WalShadow: Sub-second Postgres replication to ClickHouse from physical WAL

vmsp•about 3 hours ago

They're not using `wal_level = logical`, which has been the "friendly" way of doing CDC on Postgres since ever, but are going straight to `wal_level = replica` which, afaik, has never really been used to build something atop of except Postgres' own replication.

This is very interesting. I'd never have guessed that it'd make such a difference. I also bet this is the sort of thing that would have never end up being implemented without access to coding agents. Having to figure out these protocol-level details is no longer the huge time sink it was

saisrirampur•about 3 hours ago

Ack, thank you! The idea was to minimize the operational overhead of logical replication (slot growth, slowdowns from reorder buffering, handling advance schema changes) and reducing load on Postgres. This approach lets us purpose-build replication for ClickHouse. Postgres logical replication was primarily designed keeping in mind with Postgres as the target.

There’s also some interesting work happening in core with a similar goal of decoupling logical decoding from the Postgres process. We plan to share learnings from WalShadow with the core and hopefully help bring this to Postgres someday :) https://hacking.postgres.tv/topics/logical-decoding/

DenisM•about 3 hours ago

It’s probably brittle though? Replication implementation has to change in some ways from one version to another.

rgbrgb•about 3 hours ago

would this work with a postgres hosted on supabase? for our company brain mcp thing we mirror postgres data (along with server logs, banking data, slack, etc) in clickhouse but took a very inefficient polling approach since it works with any read-only db connection string. always looking to improve that and get rid of latency though.

ZiiS•about 2 hours ago

Not this quick but better then polling: https://supabase.com/features/supabase-pipelines

WalShadow: Sub-second Postgres replication to ClickHouse from physical WAL

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