When a password sync is 'partly done', it's a bug: converging on one canonical flow

TL;DR Two apps each had their own password-change/reset logic, plus config toggles to enable/disable backends. That combination quietly allowed partial syncs. Fix: one shared engine, one canonical order , backends mandatory (no config-disable), and every attempt written to an audit log. Lesson: for a write that spans several systems, "configurable steps" is a footgun. Make the flow fixed and make failure loud. The setup A user changes their password. Behind the scenes that single password has to land in several systems — a directory, an external database, and the app's own store. Two separate apps were doing this, each with slightly different code, and each with config flags like sync_oracle => true|false to turn backends on and off. Sounds flexible. It's actually a trap. Why configurable backends are a footgun TL;DR: a password that updates 2 of 3 systems is worse than one that updates none — because now the systems disagree and nobody gets an error. The moment a backend is optionally skippable, "skipped" and "failed" blur together. Someone flips a flag in one environment, forgets it in another, and now prod and staging run different flows. Debugging a login failure means first reverse-engineering which steps actually ran. Before After Each app had its own reset logic One shared engine, both apps call it Backends toggle via config Backends are mandatory, always run Order implicit / differed per app One canonical order: New directory → external DB → local app "Did it sync?" answered by guessing Every attempt logged with per-service status The canonical order The order isn't cosmetic. It runs most-authoritative-first, so if a downstream step fails you haven't already told the user their new password works. change/reset request | v [ New Directory ] --ok--> [ External DB ] --ok--> [ Local App store ] | | | fail fail fail | | | +----> stop, record per-service status, surface the failure Same engine, same order, both apps. A change API and a reset flow are just two entr