Designated Source of Truth

When an operation must update several systems that share no transaction — a local store, a durable log, an acknowledgment to an upstream — a crash between any two updates leaves them disagreeing, and distributed atomicity is expensive or unavailable. Instead, designate exactly one of the systems as the source of truth, treat every other system's state as a checkpoint or cache derived from it, and make recovery a reconciliation: on restart, consult the truth and repair the derived states to match. The write path is ordered so the truth is written before anything irreversible depends on it, letting the truth double as a write-ahead log.

The pattern trades transactional machinery on the hot path for repair logic on the recovery path: correctness becomes eventual-on-restart rather than atomic-per-operation, which is a good trade exactly when crashes are rare and the repair is cheap to compute by replay or comparison. Two costs are structural: the designated truth's own availability and durability become the system's deferred failure mode (the one problem the pattern relocates rather than solves), and every derived store must be safely rebuildable — nothing may live only in a checkpoint. Boundary against Atomic Phases: that pattern encloses co-located steps in one real transaction where a shared transactional store exists; this pattern is for when no such store spans the systems involved, and agreement is manufactured by hierarchy instead.