---
title: Core Concepts
description: "The building blocks of NoLag realtime infrastructure: project hierarchy, topics and pub/sub, rooms, presence, lobbies, filters, quality of service, webhooks, and access control."
---

# Core Concepts

These are the building blocks every NoLag app is made of. Read them in order for a full mental model, or jump straight to the one you need.

## In this section

- [Project Hierarchy](/docs/concepts/hierarchy) explains how Projects, Apps, Rooms, and Topics fit together.
- [Topics & Pub/Sub](/docs/concepts/topics) covers the publish-subscribe model, topic patterns, and wildcards.
- [Rooms](/docs/concepts/rooms) organize topics and manage message namespaces.
- [Presence](/docs/concepts/presence) tracks who is online and handles join and leave events in realtime.
- [Lobbies](/docs/concepts/lobbies) observe presence across many rooms at once.
- [Filters](/docs/concepts/filters) narrow message delivery to specific entities.
- [Quality of Service](/docs/concepts/qos) covers at-most-once, at-least-once, and exactly-once delivery.
- [Webhooks](/docs/concepts/webhooks) use hydration and trigger webhooks to integrate external systems.
- [Access Control](/docs/concepts/acl) provides fine-grained per-topic permissions.

## Next

Ready to build? Follow the [Quick Start](/docs/getting-started), or pick a [Guide](/docs/guides/chat-app).