Alert user-visible freshness separately from ingest health

An ingestion watermark can be healthy while users still see stale derived views. For every asynchronous projection, measure elapsed time from source acceptance or session update to the durable projection receipt, and alert on the lag distribution for a sufficiently sized cohort. Keep that alert separate from API-read latency and worker capacity: each has a different owner action. Do not page solely on queued-job count; use actionable live-work age, affected-session count, and absence of completion progress to avoid false alarms during harmless backlog.

Applies when: a dashboard depends on queued normalization or projection work after durable ingestion. Keywords: data freshness queue latency projection receipt alert hygiene worker backlog