Lemmy

Lemmy is a link aggregator for the Fediverse, similar to Reddit.

Overview

Property	Value
Language	Rust (backend), TypeScript (frontend)
Database	PostgreSQL
License	AGPL-3.0
Repository	github.com/LemmyNet/lemmy
Documentation	join-lemmy.org/docs

ActivityPub Implementation

Actor Types

Lemmy uses multiple actor types:

Person (User):

{
  "type": "Person",
  "id": "https://lemmy.ml/u/alice",
  "preferredUsername": "alice",
  "inbox": "https://lemmy.ml/u/alice/inbox"
}

Group (Community):

{
  "type": "Group",
  "id": "https://lemmy.ml/c/programming",
  "name": "!programming@lemmy.ml",
  "preferredUsername": "programming",
  "inbox": "https://lemmy.ml/c/programming/inbox",
  "followers": "https://lemmy.ml/c/programming/followers",
  "attributedTo": ["https://lemmy.ml/u/moderator"],
  "publicKey": { ... }
}

Content Types

Page (Post):

{
  "type": "Page",
  "id": "https://lemmy.ml/post/12345",
  "attributedTo": "https://lemmy.ml/u/alice",
  "to": ["https://lemmy.ml/c/programming"],
  "name": "Post Title",
  "content": "<p>Post body</p>",
  "url": "https://external-link.com",
  "commentsEnabled": true,
  "sensitive": false
}

Note (Comment):

{
  "type": "Note",
  "id": "https://lemmy.ml/comment/67890",
  "attributedTo": "https://lemmy.ml/u/bob",
  "inReplyTo": "https://lemmy.ml/post/12345",
  "content": "<p>Great post!</p>"
}

Supported Activities

Activity	Context	Notes
Create	Page, Note	Posts and comments
Update	Page, Note	Editing
Delete	Page, Note	Removal
Like	Page, Note	Upvotes
Dislike	Page, Note	Downvotes
Undo	Like, Dislike	Remove vote
Follow	Group	Join community
Announce	Page	Cross-posting
Block	Person	User blocks
Add/Remove	Moderator	Mod actions

Community Federation

Communities federate when users from other instances:

1. Search for the community
2. Subscribe (Follow the Group)
3. Posts sync to subscribers

┌──────────────┐     Follow      ┌──────────────┐
│ lemmy.ml     │◀────────────────│ lemmy.world  │
│ /c/programming│                │ (subscriber) │
└──────────────┘                 └──────────────┘
       │                                ▲
       │ Announce(Create(Page))         │
       └────────────────────────────────┘

Custom Extensions

Property	Type	Description
commentsEnabled	Boolean	Allow comments
stickied	Boolean	Pinned post
language	String	Content language
sensitive	Boolean	NSFW content

API Endpoints

Actor Endpoints

GET /u/{username}          # Person
GET /c/{community}         # Group
GET /post/{id}            # Page
GET /comment/{id}         # Note

Collections

GET /c/{community}/followers
GET /c/{community}/outbox

Voting System

Unlike Mastodon's Like-only system, Lemmy supports both upvotes and downvotes:

Upvote:

{
  "type": "Like",
  "actor": "https://lemmy.ml/u/alice",
  "object": "https://lemmy.ml/post/12345"
}

Downvote:

{
  "type": "Dislike",
  "actor": "https://lemmy.ml/u/alice",
  "object": "https://lemmy.ml/post/12345"
}

Compatibility Notes

Interoperability

• Posts from Mastodon appear as top-level posts
• Lemmy comments federate as Notes
• Groups work with platforms supporting Group actors

Known Limitations

• Images hosted on Lemmy, not embedded in AP
• Markdown converted to HTML for federation
• Some vote synchronization delays

See Also

• Lemmy Documentation
• Rust Libraries
• Server Software Overview