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Just providing reassignPartitions() was considered, with changes to partition count and replication factor inferred from partitions and brokers present in the assignments argument. This would require the caller to provide an assignment of partitions to brokers, but the AdminClient doesn't provide methods to obtain the necessary information to make an informed decision (what it the load on the brokers, how much free disk space do they have, or how many partitions are they already hosting), so it was felt better to retain alterPartitionCounts() and alterReplicationFactors() for the time beingwhereas currently it's possible to increase the partition count without specifying an assignment. It also suffered from the synchronous/asynchronous API problem.

Similarly a alterReplicationFactors() method, separate from reassignPartitions() was considered, but both require a partition to brokers assignment, and both are implemented in the same way (by writing to the /admin/reassign_partitions znode), so there didn't seem much point in making an API which distinguished them.