title: Mahout & Lucene Version Compatibility
body: <p>I am trying to use Mahout to do some analysis on the term vectors stored in my Solr/Lucene index. Unfortunately, it seems that the latest Mahout release is behind the latest Solr/Lucene release.</p>
  
  <p>My Solr/Lucene installation is 4.10.3. As far as I can tell, the latest Mahout release (1.0) expects Lucene indexes at version 4.6.1.</p>
  
  <p>When I run <code>mahout lucene.vector</code> I get the error:</p>
  
  <pre><code>Exception in thread "main" org.apache.lucene.index.IndexFormatTooNewException: Format version is not supported (resource: MMapIndexInput(path="/path/to/data/index/segments.gen")): -3 (needs to be between -2 and -2)
  </code></pre>
  
  <p>I have tried two things so far to tackle this problem:</p>
  
  <p>First, I edited my solrconfig.xml file to say:</p>
  
  <pre><code>&lt;luceneMatchVersion&gt;4.6.1&lt;/luceneMatchVersion&gt;
  </code></pre>
  
  <p>delete my indexed data, and built a clean index from the original documents. This has done nothing to fix the error.</p>
  
  <p>So secondly, I tried to change the <code>lucene.version</code> in the Mahout pom.xml file to 4.10.3 and recompile the binary to see if the capabilities had been added yet. I knew this was unlikely to work, but tried anyway.</p>
  
  <p>My question is, how do I appropriately change the Lucene version that Solr uses for writing index files if it is not the above luceneMatchVersion setting in solrconfig.xml?</p>