Federation

This document is for anyone who wants to know how Lemmy federation works, without being overly technical. It is meant provide a high-level overview of ActivityPub federation in Lemmy. If you are implementing ActivityPub yourself and want to be compatible with Lemmy, read our ActivityPub API outline.

Documentation conventions

To keep things simple, sometimes you will see things formatted like Create/Note or Delete/Event or Undo/Follow. The thing before the slash is the Activity, and the thing after the slash is the Object inside the Activity, in an object property. So these are to be read as follows:

  • Create/Note: a Create activity containing a Note in the object field
  • Delete/Event: a Delete activity containing an Event in the object field
  • Undo/Follow: an Undo activity containing a Follow in the object field

In Lemmy we use some specific terms to refer to ActivityPub items. They are essentially our specific implementations of well-known ActivityPub concepts:

  • Community: Group
  • User: Person
  • Post: Page
  • Comment: Note

This document has three main sections:

  • Federation philosophy lays out the general model of how this is intended to federate
  • User Activities describes which actions that a User can take to interact
  • Community Activities describes what the Community does in response to certain User actions

Federation philosophy

The primary Actor in Lemmy is the Community. Each community resides on a single instance, and consists of a list of Posts and a list of followers. The primary interaction is that of a User sending a Post or Comment related activity to the Community inbox, which then announces it to all its followers.

Each Community has a specific creator User, who is responsible for setting rules, appointing moderators, and removing content that violates the rules.

Besides moderation on the community level, each instance has a set of administrator Users, who have the power to do site-wide removals and bans.

Users follow Communities that they are interested in, in order to receive Posts and Comments. They also vote on Posts and Comments, as well as creating new ones. Comments are organised in a tree structure and commonly sorted by number of votes. Direct messages between Users are also supported.

Users can not follow each other, and neither can Communities follow anything.

Our federation implementation is already feature complete, but so far we haven't focused at all on complying with the ActivityPub spec. As such, Lemmy is likely not compatible with implementations which expect to send and receive valid activities. This is something we plan to fix in the near future. Check out #698 for an overview of our deviations.

User Activities

Follow a Community

Each Community page has a "Follow" button. Clicking this triggers a Follow activity to be sent from the user to the Community inbox. The Community will automatically respond with an Accept/Follow activity to the user inbox. It will also add the user to its list of followers, and deliver any activities about Posts/Comments in the Community to the user.

Unfollow a Community

After following a Community, the "Follow" button is replaced by "Unfollow". Clicking this sends an Undo/Follow activity to the Community inbox. The Community removes the User from its followers list and doesn't send any activities to it anymore.

Create a Post

When a user creates a new Post in a given Community, it is sent as Create/Page to the Community inbox.

Create a Comment

When a new Comment is created for a Post, both the Post ID and the parent Comment ID (if it exists) are written to the in_reply_to field. This allows assigning it to the correct Post, and building the Comment tree. It is then sent to the Community inbox as Create/Note

The origin instance also scans the Comment for any User mentions, and sends the Create/Note to those Users as well.

Edit a Post

Changes the content of an existing Post. Can only be done by the creating User.

Edit a Comment

Changes the content of an existing Comment. Can only be done by the creating User.

Likes and Dislikes

Users can like or dislike any Post or Comment. These are sent as Like/Page, Dislike/Note etc to the Community inbox.

Deletions

The creator of a Post, Comment or Community can delete it. It is then sent to the Community followers. The item is then hidden from all users.

Removals

Mods can remove Posts and Comments from their Communities. Admins can remove any Posts or Comments on the entire site. Communities can also be removed by admins. The item is then hidden from all users.

Removals are sent to all followers of the Community, so that they also take effect there. The exception is if an admin removes an item from a Community which is hosted on a different instance. In this case, the removal only takes effect locally.

Revert a previous Action

We don't delete anything from our database, just hide it from users. Deleted or removed Communities/Posts/Comments have a "restore" button. This button generates an Undo activity which sets the original delete/remove activity as object, such as Undo/Remove/Post or Undo/Delete/Community.

Clicking on the upvote button of an already upvoted post/comment (or the downvote button of an already downvoted post/comment) also generates an Undo. In this case and Undo/Like/Post or Undo/Dislike/Comment.

Create private message

User profiles have a "Send Message" button, which opens a dialog permitting to send a private message to this user. It is sent as a Create/Note to the user inbox. Private messages can only be directed at a single User.

Edit private message

Update/Note changes the text of a previously sent message

Delete private message

Delete/Note deletes a private message.

Restore private message

Undo/Delete/Note reverts the deletion of a private message.

Community Activities

The Community is essentially a bot, which will only do anything in reaction to actions from Users. The User who first created the Community becomes the first moderator, and can add additional moderators. In general, whenever the Community receives a valid activity in its inbox, that activity is forwarded to all its followers.

Accept follow

If the Community receives a Follow activity, it automatically responds with Accept/Follow. It also adds the User to its list of followers.

Unfollow

Upon receiving an Undo/Follow, the Community removes the User from its followers list.

Announce

If the Community receives any Post or Comment related activity (Create, Update, Like, Dislike, Remove, Delete, Undo), it will Announce this to its followers. For this, an Announce is created with the Community as actor, and the received activity as object. Following instances thus stay updated about any actions in Communities they follow.

Delete Community

If the creator or an admin deletes the Community, it sends a Delete/Group to all its followers.