DuckDB’s Quack Protocol Enables Client-Server with Multiple Concurrent Writers

DuckDB has released the Quack remote protocol, enabling DuckDB instances to communicate in a client-server setup with support for multiple concurrent writers. The protocol is designed to be simple, fast, and builds on proven technologies like HTTP.

While DuckDB’s in-process architecture excels for single-process data science workloads, concurrent writes to the same database file from multiple processes were a pain point. Common workarounds included custom RPC solutions, Arrow Flight SQL, MotherDuck’s proprietary protocol, or even switching to PostgreSQL (optionally running DuckDB via pg_duckdb). Quack is DuckDB’s official first-party solution.

How It Works

Both DuckDB instances need the Quack extension, currently available in the core_nightl (nightly) build. The protocol is symmetric: any DuckDB instance can act as client or server. The server exposes an HTTP endpoint; the client connects to it. No special server binary is required—just a running DuckDB process with the extension loaded.

Key design choices:

• Protocol is built on HTTP, making it firewall-friendly and debuggable with standard tools.
• Supports bulk operations and small transactions with low latency.
• No legacy constraints—designed from scratch in 2026, learning from Arrow Flight SQL and others.

Use Cases

• Multiple telemetry-collecting processes inserting into the same DuckDB database.
• A dashboard querying the same tables concurrently without file-locking issues.
• Enabling DuckDB in traditional client-server deployments without external middleware.

This release marks a shift for DuckDB, which previously emphasized its in-process nature. The team acknowledges user demand as the driver: “We see DuckDB as a universal data wrangling tool. If this means having a client-server protocol in addition to the in-process capabilities – fine.”

Current status: Extension is available in the nightly build. Expect a stable release in the coming months.

For a deep dive into the protocol internals, including the full research paper reference and a step-by-step setup guide, read the original announcement.