# Zero Sync Engine Architecture and ZQL Mechanics

> An exhaustive technical guide to Rocicorp's Zero sync engine, zero-cache Postgres WAL replication, zero-client IndexedDB execution, and ZQL query streaming.

Author: Aaron Boodman
Published: 2026-06-15 | Modified: 2026-07-25

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Zero, developed by Rocicorp and released in 1.0 form in June 2026, is a general-purpose, query-driven synchronization engine for web applications<a href="#cite-1" class="cite-link">[1]</a>. Founded by Aaron Boodman, Zero combines client-side IndexedDB caching (zero-client) with a cloud middleware replica (zero-cache) that monitors PostgreSQL Write-Ahead Logs (WAL)<a href="#cite-1" class="cite-link">[1]</a>. By executing client queries using Zero Query Language (ZQL) through Incremental View Maintenance (IVM), Zero achieves zero-millisecond read response times while preserving server-side authorization authority<a href="#cite-2" class="cite-link">[2]</a>.

## Systemic Architectural Components

Unlike legacy sync frameworks that force applications to download an entire database up-front or manually manage REST/GraphQL caches, Zero introduces query-driven background streaming<a href="#cite-1" class="cite-link">[1]</a>. The architecture comprises three primary layers:

1. **PostgreSQL Database**: The authoritative central source of truth storing persistent application records<a href="#cite-1" class="cite-link">[1]</a>.
2. **zero-cache Middleware**: A stateful cloud node that uses Postgres logical replication (`wal_level = logical`) to maintain a local SQLite read replica. It validates incoming client ZQL query parameters against server authorization rules<a href="#cite-1" class="cite-link">[1]</a>.
3. **zero-client Engine**: A JavaScript runtime library compiled into the client web app. It manages a client-side IndexedDB store, executing queries locally in zero milliseconds while maintaining an active WebSocket connection to `zero-cache`<a href="#cite-1" class="cite-link">[1]</a>.

```
[ Browser (zero-client) ] <-- 0ms ZQL Read --> [ Local IndexedDB ]
            ^                                          |
            |--- Incremental Row Streaming (WS) -------|
            v
[ Cloud (zero-cache) ]    <-- Logical Replication -- [ Postgres WAL ]
```

## ZQL Execution and Optimistic Mutations

When an application invokes a query via `useQuery(zqlQuery)`, the query executes against local memory first, returning matching local records immediately within the next frame<a href="#cite-1" class="cite-link">[1]</a>. In parallel, `zero-client` sends the ZQL subscription parameters to `zero-cache`, which validates authorization, executes the query against its SQLite replica, and streams missing or updated rows over WebSockets<a href="#cite-1" class="cite-link">[1]</a>.

```typescript
// ZQL Query Example in Client Runtime
const zqlQuery = z.issue
  .where('status', '=', 'open')
  .related('assignee')
  .limit(50);

const [issues] = useQuery(zqlQuery); // 0ms execution from local cache
```

Write operations execute locally using optimistic mutations, sending mutation intents to the backend server where business rules reconcile changes before broadcasting authoritative state back down to all subscribed clients<a href="#cite-2" class="cite-link">[2]</a>.


## Frequently Asked Questions

### Q: What is Rocicorp Zero?
Zero, tagged 1.0 stable in June 2026 by Rocicorp, is a query-driven synchronization engine for web apps that connects client-side IndexedDB caching (zero-client) with a Postgres WAL replication middleware (zero-cache).

### Q: How does zero-cache sync with PostgreSQL?
zero-cache connects to Postgres using logical replication (wal_level = logical), listening to Write-Ahead Log changes to update an internal SQLite replica and stream row diffs to subscribed clients over WebSockets.

### Q: What is ZQL (Zero Query Language)?
ZQL is a declarative TypeScript query API that executes instantly in local memory while automatically subscribing the client to real-time server diffs validated against backend authorization rules.