ATProto gets PDS ActivityPub support

aschrijver · June 9, 2025, 9:48pm

This just in:

github.com/bluesky-social/atproto

PDS ActivityPub support 😉

main ← mikekasprzak:activitypub-mainline

opened 03:58AM - 09 Jun 25 UTC

mikekasprzak

+2270 -1

No joke, this PR adds preliminary ActivityPub (i.e. Mastodon) support to the PDS…. :exploding_head: **IMPORTANT**: You shouldn't expect full interoperability with this alone! At most, you should expect that your public posts get shared with your followers on Mastodon (i.e. an outbox). While it should be capable of receiving likes and replies, there's no current way to expose those to the Bluesky apps. --- ## :warning: IMPORTANT: THIS IS A PROOF OF CONCEPT/WORK IN PROGRESS :warning: Likes are appreciated, but please **ONLY POST** if you have something to say regarding this PR. :warning: No feature requests! No asking when it'll be ready! Be kind to my alerts, thanks! :warning: --- You might have seen [this skeet](https://bsky.app/profile/distraction.engineer/post/3lql7jiqaac2a): ![image](https://github.com/user-attachments/assets/8a37a773-159c-44f6-a167-bac865a0340e) At the time of this writing, you should be able to search-for and find `@chaos.distraction.engineer` on Bluesky and `@chaos@distraction.engineer` on Mastodon/ActivityPub. This is my modified PDS server `manhattan.distraction.engineer` that's able to communicate using both the AtProto and ActivityPub protocols... err, well the ActivityPub part is a work in progress. :warning: **YOU CAN'T CURRENTLY FOLLOW THE MASTODON USER, ONLY THE BLUESKY USER** :warning: This draft PR is a place for me to share my PDS changes thus far. This isn't my primary branch (that would be [this one over here](https://github.com/mikekasprzak/atproto/tree/activitypub)), but in that one I've had to break a number of things to make my own [docker images](https://github.com/mikekasprzak/pds/pkgs/container/pds) available for testing. **I DON'T RECOMMEND USING MY DOCKER IMAGES** ## How to build :warning: Advanced users only! Not recommended for production! :warning: * Install these codegen patches. These changes were necessary to author the Lexicon files correctly. * #3932 * #3938 * alternatively my [testing branch](https://github.com/mikekasprzak/atproto/tree/activitypub-testing) has these patches applied * run `pnpm install` to install the npm packages * run `pnpm precodegen` to build the codegen tool * run `pnpm install` to update the npm packages and point at our built version of the codegen tool * run `pnpm codegen` to generate code * run `pnpm build` or `pnpm dev` ### Testing Add an entry for `test` to your `/etc/hosts` file. It should look something like this: ```bash 127.0.0.1 localhost ::1 localhost 127.0.1.1 pop-os.localdomain mypcname 127.0.0.1 alice.test bob.test carla.test triage.test mod.test admin-mod.test 127.0.0.1 test ``` To confirm it's working, ping it. ```bash ping test # PING test (127.0.0.1) 56(84) bytes of data. # 64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.096 ms # 64 bytes from localhost (127.0.0.1): icmp_seq=2 ttl=64 time=0.090 ms # 64 bytes from localhost (127.0.0.1): icmp_seq=3 ttl=64 time=0.083 ms # ... ``` Start the development server. ```bash pnpm dev ``` Wait a while, as it needs to start and restart `dist/bin.js` several times before its happy. Then, to confirm that the local server is running, in a separate tab perform a webfinger request on one of the test users. As far as the development environment is concerned, the dummy user `alice.test` is considered to have the ActivityPub handle `alice@test`. ```bash curl http://test:2583/.well-known/webfinger?resource=acct:alice@test # Or the pretty version curl -s http://test:2583/.well-known/webfinger?resource=acct:alice@test|jq ``` If it's working, you'll get a response like so: ```javascript { "subject": "acct:alice@test", "links": [ { "rel": "self", "type": "application/activity+json", "href": "http://test:2583/xrpc/org.w3.activitypub.getActor?repo=did:plc:cltjd4ihxplfp76w3szxc5l2" } ] } ``` :warning: Not all endpoints work! See below for a list of what's working or not! :warning: ### Deploying :warning: NOT RECOMMENDED! ALSO NOTE: MY DOCKER IMAGES ARE NOT UP TO DATE! :warning: Deploy this as you would any PDS server (i.e. adding `_atproto` TXT records to your domain). Assuming you're not doing anything out of the ordinary (i.e. using names under a different subdomain), it should just work. :crossed_fingers: :warning: Right now ActivityPub support is always enabled, but it may not be if this is merged into mainline. :warning: If you want to change the root domain used by your users, you can set `PDS_HOSTNAME_ALT` in your `.env` file. Doing this will also require setting an HTTP 301 redirect for `newdomain.com/.well-known/webfinger` (or the whole `/.well-known/` folder). As an example, my PDS server is on <https://manhattan.distraction.engineer>. Typical users are created as subdomains, so `chaos.manhattan.distraction.engineer`. I set my alt domain to <https://distraction.engineer>, which is a regular static website hosted on Cloudflare. I was able to make my `/.well-known/` folder redirect by creating a file `_redirects` containing the following script. ```bash /.well-known/* https://manhattan.distraction.engineer/.well-known/:splat 301 ``` I then changed my PDS username from `chaos.manhattan.distraction.engineer` to `chaos.distraction.engineer`, which appears as `chaos@distraction.engineer` on Mastodon. :partying_face: ## Background This wacky ride started [at the end of may](https://bsky.app/profile/distraction.engineer/post/3lqdcxzzbw22j). About 3 days later I was able to make the skeet [snapshotted above](https://bsky.app/profile/distraction.engineer/post/3lql7jiqaac2a). While I originally planned to "staple-on" Mastodon-like endpoints to the PDS server, as I grew more familiar with the PDS internals, I realized I could make it work the same as the rest of the server: using `xrpc`. That's what I've been working on since, and what leads me to this PR. ## TODO * [x] Rewrite code to use Lexicons and run over `xrpc` * [x] Build core Lexicons for the ActivityStreams, ActivityPub, and JoinMastodon namespaces * [x] Webfinger (`/.well-known/webfinger?resource=`) * [x] Add `/xrpc/org.w3.activitypub.getActor` endpoint * [x] Return an `Activity#Create` document with an `Actor#Person` inside * [x] Use `icon` and `image` fields for avatar and profile image respectfully * [x] Add `/xrpc/org.w3.activitypub.getOutbox` endpoint * [x] GET version (public) * [x] When no page is specified, return an `OrderedCollection` * [ ] Use correct number of items * [ ] When page is specified, return an `OrderedCollectionPage` * [ ] Add Paging * [ ] POST version (private) * [ ] Add `/xrpc/org.w3.activitypub.getResource` endpoint * [ ] Allow fetching posts (Notes) by id * [ ] Allow fetching activities by id * [ ] TBD: Should this endpoint handle fetching collections too? i.e. likes, followers, etc. * [ ] If so, add Paging * [ ] Add `/xrpc/org.w3.activitypub.pubInbox` endpoint * [ ] Accept follower requests. * [ ] Store followers ... somewhere * [ ] Add support for the `sharedInbox` endpoint (for delivering to all members of a server) * [ ] Send (forward) messages to followers * [ ] Consider switching endpoints over to the `org.joinmastodon` namespace as TECHNICALLY I want to support Mastodon style ActivityPub, as that's the most popular. Mastodon isn't necessarily the most "spec compliant", and including "Mastodon hacks" in `org.w3.activitypub` doesn't feel right. * [ ] Public and Private keys (RSA) * [ ] Generate keypairs * [ ] Import keypairs * [ ] Store and fetch your keys in such a way that they persist in your backups * [ ] Expose public key via getActor * [ ] Expose public key via webfinger? * [ ] Explore exposing the public key via https://plc.directory * [ ] Sign messages sent * [ ] ... ### Known Bugs * [ ] `/xrpc/org.w3.activitypub.getOutbox` unnecessarily includes at AtProto `$type` field * [ ] ... ## Wrapup I'll be updating this PR as I go, and expanding on things. It's a draft PR, so please cut me some slack. :wink:

WisTex · June 9, 2025, 11:29pm

This is awesome.

Mike said:

Consider switching endpoints over to the org.joinmastodon namespace as TECHNICALLY I want to support Mastodon style ActivityPub, as that’s the most popular. Mastodon isn’t necessarily the most “spec compliant”, and including “Mastodon hacks” in org.w3.activitypub doesn’t feel right.

I am not sure how to contact Mike, but I did want to mention that many platforms are purposefully moving away from how Mastodon implements ActivityPub. It may be the most popular now, but platforms that support threaded conversations such as blogs, forums, and Facebook-style platforms are creating their own FEPs and purposefully deviating from what Mastodon implements.

Even if you mostly want to support Mastodon, it may be wise to look at how other platforms are using ActivityPub too, especially in the threaded conversation space. After all, if every WordPress blog turned on ActivityPub support, they would instantly pass Mastodon in number of instances installed. I fully expect fediverse-powered blogs and forums to grow at a far larger rate than people expect.

Just something to consider. Both Mastodon and Bluesky are influenced by Twitter, so it makes sense to focus on Mastodon’s way of doing things. But please don’t leave the rest of us out.

aschrijver · June 13, 2025, 4:12pm

This is something to ping to @eprodrom and @codenamedmitri to bring to W3C attention.

PS. In the delightful-fediverse-experience curated list repo which I just gave a complete overhaul (so check it out), I created a new taxonomy that also de-emphasizes “alternative social media” where adoption is already high, in favor of other social networking areas, esp. innovative ones. See:

PoV · June 13, 2025, 10:37pm

Hi. Have any of the alternatives documented their changes or implementations in a meaningful way? Got links?

While I’m implementing ActivityPub and ActivityStreams as general as I can, there’s a bit too much ambiguity to just follow ActivityPub as it is written. I’m also focused on Microblogging, in essence exposing Bluesky to the Fediverse (300 character/3000 grapheme limits and all). Logically that makes Mastodon my most relevant reference as it’s the most similar.

Notably, the Mastodon spec calls out “Notes” as a focus document type for microblogging, adds extensions like “features posts” on your user page, and allows you to setup domain aliasing using 301 redirects on webfinger requests. This is what I primarily mean by following Mastodon, as they’ve defined a specification I can reference, and reach a large population of microblogging users. I have no intention of implementing everything, but their spec seemed a good starting place.

aschrijver · June 14, 2025, 9:22am

There is a best-practice that a bunch of fediverse projects follow, to add a FEDERATION.md to the root of their repo, to describe how ActivityPub is extended and implemented. Here is a (likely not complete) list in the developer tutorial wiki (hint to all: add missing projects, it is a wiki anyone can edit) pinned to this forum.

There’s a FEP that relates to this best-practice. See:

PoV · June 17, 2025, 4:26pm

Thanks for the links!

FWIW I decided not to move to org.joinmastodon endpoints, and instead decided to “enhance” my org.w3.activitypub objects with extensions. The scope of this is still only for microblogging, but adding “Mastodon Client App” support is on the roadmap now. Generally speaking, making a post posts to both networks, but replies (and likes) will be per network. Sharing/retweeting/reskeeting and quoting support is up in the air, but it will likely be per network only. TBD if an exception can be made for posts we know exist on both networks.

Per a different conversation, I’ve since added FEP ef61 “Portable Object” support. Currently I have IDs working, and actors can optionally emit ap://URIs. For the DID’s, I’m utilizing a nifty did:key encoding called “multidid” to convert AtProto DID’s into ActivityPub DIDs, which I then base58 encode as a string. This is a fully reversible operation, so my /.well-known/apgateway endpoint decodes the did:key directly into AtProto DID (did:plc, did:web, etc), which I use to lookup that users data. Quite elegant if I do say so.

More can be found at the github link (I don’t have permission to post more pictures).

At the moment my apgateway isn’t doing the “tunneling” part, but I like how the ap:// URI’s actually shortened my xrpc requests.