• Summary
• Description of the related tech and terms
  • The Fediverse
  • ActivityPub
• Motivation
  • Goals
  • Non-Goals
• Proposal
• Design and implementation details
  • Production readiness
  • The social following part
    • Endpoints
    • Serializers
    • Subscription
    • Unfollow
    • Sending activities out
    • Webfinger
    • HTTP signatures
  • Host allowlist and denylist
  • Limits and rollout
  • The cross-instance issues and merge requests part

ActivityPub support

Summary

The end goal of this proposal is to build interoperability features into GitLab so that it’s possible on one instance of GitLab to open a merge request to a project hosted on an other instance, merging all willing instances in a global network.

To achieve that, we propose to use ActivityPub, the w3c standard used by the Fediverse. This will allow us to build upon a robust and battle-tested protocol, and it will open GitLab to a wider community.

Before starting implementing cross-instance merge requests, we want to start with smaller steps, helping us to build up domain knowledge about ActivityPub and creating the underlying architecture that will support the more advanced features. For that reason, we propose to start with implementing social features, allowing people on the Fediverse to subscribe to activities on GitLab, for example to be notified on their social network of choice when their favorite project hosted on GitLab makes a new release. As a bonus, this is an opportunity to make GitLab more social and grow its audience.

Description of the related tech and terms

Feel free to jump to Motivation 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.

Motivation

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.

Goals

• allowing to share interesting events on ActivityPub based social media
• allowing to open an issue and discuss it from one instance to an other
• allowing to fork a project from one instance to an other
• allowing to open a merge request, discuss it and merge it from one instance to an other
• allowing to perform a network wide search?

Non-Goals

• federation of private resources
• allowing to perform a network wide search?

Proposal

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 useful.

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 forks to enable forking a project from an other instance.
3. Implement ActivityPub for cross-instance discussions to enable discussing issues and merge requests from another instance:
   1. In issues.
   2. In merge requests.
4. Implement ActivityPub to submit cross-instance merge requests to enable submitting merge requests to other instances.
5. Implement cross-instance search to enable discovering projects on other instances.

It’s open to discussion if this last step should be included at all. Currently, in most Fediverse apps, when you want to display a resource from an instance that your instance does not know about (typically a user you want to follow), you paste the URL of the resource in the search box of your instance, and it fetches and displays the remote resource, now actionable from your instance. We plan to do that at first.

The question is : do we keep it at that? This UX has severe frictions, especially for users not used to Fediverse UX patterns (which is probably most GitLab users). On the other hand, distributed search is a subject complicated enough to deserve its own blueprint (although it’s not as complicated as it used to be, now that decentralization protocols and applications worked on it for a while).

Design and implementation details

First, it’s a good idea to get familiar with the specifications of the three standards we’re going to use:

• ActivityPub defines the HTTP requests happening to implement federation.
• ActivityStreams defines the format of the JSON messages exchanged by the users of the protocol.
• Activity Vocabulary defines the various messages recognized by default.

Feel free to ping @oelmekki if you have questions or find the documents too dense to follow.

Production readiness

TBC

The social following part

This part is laying the ground work allowing to add new ActivityPub actors to GitLab.

There are 5 actors we want to implement:

• the releases actor, to be notified when given project makes a new release
• the topic actor, to be notified when a new project is added to a topic
• the project actor, regarding all activities from a project
• the group actor, regarding all activities from a group
• the user actor, regarding all activities from a user

We’re only dealing with public resources for now. Allowing federation of private resources is a tricky subject that will be solved later, if it’s possible at all.

Endpoints

Each actor needs 3 endpoints:

• the profile endpoint, containing basic info, like name, description, but also including links to the inbox and outbox
• the outbox endpoint, allowing to show previous activities for an actor
• the inbox endpoint, on which to post to submit follow and unfollow requests (among other things we won’t use for now).

The controllers providing those endpoints are in app/controllers/activity_pub/. It’s been decided to use this namespace to avoid mixing the ActivityPub JSON responses with the ones meant for the frontend, and also because we may need further namespacing later, as the way we format activities may be different for one Fediverse app, for an other, and for our later cross-instance features. Also, this namespace allow us to easily toggle what we need on all endpoints, like making sure no private project can be accessed.

Serializers

The serializers in app/serializers/activity_pub/ are the meat of our implementation, are they provide the ActivityStreams objects. The abstract class ActivityPub::ActivityStreamsSerializer does all the heavy lifting of validating developer provided data, setting up the common fields and providing pagination.

That pagination part is done through Gitlab::Serializer::Pagination, which uses offset pagination. We need to allow it to do keyset pagination.

Subscription

Subscription to a resource is done by posting a Follow activity to the actor inbox. When receiving a Follow activity, we should generate an Accept or Reject activity in return, sent to the subscriber’s inbox.

The general workflow of the implementation is as following:

• A POST request is made to the inbox endpoint, with the Follow activity encoded as JSON
• if the activity received is not of a supported type (e.g. someone tries to comment on the activity), we ignore it ; otherwise:
• we create an ActivityPub::Subscription with the profile URL of the subscriber
• we queue a job to resolve the subscriber’s inbox URL
  • in which we perform a HTTP request to the subscriber profile to find their inbox URL (and the shared inbox URL if any)
  • we store that URL in the subscription record
• we queue a job to accept the subscription
  • in which we perform a HTTP request to the subscriber inbox to post an Accept activity
  • we update the state of the subscription to :accepted

ActivityPub::Subscription is a new abstract model, from which inherit models related to our actors, each with their own table:

• ActivityPub::ReleasesSubscription, table activity_pub_releases_subscriptions
• ActivityPub::TopicSubscription, table activity_pub_topic_subscriptions
• ActivityPub::ProjectSubscription, table activity_pub_project_subscriptions
• ActivityPub::GroupSubscription, table activity_pub_group_subscriptions
• ActivityPub::UserSubscription, table activity_pub_user_subscriptions

The reason to go with a multiple models rather than, say, a simpler actor enum in the Subscription model with a single table is because we needs specific associations and validations for each (an ActivityPub::ProjectSubscription belongs to a Project, an ActivityPub::UserSubscription does not). It also gives us more room for extensibility in the future.

Unfollow

When receiving an Undo activity mentioning previous Follow, we remove the subscription from our database.

We are not required to send back any activity, so we don’t need any worker here, we can directly remove the record from database.

Sending activities out

When specific events (which ones?) happen related to our actors, we should queue events to issue activities on the subscribers inboxes (the activities are the same than we display in the actor’s outbox).

We’re supposed to deduplicate the subscriber list to make sure we don’t send an activity twice to the same person - although it’s probably better handled by a uniqueness validation from the model when receiving the Follow activity.

More importantly, we should group requests for a same host : if ten users are all on https://mastodon.social/, we should issue a single request on the shared inbox provided, adding all the users as recipients, rather than sending one request per user.

Webfinger

Mastodon requires instance to implement the Webfinger protocol. This protocol is about adding an endpoint at a well known location which allows to query for a resource name and have it mapped to whatever URL we want (so basically, it’s used for discovery). Mastodon uses this to query other fediverse apps for actor names, in order to find their profile URLs.

Actually, GitLab already implements the Webfinger protocol endpoint through Doorkeeper (this is the action that maps to its route), implemented in GitLab in JwksController.

There is no incompatibility here, we can just extend this controller. Although, we’ll probably have to rename it, as it won’t be related to Jwks alone anymore.

One difficulty we may have is that contrary to Mastodon, we don’t only deal with users. So we need to figure something to differentiate asking for a user from asking for a project, for example. One obvious way would be to use a prefix, like user-<username>, project-<project_name>, etc. I’m pondering that from afar, while we haven’t implemented much code in the epic and I haven’t dig deep into Webfinger’s specs, this remark may be deprecated when we reach actual implementation.

HTTP signatures

Mastodon requires HTTP signatures, which is yet an other standard, in order to make sure no spammer tries to impersonate a given server.

This is asymmetrical cryptography, with a private key and a public key, like SSH or PGP. We will need to implement both signing requests, and verifying them. This will be of considerable help when we’ll want to have various GitLab instances communicate later in the epic.

Host allowlist and denylist

To give GitLab instance owners control over potential spam, we need to allow to maintain two mutually exclusive lists of hosts:

• the allowlist : only hosts mentioned in this list can be federated with.
• the denylist : all hosts can be federated with but the ones mentioned in that list.

A setting should allow the owner to switch between the allowlist and the denylist. In the beginning, this can be managed in rails console, but it will ultimately need a section in the admin interface.

Limits and rollout

In order to control the load when releasing the feature in the first months, we’re going to set gitlab.com to use the allowlist and rollout federation to a few Fediverse servers at a time, so that we can see how it takes the load progressively, before ultimately switching to denylist (note: there are some ongoing discussions regarding if federation should be activated on gitlab.com or not).

We also need to implement limits to make sure the federation is not abused:

• limit to the number of subscriptions a resource can receive.
• limit to the number of subscriptions a third party server can generate.

The cross-instance issues and merge requests part

We’ll wait to be done with the social following part before designing this part, to have ground experience with ActivityPub.