AmbryReplica getReplicaForPartitionOnNode(DataNodeId dataNodeId, String partitionString) {
    // Note: partitionString here is now from partitionId.toPathString()
    AmbryDataNode ambryDataNode = getDataNodeId(dataNodeId.getHostname(), dataNodeId.getPort());
    return ambryDataNodeToAmbryReplicas.getOrDefault(ambryDataNode, new ConcurrentHashMap<>()).get(partitionString);
  }
Return the unique {@link AmbryReplica} for a {@link AmbryPartition} on a {@link AmbryDataNode}.
@param dataNodeId the {@link DataNodeId} on which the ambry replica resides.
@param partitionString the partition id string associated with the {@link AmbryPartition}.
@return the {@link AmbryReplica} associated with the given parameters.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/HelixClusterManager.java