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Lemmy

Decentralized Reddit alternative. Link aggregation and discussion forums in the Fediverse.

Overview​

Lemmy is a free, open-source social news aggregator and forum that federates via ActivityPub. It provides a Reddit-like experience where anyone can run their own instance while staying connected to the broader network.

Key Features​

Federation​

┌─────────────────────────────────────────────────────────┐
│                    Lemmy Network                         │
├─────────────────────────────────────────────────────────┤
│                                                          │
│  ┌───────────────┐        ┌───────────────┐             │
│  │  lemmy.world  │◄──────►│ lemmy.ml      │             │
│  │   Instance A  │   AP   │  Instance B   │             │
│  └───────┬───────┘        └───────┬───────┘             │
│          │                        │                      │
│          └────────────────────────┘                      │
│                       │                                  │
│             ActivityPub Protocol                        │
│                       │                                  │
│          ┌────────────┴────────────┐                    │
│          │                         │                    │
│  ┌───────▼───────┐        ┌───────▼───────┐            │
│  │   Mastodon    │        │     Kbin      │            │
│  │  (comments)   │        │ (compatible)  │            │
│  └───────────────┘        └───────────────┘            │
│                                                          │
│  Subscribe to communities on any instance               │
│  Mastodon users can participate in discussions          │
│                                                          │
└─────────────────────────────────────────────────────────┘

Community Structure​

┌─────────────────────────────────────────┐
│        Community: !technology@lemmy.ml   │
├─────────────────────────────────────────┤
│                                          │
│  📌 Pinned: Community Rules             │
│                                          │
│  🔥 Hot Posts                           │
│  ├── [1234↑] New open source project    │
│  ├── [567↑]  Linux kernel update        │
│  └── [234↑]  Self-hosting guide         │
│                                          │
│  Moderators: @alice, @bob               │
│  Subscribers: 45,678                     │
│                                          │
│  [Subscribe] [Create Post]              │
│                                          │
└─────────────────────────────────────────┘

Content Features​

FeatureDescription
PostsLinks or text posts
CommentsThreaded discussions
VotingFull scores (+/-) visible
CommunitiesTopic-focused groups
ModerationPer-community moderators
Cross-postingShare to multiple communities
Saved postsBookmark content
BlockingUsers and communities

Interface​

  • Clean design — Mobile-friendly responsive UI
  • Multiple themes — Light, dark, solarized
  • i18n — Many languages supported
  • Markdown — Full formatting support

Self-Hosting​

Requirements​

  • Docker and Docker Compose
  • PostgreSQL
  • Domain name with HTTPS

Docker Installation​

# docker-compose.yml
version: '3'
services:
  lemmy:
    image: dessalines/lemmy:latest
    depends_on:
      - postgres
      - pictrs
    ports:
      - "8536:8536"
    volumes:
      - ./lemmy.hjson:/config/config.hjson
    environment:
      RUST_LOG: "warn,lemmy_server=info,lemmy_api=info"

  lemmy-ui:
    image: dessalines/lemmy-ui:latest
    depends_on:
      - lemmy
    ports:
      - "1234:1234"
    environment:
      LEMMY_UI_LEMMY_INTERNAL_HOST: lemmy:8536
      LEMMY_UI_LEMMY_EXTERNAL_HOST: lemmy.example.com

  postgres:
    image: postgres:15-alpine
    volumes:
      - ./postgres:/var/lib/postgresql/data
    environment:
      POSTGRES_USER: lemmy
      POSTGRES_PASSWORD: password
      POSTGRES_DB: lemmy

  pictrs:
    image: asonix/pictrs:0.4
    volumes:
      - ./pictrs:/mnt

Configuration​

// lemmy.hjson
{
  database: {
    host: postgres
    user: lemmy
    password: password
    database: lemmy
  }
  hostname: "lemmy.example.com"
  email: {
    smtp_server: "smtp.example.com"
    smtp_login: "noreply@example.com"
    smtp_password: "password"
    smtp_from_address: "noreply@example.com"
  }
  setup: {
    admin_username: "admin"
    admin_password: "changeme"
    admin_email: "admin@example.com"
    site_name: "My Lemmy Instance"
  }
}

Ansible Deployment​

git clone https://github.com/LemmyNet/lemmy-ansible.git
cd lemmy-ansible
# Configure inventory and vars
ansible-playbook lemmy.yml

Technical Stack​

ComponentTechnology
BackendRust (Actix-web)
FrontendTypeScript (Inferno)
DatabasePostgreSQL + Diesel
FederationActivityPub
LicenseAGPL-3.0

Why Rust?​

  • Performance — Fast and memory-efficient
  • Safety — Memory-safe by design
  • Reliability — Strong type system

Mobile Apps​

PlatformApps
AndroidJerboa (official), Liftoff, Thunder
iOSMlem, Thunder, Voyager
Cross-platformVoyager (PWA)

Moderation​

Hierarchy​

Site Admins
    │
    ├── Site-wide policies
    ├── Instance federation
    └── Global bans
          │
Community Moderators
    │
    ├── Community rules
    ├── Post/comment removal
    └── Community bans

Tools​

ToolDescription
ReportsUser flagging system
RemovalHide content
BansUser restrictions
FederationAllow/block instances
Auto-modSlur filter, etc.

Comparison with Reddit​

AspectLemmyReddit
OwnershipDistributedCentralized
Open sourceYesNo
AlgorithmNoneHeavy
AdsNoneMany
APIFreePaid/restricted
ModerationDistributedCentralized
Self-hostingYesNo

Finding Instances​

Popular instances:

InstanceFocus
lemmy.worldGeneral
lemmy.mlTech
beehaw.orgCommunity
programming.devDevelopers

Growth​

After Reddit's 2023 API changes, Lemmy saw significant growth:

  • Pre-June 2023: ~100 instances
  • Post-June 2023: 1,500+ instances
  • Monthly active users: 60,000+

See Also​