Out-of-Order Event
An event that reaches a processing system after records whose source event time is later, so arrival order no longer matches the original temporal order.
Network delay, partitions, retries, offline producers, and parallel delivery paths can all reorder a stream. Treating ingestion order as event-time order can therefore corrupt windows, ordering assumptions, or state updates.
The remedy depends on semantics: bounded reordering buffers, watermarks, idempotent state transitions, or a separate late-data path are common options. Resilient backfill for date-partitioned data encounters a related ordering problem when historical data re-enters a live pipeline.