Eventual Coherence: Preserving Causal Validity in Replicated Systems

The delete-update conflict, where one device deletes a resource while another concurrently updates it, is an underdetermined challenge in multi-device coordination. Existing approaches treat it as a synchronization problem rather than a semantics problem and so sacrifice at least one of three properties: availability through coordination, causal validity through last-write-wins semantics, or storage boundedness through conflict-free replicated data types. We argue this tradeoff arises from a category error: treating topology operations and content operations as equivalent and subject to uniform reconciliation semantics. Eventual coherence is the correctness criterion we introduce to resolve it: each replica is evaluated against its own causal history rather than required to agree with the others. Control plane operations establish resource topology and reconcile at synchronization boundaries; data plane operations modify resource content and propagate in near-real time. Reconciliation is governed by two principles: local deletion finality (a device that has deleted a resource permanently discards subsequent updates to it) and server-level upsert semantics (a server receiving an update for a deleted resource restores it, so devices with active causal participation keep receiving updates). Neither device is forced to agree with the other; each retains the outcome its own causal history entitles it to, and tombstone storage is bounded by outstanding causal divergence rather than accumulated deletion history. The protocol rests entirely on Lamport clock ordering, requiring no vector clocks, no operational transformation, and no physical clock synchronization, and is implementable over standard HTTP. We prove the protocol guarantees per-device causal validity, convergence to causally valid states, update preservation, and local deletion stability, and that inter- replica agreement and per-device causal validity are formally orthogonal criteria, making eventual coherence and eventual consistency incomparable rather than points on the same hierarchy. We validate the protocol against Polyglot, a proof-of-concept multi-device conversation management system.