In simple words: HDFS is good for file-oriented replication. Solr is good for index replication.
Consequently, if atomic file update operations of an application (like Solr) are not atomic on a file level, HDFS is not adequate - like for Solr with live index updates. Running Solr on HDFS (as a file system) will pose limitations due to HDFS properties. Indexing, however, still won't use Hadoop. If you produce indexes and distribute them as finalized, read-only structures (e.g., through Hadoop jobs), HDFS is fine. Solr does not need to be much aware of HDFS. The third one in the picture is records-based replication to be handled by Hbase, Cassandra or Zookeeper, depending on requirements. Cheers, Jürgen
