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The segment allocation algorithm reuses an already allocated pending segment if the new allocation request is made for the same parameters: datasource sequence name same interval same value of skipSegmentLineageCheck (false for batch append, true for streaming append) same previous segment id (used only when skipSegmentLineageCheck = false) The above parameters can thus uniquely identify a pending segment (enforced by the UNIQUE constraint on the sequence_name_prev_id_sha1 column in druid_pendingSegments metadata table). This reuse is done in order to allow replica tasks (in case of streaming ingestion) to use the same set of segment IDs. allow re-run of a failed batch task to use the same segment ID and prevent unnecessary allocations