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Introducing WalShadow: Sub-second Postgres replication to ClickHouse from physical WAL
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ClickHouse released WalShadow, an open-source engine that decodes Postgres physical WAL outside the source database and writes ClickHouse-native blocks directly, achieving roughly 200 ms commit-to-visible latency and 289,000 rows per second in benchmarks.
WalShadow eliminates the need for logical replication slots, Kafka, and intermediate serialization formats, reducing operational overhead and source-database resource consumption compared to traditional CDC pipelines. For teams running Postgres alongside ClickHouse for analytics, it offers a replication path with latency and throughput closer to a physical standby than logical-decoding-based tools. The tradeoff is that it requires access to physical WAL, which most managed Postgres providers do not expose, limiting standalone use to self-managed Postgres or ClickHouse Managed Postgres.
Written by elseif from the cluster below · every claim links back to a sourceThe three things worth knowing
WalShadow consumes Postgres physical WAL directly rather than using logical replication, decoding changes outside the source database in parallel Rust-based decoders.
Benchmarks on c8i.2xlarge instances showed approximately 200 ms commit-to-visible latency and 289,000 rows per second sustained throughput.
Physical WAL access is required, so WalShadow only works with self-managed Postgres or ClickHouse Managed Postgres, where it is in private preview.
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