PostgreSQL Logical Replication's Decentralized Design Complicates Monitoring Builds

Engineers building a PostgreSQL logical replication monitoring system across versions 10 through 17 encountered a fundamental architectural challenge: replication metadata is split between publisher and subscriber, with neither side holding the complete picture. Publishers track active WAL senders and lag metrics, while subscribers store subscription state and sync progress, forcing developers to query multiple instances to assemble a full topology view. Lag data exists exclusively on the publisher side, meaning an offline subscriber leaves lag values entirely invisible and indistinguishable from other failure states. In one production environment, a single publication served up to 12 subscribers across different hosts, requiring a two-tier caching strategy to maintain an accurate topology. The article argues these are deliberate design decisions that prioritize publisher scalability, but their implications only become apparent when building monitoring or observability tooling on top of replication.