Event-driven backfill needs a reseed-from-state companion
Designing side-effects that fire on a state-change event (a link added, a person created, a connection established).
Hooks that fire on edge events — when X is added, when Y is connected — silently miss the case where pre-existing items in the system should also trigger the same side-effect. Concrete example: a lazy-backfill where adding an email link to a person triggers a search across every IMAP inbox. Works perfectly for new links added after the second inbox is connected. But every link that existed before the second inbox came online never gets searched in it. The trigger condition is link-added, the new state is inbox-added. Build a reseed-from-current-state companion that iterates the existing items and enqueues the same side-effect for each. Same handler, same queue, different driver. Without it the system seems consistent until you trace why an inbox never produces results — then the asymmetry surfaces.
Whenever a hook is wired to event A triggering side-effect on field B, ask immediately: what happens when a new instance of A's PARTNER state arrives later? Build a reseed CLI or admin endpoint that iterates over current B and enqueues the side-effect once for each. Cost is small; without it the system silently misses entire categories of bootstrap.