Buckets records by a partition key. getKey is accepted to keep the call shape symmetric with
the rest of the diff pipeline (callers thread a stable record identity through partition + rollup);
partitioning itself only needs getPartition. Returns a Map keyed by partition string, each value
the array of records that fall in that partition. Records arrive in the order encountered — order
is preserved within a bucket and never relied upon by the rollup (see partitionRollupHash).
Buckets records by a partition key.
getKeyis accepted to keep the call shape symmetric with the rest of the diff pipeline (callers thread a stable record identity through partition + rollup); partitioning itself only needsgetPartition. Returns a Map keyed by partition string, each value the array of records that fall in that partition. Records arrive in the order encountered — order is preserved within a bucket and never relied upon by the rollup (seepartitionRollupHash).