• What
  • The Fediverse
  • ActivityPub
• Why
• How

ActivityPub

Usage of ActivityPub in GitLab is governed by the GitLab Testing Agreement.

The goal of those documents is to provide an implementation path for adding Fediverse capabilities to GitLab.

This page describes the conceptual and high level point of view, while sub-pages discuss implementation in more technical depth (as in, how to implement this in the actual rails codebase of GitLab).

This feature requires two feature flags:

• activity_pub: Enables or disables all ActivityPub-related features.
• activity_pub_project: Enables and disable ActivityPub features specific to projects. Requires the activity_pub flag to also be enabled.

What

Feel free to jump to the Why section if you already know what ActivityPub and the Fediverse are.

Among the push for decentralization of the web, several projects tried different protocols with different ideals behind their reasoning. Some examples:

• Secure Scuttlebutt (or SSB for short)
• Dat
• IPFS,
• Solid

One gained traction recently: ActivityPub, better known for the colloquial Fediverse built on top of it, through applications like Mastodon (which could be described as some sort of decentralized Facebook) or Lemmy (which could be described as some sort of decentralized Reddit).

ActivityPub has several advantages that makes it attractive to implementers and could explain its current success:

• It’s built on top of HTTP. You don’t need to install new software or to tinker with TCP/UDP to implement ActivityPub, if you have a webserver or an application that provides an HTTP API (like a rails application), you already have everything you need.
• It’s built on top of JSON. All communications are basically JSON objects, which web developers are already used to, which simplifies adoption.
• It’s a W3C standard and already has multiple implementations. Being piloted by the W3C is a guarantee of stability and quality work. They have profusely demonstrated in the past through their work on HTML, CSS or other web standards that we can build on top of their work without the fear of it becoming deprecated or irrelevant after a few years.

The Fediverse

The core idea behind Mastodon and Lemmy is called the Fediverse. Rather than full decentralization, those applications rely on federation, in the sense that there still are servers and clients. It’s not P2P like SSB, Dat and IPFS, but instead a galaxy of servers chatting with each other instead of having central servers controlled by a single entity.

The user signs up to one of those servers (called instances), and they can then interact with users either on this instance, or on other ones. From the perspective of the user, they access a global network, and not only their instance. They see the articles posted on other instances, they can comment on them, upvote them, etc.

What happens behind the scenes: their instance knows where the user they reply to is hosted. It contacts that other instance to let them know there is a message for them - somewhat similar to SMTP. Similarly, when a user subscribes to a feed, their instance informs the instance where the feed is hosted of this subscription. That target instance then posts back messages when new activities are created. This allows for a push model, rather than a constant poll model like RSS. Of course, what was just described is the happy path; there is moderation, validation and fault tolerance happening all the way.

ActivityPub

Behind the Fediverse is the ActivityPub protocol. It’s a HTTP API attempting to be as general a social network implementation as possible, while giving options to be extendable.

The basic idea is that an actor sends and receives activities. Activities are structured JSON messages with well-defined properties, but are extensible to cover any need. An actor is defined by four endpoints, which are contacted with the application/ld+json; profile="https://www.w3.org/ns/activitystreams" HTTP Accept header:

• GET /inbox: used by the actor to find new activities intended for them.
• POST /inbox: used by instances to push new activities intended for the actor.
• GET /outbox: used by anyone to read the activities created by the actor.
• POST /outbox: used by the actor to publish new activities.

Among those, Mastodon and Lemmy only use POST /inbox and GET /outbox, which are the minimum needed to implement federation:

• Instances push new activities for the actor on the inbox.
• Reading the outbox allows reading the feed of an actor.

Additionally, Mastodon and Lemmy implement a GET / endpoint (with the mentioned Accept header). This endpoint responds with general information about the actor, like name and URL of the inbox and outbox. While not required by the standard, it makes discovery easier.

While a person is the main use case for an actor, an actor does not necessarily map to a person. Anything can be an actor: a topic, a subreddit, a group, an event. For GitLab, anything with activities (in the sense of what GitLab means by “activity”) can be an ActivityPub actor. This includes items like projects, groups, and releases. In those more abstract examples, an actor can be thought of as an actionable feed.

ActivityPub by itself does not cover everything that is needed to implement the Fediverse. Most notably, these are left for the implementers to figure out:

• Finding a way to deal with spam. Spam is handled by authorizing or blocking (“defederating”) other instances.
• Discovering new instances.
• Performing network-wide searches.

Why

Why would a social media protocol be useful for GitLab? People want a single, global GitLab network to interact between various projects, without having to register on each of their hosts.

Several very popular discussions around this have already happened:

• Share events externally via ActivityPub
• Implement cross-server (federated) merge requests
• Distributed merge requests.

The ideal workflow would be:

1. Alice registers to her favorite GitLab instance, like gitlab.example.org.
2. She looks for a project on a given topic, and sees Bob’s project, even though Bob is on gitlab.com.
3. Alice selects Fork, and the gitlab.com/Bob/project.git is forked to gitlab.example.org/Alice/project.git.
4. She makes her edits, and opens a merge request, which appears in Bob’s project on gitlab.com.
5. Alice and Bob discuss the merge request, each one from their own GitLab instance.
6. Bob can send additional commits, which are picked up by Alice’s instance.
7. When Bob accepts the merge request, his instance picks up the code from Alice’s instance.

In this process, ActivityPub would help in:

• Letting Bob know a fork happened.
• Sending the merge request to Bob.
• Enabling Alice and Bob to discuss the merge request.
• Letting Alice know the code was merged.

It does not help in these cases, which need specific implementations:

• Implementing a network-wide search.
• Implementing cross-instance forks. (Not needed, thanks to Git.)

Why use ActivityPub here rather than implementing cross-instance merge requests in a custom way? Two reasons:

1. Building on top of a standard helps reach beyond GitLab. While the workflow presented above only mentions GitLab, building on top of a W3C standard means other forges can follow GitLab there, and build a massive Fediverse of code sharing.
2. An opportunity to make GitLab more social. To prepare the architecture for the workflow above, smaller steps can be taken, allowing people to subscribe to activity feeds from their Fediverse social network. Anything that has a RSS feed could become an ActivityPub feed. People on Mastodon could follow their favorite developer, project, or topic from GitLab and see the news in their feed on Mastodon, hopefully raising engagement with GitLab.

How

The idea of this implementation path is not to take the fastest route to the feature with the most value added (cross-instance merge requests), but to go on with the smallest useful step at each iteration, making sure each step brings something immediately.

1. Implement ActivityPub for social following. After this, the Fediverse can follow activities on GitLab instances.
   1. ActivityPub to subscribe to project releases.
   2. ActivityPub to subscribe to project creation in topics.
   3. ActivityPub to subscribe to project activities.
   4. ActivityPub to subscribe to group activities.
   5. ActivityPub to subscribe to user activities.
2. Implement cross-instance search to enable discovering projects on other instances.
3. Implement cross-instance forks to enable forking a project from an other instance.
4. Implement ActivityPub for cross-instance discussions to enable discussing issues and merge requests from another instance:
   1. In issues.
   2. In merge requests.
5. Implement ActivityPub to submit cross-instance merge requests to enable submitting merge requests to other instances.

For now, see how to implement an ActivityPub actor.