---
title: Real-Time Sync with Zero
description: How SurfSense uses Rocicorp Zero for instant real-time data synchronization
---

# Real-Time Sync with Zero

SurfSense uses [Rocicorp Zero](https://zero.rocicorp.dev/) for real-time data synchronization. Zero continuously replicates data from PostgreSQL to a local cache on each client, enabling instant UI updates for notifications, documents, connectors, chat messages, and comments.

## How It Works

Zero runs a **zero-cache** server that sits between PostgreSQL and the browser:

1. **zero-cache** replicates data from PostgreSQL into a local SQLite replica using logical replication
2. The browser connects to zero-cache via WebSocket and syncs relevant data locally
3. When data changes in PostgreSQL (e.g., a new notification), zero-cache pushes the update to all connected clients instantly
4. Queries run against local data first for instant results, then update when server data arrives

## Architecture

| Component | Role |
|-----------|------|
| **PostgreSQL** | Source of truth (with `wal_level=logical`) |
| **zero-cache** | Replicates Postgres → SQLite, serves client sync via WebSocket |
| **Browser** | Stores synced data locally, runs queries against local cache |

## Configuration

### Docker Deployment

zero-cache is included in the Docker Compose setup. The key environment variables are:

| Variable | Description | Default |
|----------|-------------|---------|
| `ZERO_CACHE_PORT` | Port for the zero-cache service | `5929` (prod) / `4848` (dev) |
| `ZERO_ADMIN_PASSWORD` | Password for the zero-cache admin UI and `/statz` endpoint | `surfsense-zero-admin` |
| `ZERO_UPSTREAM_DB` | PostgreSQL connection URL for replication | Built from `DB_*` vars |
| `NEXT_PUBLIC_ZERO_CACHE_URL` | URL the frontend uses to connect to zero-cache | `http://localhost:<ZERO_CACHE_PORT>` |

### Manual / Local Development

If running the frontend outside Docker (e.g., `pnpm dev`), you need:

1. A running zero-cache instance pointing at your PostgreSQL database
2. `NEXT_PUBLIC_ZERO_CACHE_URL` set in your `.env` file (default: `http://localhost:4848`)

### Custom Domain / Reverse Proxy

When deploying behind a reverse proxy, set `NEXT_PUBLIC_ZERO_CACHE_URL` to your public zero-cache URL (e.g., `https://zero.yourdomain.com`). The zero-cache service must be accessible via WebSocket from the browser.

### Database Requirements

zero-cache connects to PostgreSQL using logical replication. The database must meet these requirements:

1. **`wal_level = logical`** — already configured in the bundled `postgresql.conf`
2. **The database user must have `REPLICATION` privilege** — required for creating logical replication slots

In the default Docker setup, the `surfsense` user is a PostgreSQL superuser and has all required privileges automatically.

**For managed databases** (RDS, Supabase, Cloud SQL, etc.) where the app user may not be a superuser, you need to grant replication privileges:

```sql
ALTER USER surfsense WITH REPLICATION;
GRANT CREATE ON DATABASE surfsense TO surfsense;
```

The `REPLICATION` privilege allows zero-cache to create a logical replication slot for streaming changes. The `CREATE` privilege allows zero-cache to create internal schemas (`zero`, `zero_0`) for its metadata.

## Synced Tables

Zero syncs the following tables for real-time features:

| Table | Used By |
|-------|---------|
| `notifications` | Inbox (comments, document processing, connector status) |
| `documents` | Document list, processing status indicators |
| `folders` | Nested folder tree for organizing documents |
| `search_source_connectors` | Connector status, indexing progress |
| `new_chat_messages` | Live chat message sync for shared chats |
| `chat_comments` | Real-time comment threads on AI responses |
| `chat_session_state` | Collaboration indicators (who is typing) |

## Troubleshooting

- **zero-cache not starting**: Check `docker compose logs zero-cache`. Ensure PostgreSQL has `wal_level=logical` (configured in `postgresql.conf`).
- **Frontend not syncing**: Open DevTools → Console and check for WebSocket connection errors. Verify `NEXT_PUBLIC_ZERO_CACHE_URL` matches the running zero-cache address.
- **Stale data after restart**: zero-cache rebuilds its SQLite replica from PostgreSQL on startup. This may take a moment for large databases.

## Learn More

- [Rocicorp Zero Documentation](https://zero.rocicorp.dev/docs)
- [Zero Schema Reference](https://zero.rocicorp.dev/docs/schema)
- [Zero Deployment Guide](https://zero.rocicorp.dev/docs/deployment)