We tried to use katta and for what its value - we found it very stable, relatively easy to manage (compared to managing a simple vanilla lucine)
The only mistake I can think of is the lack of real-time updates - when we tested it (about 9-10 months ago), it meant updating the index using a separate process (working with hadoop or whatever you have) .. . and replacement with a live index, this was a violation of the transaction for us.
If you are studying distributed lucene, you should really try ElasticSearch or Solandra
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