kb-nodebb

name	kb-nodebb
description	Background knowledge about NodeBB, the modern open-source forum software built on Node.js with native ActivityPub federation (core feature since v4.0.0, January 2025). Covers the three actor types (Person user at /u/username, Group category at /category/<cid> implementing FEP-1b12 Group Federation with Announce relay pattern, Application instance actor), the two-level content hierarchy (category as audience → topic as context → post), how topics map to resolvable OrderedCollection contexts per FEP-7888 and posts are Note/Article objects, extensive FEP support (12 implemented: FEP-1b12 Group Federation, FEP-7888 context property, FEP-f228 Backfilling Conversations, FEP-4f05 Soft Deletion, FEP-9098 Custom emojis, FEP-b2b8 Long-form Text, FEP-fe34 Origin-based security, FEP-ae0c Relay Protocols, FEP-11dd Context Ownership, FEP-f15d Context Relocation, FEP-fb2a Actor metadata, FEP-0151 NodeInfo), the tech stack (Node.js/WebSocket/Socket.IO/ MongoDB or Redis or PostgreSQL, GPL-3.0), content discovery (follow- based, URL paste to search, Uncategorized catch-all with auto-pruning, no relay support), per-category federation control with domain allow/deny lists, content pruning (30 days remote content, 7 days remote users), cross-posting between categories (v4.8.0+), hooks-based plugin system (filter/action/static/response hooks), and comparison with Discourse (core AP vs plugin AP). Load when the user asks about NodeBB; implementing a NodeBB-compatible ActivityPub server; how NodeBB categories federate as Group actors; NodeBB's context and audience properties; FEP-7888 conversational contexts in NodeBB; federating with NodeBB instances; how forum software works in the Fediverse; NodeBB's content pruning of remote data; NodeBB vs Discourse federation; or building software that interoperates with NodeBB forums.
user-invocable	false

NodeBB — Complete Reference

Overview

NodeBB is a modern, open-source forum software built on Node.js with native WebSocket real-time updates. Founded in 2014 by Julian Lam, Andrew Rodrigues, and Barış Soner Üşaklı in Toronto, Canada. Licensed GPL-3.0.

Starting with v4.0.0 (released January 20, 2025), NodeBB includes ActivityPub federation as a core feature — not a plugin. This makes NodeBB the first major traditional forum software to ship ActivityPub in its core. The implementation was funded by the NLNet Foundation (NGI Zero Core fund, approved August 2023).

NodeBB organizes content as forum topics (not ephemeral posts). Content is presented chronologically with active topics "bumped" to the top when receiving replies. This is fundamentally different from microblogging platforms — NodeBB brings the structured discussion forum paradigm into the Fediverse.

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1. Technology Stack

Component	Technology
Backend	Node.js
Real-time	WebSockets (Socket.IO)
Database	MongoDB (default), Redis, or PostgreSQL
Frontend	Server-rendered templates + client-side JS
License	GPL-3.0

Database Architecture

NodeBB provides a unified database abstraction layer (hashes, sets, sorted sets, lists) that works identically across all three backends. Data is stored as key-value objects (e.g., user:<uid>).

Multi-database support: Sessions can be separated into a different database instance (e.g., Redis for sessions, MongoDB for data). Redis pub-sub is used for inter-process communication regardless of primary database, enabling horizontal scaling across multiple Node.js processes.

Plugin System

Hooks-based architecture with four hook types:

Type	Behavior
Filters	Take input, modify, return changed value
Actions	Execute side effects, return nothing
Static	Like actions but NodeBB waits for completion
Response	Return HTTP responses

Plugins can mount API routes at /api/v3/plugins. Widget system supports template-and-location based widget areas. Large npm-based plugin ecosystem (SSO, media embedding, advertising, emoji, push notifications, rich-text, galleries, etc.).

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2. ActivityPub Implementation

Why Core, Not Plugin

ActivityPub required extensive architectural changes that couldn't be accomplished via a plugin. It was built directly into the NodeBB core.

Configuration

• Global toggle: ACP > Settings > Federation (ActivityPub)
• New v4 installs: federation enabled by default, all categories federate
• Upgrades from v3: federation disabled by default (opt-in)
• Per-category control: the "fediverse" pseudo-user is granted viewing/posting privileges on a per-category basis
• Domain filtering: allow-list or deny-list modes

Development Timeline

Date	Milestone
Mid-2023	Julian conceives inter-NodeBB communication idea
Aug 2023	NLNet Foundation funding approved
Jun 2024	v4.0.0-alpha
Sep 2024	v4.0.0-beta.1
Nov 2024	RC1
Jan 20, 2025	v4.0.0 stable
2025–2026	Continued releases through v4.9.0+

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3. Actor Types

Actor	Type	Path	Purpose
User	Person	/u/username	Individual user accounts
Category	Group	/category/<cid>	Forum categories, Group Federation
Instance	Application	(instance-level)	Server-level actor

Category as Group Actor (FEP-1b12)

Each category identifies itself as a Group actor. Categories:

• Maintain follow relationships with other actors and remote users
• Can follow other Group actors from different forums
• Federate content via the Announce relay pattern
• Have a unique handle — a "slugified" version of the category name (e.g., "Category About Cats" → category-about-cats)

When a user posts content (new topic or reply), the category wraps it in an Announce activity and distributes to all followers:

User creates post → Create{Note} sent to Category
Category wraps → Announce{Create{Note}} sent to all followers
Remote instances receive → display post in corresponding category

This is the same pattern used by Lemmy communities and Kbin/Mbin magazines. Remote users can follow a NodeBB category the same way they follow a Lemmy community.

The Uncategorized Pseudo-Category

Serves as a catch-all for manually discovered remote content (via URL paste into search). Content lands here automatically before admins move it to appropriate local categories. The Uncategorized category does NOT use the Announce pattern and has automated pruning.

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4. Content Model and Conversational Context

Two-Level Hierarchy

NodeBB maps forum structure to ActivityPub using a two-level hierarchy:

Category (audience) → Topic (context) → Post
     Group actor        OrderedCollection    Note/Article

Every post carries both properties:

• audience → points to the category (Group actor)
• context → points to the topic (resolvable URL returning an OrderedCollection per FEP-7888)

Object Types

NodeBB Concept	AP Object	Notes
Topic	context (OrderedCollection)	Resolvable collection of posts
Post / Reply	Note or Article	Within a context, with inReplyTo for threading
Category	Group actor	Uses audience property

NodeBB builds topics from incoming ActivityPub activities (Create, Question, Listen) and objects (Note, Article).

Context Resolution (FEP-7888)

When NodeBB encounters an object with a context property:

1. Primary: Resolvable context — resolves the URL to retrieve a Collection, parses all objects, organizes them into a single topic. Supports synchronization headers for freshness verification.

2. Fallback: Reply chain traversal — traverses inReplyTo chain upward to the root node, organizes chain objects into a topic.

Reply chain traversal has limitations:

• If any object in the chain becomes unresolvable, the chain breaks
• Only follows the direct reply path, missing sibling branches

Outboxes

User and category outboxes (v4.10.0+) return paginated OrderedCollection containing activities (Create, Like, etc.) with first, last, prev, next navigation.

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5. FEP Support

NodeBB evaluates FEPs using four positions (mirroring Mozilla's Specification Positions): positive, neutral, defer, negative.

Implemented

FEP	Title
FEP-0151	NodeInfo in Fediverse Software
FEP-11dd	Context Ownership and Inheritance
FEP-1b12	Group Federation
FEP-4f05	Soft Deletion
FEP-7888	Demystifying the context property
FEP-9098	Custom emojis
FEP-ae0c	Fediverse Relay Protocols (Mastodon and LitePub)
FEP-b2b8	Long-form Text
FEP-f15d	Context Relocation and Removal
FEP-f228	Backfilling Conversations
FEP-fb2a	Actor metadata
FEP-fe34	Origin-based security model

Partially Implemented

FEP-7458 (Using the replies collection)

Positive (Not Yet Implemented)

FEP-044f, FEP-8b32, FEP-9967, FEP-c0e0

NodeBB is an active participant in the SWICG Forum and Threaded Discussions Task Force, promoting threaded discussion formats in the Fediverse.

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6. Content Discovery and Pruning

Discovery Methods

1. Follow users/categories — search for handles (e.g., [e-mail])
2. URL paste — paste a remote content URL into NodeBB's search bar to pull it in; content lands in Uncategorized
3. Cross-posting (v4.8.0+) — topics from remote categories can be added to local categories

No Relay Support

NodeBB explicitly does NOT support relays. This is intentional — relays "do not guarantee the quality of incoming content." The design prioritizes relationship-based content distribution over algorithmic or broadcast discovery.

Content Pruning

NodeBB automatically manages remote content to prevent database bloat:

Setting	Default	Condition for removal
Remote content retention	30 days	Topics with no local engagement (votes or replies)
Remote user retention	7 days	Accounts with no posts and no follow relationships

Both settings are configurable via the admin panel.

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7. NodeBB vs Discourse (Federation)

Aspect	NodeBB	Discourse
Language	Node.js	Ruby on Rails
AP integration	Core feature (v4+)	Plugin (discourse-activity-pub)
AP since	January 2025	2023 (plugin)
Actor model	Group (categories)	Group (categories)
Context support	FEP-7888 (resolvable)	Separate approach
FEPs implemented	12	Fewer
Real-time	Native WebSockets	Long polling / MessageBus
Database	MongoDB/Redis/PostgreSQL	PostgreSQL only
License	GPL-3.0	GPL-2.0

Both bring threaded forum discussions to the Fediverse, using categories as Group actors. NodeBB's core integration means tighter coupling with the ActivityPub protocol and more extensive FEP adoption. Discourse's plugin approach allows easier opt-in/opt-out but has less deep integration.

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8. Interoperability Notes for Implementers

1. Categories are Group actors — follow a NodeBB category by sending Follow to the Group actor at /category/<cid>. The category relays all content via Announce, identical to Lemmy communities. Category handles are slugified names (e.g., general-discussion).

2. Posts carry both context and audience — context points to the topic (resolvable OrderedCollection), audience points to the category (Group actor). This two-level hierarchy is NodeBB-specific and differs from Lemmy's single-level approach.

3. Topics are resolvable collections — if your software encounters a NodeBB context URL, resolve it to get the full topic as an OrderedCollection. This is the recommended way to backfill conversations (per FEP-7888 and FEP-f228).

4. Note and Article objects accepted — NodeBB processes both Note and Article types when building topics. Send whichever is appropriate for your content.

5. No relay federation — NodeBB will not participate in relay subscriptions. Content exchange requires explicit follow relationships.

6. Remote content is pruned — NodeBB deletes remote content after 30 days (configurable) if it has no local engagement. If your software references old NodeBB content, it may have been pruned from remote instances that imported it. Similarly, content your software sends to NodeBB will be pruned if no local user engages with it.

7. Cross-posting — since v4.8.0, a single topic can appear in multiple local categories. This means the same remote content may be accessible via different category Group actors on the same instance.

8. Soft deletion (FEP-4f05) — NodeBB supports soft deletion. When content is deleted, a Delete activity with a Tombstone object is sent rather than simply disappearing.

9. Custom emojis (FEP-9098) — NodeBB supports federated custom emoji. Remote emoji in posts and chats are rendered (since v4.7.0).

10. Content in Uncategorized has no audience — manually discovered content landing in the Uncategorized pseudo-category may not have a proper audience property pointing to a Group actor. This content is not Announced to followers.