zerolite

Drop-in replacement for the Docker-based development backend that Rocicorp's Zero requires. Instead of running PostgreSQL, zero-cache, and MinIO in Docker containers, zerolite bundles everything into a single process using PGlite (PostgreSQL compiled to WASM).

The goal is simple: bun install && bun dev with zero system dependencies.

How it works

zerolite starts four things in one process:

1. A PGlite instance (full PostgreSQL 16 running in-process via WASM)
2. A TCP proxy that speaks the PostgreSQL wire protocol, including logical replication
3. A zero-cache child process that connects to the proxy thinking it's a real Postgres server
4. A minimal S3-compatible HTTP server for file uploads

The trick is in the TCP proxy. zero-cache needs logical replication to stay in sync with the upstream database. PGlite doesn't support logical replication natively, so zerolite fakes it. Every mutation is captured by triggers into a changes table, then encoded into the pgoutput binary protocol and streamed to zero-cache through the replication connection. zero-cache can't tell the difference.

The proxy also handles multi-database routing. zero-cache expects three separate databases (upstream, CVR, change), but PGlite is a single database. zerolite maps database names to schemas, so zero_cvr becomes the zero_cvr schema and zero_cdb becomes zero_cdb.

Install

npm install zerolite

or with bun:

bun add zerolite

You also need @rocicorp/zero installed in your project for the zero-cache binary.

Usage

import { startZeroLite } from 'zerolite'

const { config, stop } = await startZeroLite({
  pgPort: 6434,
  zeroPort: 5849,
  s3Port: 10201,
  migrationsDir: 'src/database/migrations',
  seedFile: 'src/database/seed.sql',
})

// your app connects to zero-cache at localhost:5849
// database is at postgresql://user:password@localhost:6434/postgres

// when done
await stop()

All options are optional and have sensible defaults. See src/config.ts for the full list.

What gets faked

The proxy intercepts several things to convince zero-cache it's talking to a real PostgreSQL server with logical replication enabled:

• IDENTIFY_SYSTEM returns a fake system ID and timeline
• CREATE_REPLICATION_SLOT persists slot info in a local table and returns a valid LSN
• START_REPLICATION enters streaming mode, encoding changes as pgoutput binary messages
• current_setting('wal_level') always returns logical
• pg_replication_slots queries are redirected to a local tracking table
• SET TRANSACTION SNAPSHOT is silently accepted (PGlite doesn't support imported snapshots)
• ALTER ROLE ... REPLICATION returns success
• READ ONLY is stripped from transaction starts to avoid PGlite serialization issues

The pgoutput encoder produces spec-compliant binary messages: Begin, Relation, Insert, Update, Delete, Commit, and Keepalive. All column values are encoded as text (typeOid 25), which zero-cache handles fine since it re-maps types downstream anyway.

Tests

78 tests across 5 test files covering the full stack from binary encoding to TCP-level integration:

bun test

The test suite includes a zero-cache compatibility layer that decodes pgoutput messages into the same typed format that zero-cache's PgoutputParser produces, validating end-to-end compatibility.

Limitations

This is a development tool. It is not suitable for production use.

• PGlite runs single-threaded. All queries are serialized through a mutex. This is fine for development but would be a bottleneck under real load.
• Column types are all encoded as text in the replication stream. Zero-cache handles this, but other pgoutput consumers might not.
• Triggers add overhead to every write. Again, fine for development.
• PGlite stores data on the local filesystem. No replication, no backups, no high availability.
• The S3 server is just a filesystem wrapper. No multipart uploads, no ACLs, no versioning.

Project structure

src/
  index.ts              main entry, orchestrates startup
  config.ts             configuration with defaults
  pg-proxy.ts           tcp proxy with query rewriting
  pglite-manager.ts     pglite instance and migration runner
  s3-local.ts           minimal s3 http server
  replication/
    handler.ts          replication protocol state machine
    pgoutput-encoder.ts binary pgoutput message encoder
    change-tracker.ts   trigger installation and change reader

License

MIT