Asynchronous database replication can cause users to read progressively older values when successive reads hit replicas with different replication lags — a phenomenon likened to traveling through a wormhole. Monotonic reads is a consistency guarantee that ensures a user always sees data moving forward in time. The practical solution is to make each user's reads sticky to a specific replica, achieved by hashing the user ID to deterministically select a replica rather than picking one at random.
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