KIP-1127 — Flexible Windows for Late Arriving Data

Discussion Streams

Adds a new `FlexibleWindows` window type to Kafka Streams that retains open windows indefinitely until explicitly closed by a grace period expiry or new records, rather than discarding late-arriving records that fall outside a fixed window end time. Existing Kafka Streams window types (tumbling, hopping, sliding, session) all rely on event-time boundaries and discard truly late data, which breaks use cases such as micro-batching by offset order where no record should ever be dropped.

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