@Override
    public PartitionId getRandomFullyWritablePartition(String partitionClass, List<PartitionId> partitionsToExclude) {
      return null;
    }
Mock clusterMap used when one wants the inputStream input for getPartitionIdFromStream
to be read and have constructed a MockPartitionId from the input
ambry-replication/src/test/java/com/github/ambry/replication/BlobIdTransformerTest.java