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Planet PostgreSQL0 Hot0 comments17 min read
The Shadow Knows
ClickHouse's newly released WalShadow tool replicates PostgreSQL data into ClickHouse by parsing the raw physical WAL stream instead of using PostgreSQL's logical decoding subsystem. The piece dissects what logical decoding actually provides — historic catalog snapshots via cmin/cmax resolution, transaction reassembly across interleaved WAL records, TOAST reconstruction, and a consistent exported-snapshot start point — and shows that WalShadow reimplements most of this machinery itself, including running a shadow PostgreSQL instance as its catalog authority. It highlights documented gaps in WalShadow (silent divergence risks on bootstrap and unchanged-TOAST updates, no cross-table transaction ordering, unsupported prepared transactions) and argues the deeper structural risk: physical WAL and heap tuple formats are not stable interfaces, unlike the versioned logical replication protocol. A recent renaming of TOAST-related internals ahead of PostgreSQL 19 illustrates how these internal formats shift without notice. WalShadow's daemon refuses to run against unaudited major versions, meaning users' ability to upgrade PostgreSQL becomes gated by the vendor's audit schedule. The recommendation is to use logical decoding unless you are, or are working with, a vendor who controls both the source server and the replication consumer.
Source: https://postgr.es/p/9vu. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.
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