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

ClickHouse has announced WalShadow, an open-source replication engine that transfers PostgreSQL data to ClickHouse by consuming physical write-ahead log (WAL) streams rather than using logical replication. It decodes changes outside the source database, tracks schema through a shadow PostgreSQL catalog, batches records into ClickHouse-native blocks, and inserts them concurrently, aiming to reduce source overhead while supporting initial loads, recovery, switchovers, and schema changes. In reported benchmarks, WalShadow delivered roughly 200-millisecond commit-to-visible latency and sustained 289,000 rows per second, compared with about 10 seconds and 120,000 rows per second for PeerDB under the tested conditions. Because managed PostgreSQL services generally do not expose physical WAL, WalShadow is also entering private preview for ClickHouse Managed Postgres, where ClickHouse can integrate it into the replication layer. The launch builds on ClickHouse’s broader effort to combine PostgreSQL transactional workloads with ClickHouse analytics in a more unified data stack.

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