public domain with HTTPS already setup. Because your code is still running locally, you have access to all your usual debugging tools and can iterate rapidly.
• Testing against other implementations running locally (be it on your machine or inside a VM/container) lets you access debug logs and see what the other server is actually receiving, which can be quite useful.
• Darius Kazemi also wrote an application that lets you send ActivityPub objects directly to other servers, which is useful for testing your application against outside data without polluting other people’s instances.
• Ted Unangst also has his own compilation of AP-related links.