A microservice that ran while nothing used it
A Rust thumbnail service wrote 200px and 800px variants into documents.thumbnails_json that every resolver ignored in favour of the full-res original.
Built, and nothing used it
Shipped, typechecked, dependency-injected, unreachable. Nothing ever called it.
The service generates 200px and 800px variants on every upload and stores them in documents.thumbnails_json. Every organisation-facing resolver — resolveLogoUrl, resolveCoverUrl, gallery preview, full gallery, staff photos — signed document.storage_key, the full-res original. Only the Documents Center path ever asked for a variant.
So it was deployed, healthy, burning CPU on every upload, and its entire output was dead weight in storage. It surfaced as a Lighthouse complaint about image delivery, not as anything anyone would recognise as "the thumbnailer isn't wired up."
Two things fell out. An ordering bug in getSignedUrlWithoutAccessCheck returned external HTTP URLs before thumbnail resolution ran, so an externally hosted image could never serve a variant even after backfilling — a three-line fix. And the seeded imagery is external Unsplash URLs the service can't touch at all, which needed client-side URL-param resizing instead.
What changed in how I work: the fix included per-image logging on both sides, because the failure mode wasn't a crash, it was silence. A service with no per-request observability can be bypassed indefinitely and still look green on its health check.