pg_crdt - an experimental CRDT extension for Postgres

Summary

Supabase announces pg_crdt, an experimental Postgres extension that adds CRDT support as a data type for collaborative and offline-oriented applications. It frames CRDTs as structures whose updates can be merged in any order to reach the same final state, avoiding the last-write-wins behavior of ordinary arrays. A content column can use crdt.ydoc, backed by Yjs, while the extension also supports Automerge through Rust libraries wrapped with pgx. The post contrasts storing CRDTs directly in Postgres with making Realtime an authoritative server, favoring the database-centered approach for simpler architecture and broader tooling, while noting that the decision is unsettled and pg_crdt is not released on the Supabase platform. Known limitations include full-document WAL broadcasts, increased WAL and dead tuples for frequently updated documents, and serialization overhead for large CRDT values.

Takeaways

1. 1

   CRDT updates are commutative and idempotent in pg_crdt, so they can be applied in different orders or multiple times while producing the same result.

2. 2

   The extension supports Yjs and Automerge, whose Rust implementations can run in both browser and Postgres environments through the extension.

3. 3

   Postgres integration currently has major costs: WAL broadcasts include the complete document, frequent updates create substantial WAL and dead tuples, and large CRDT values add serialization overhead.