title: Solr 4.2 - what is _version_field?
body: <p>I am getting below error in my solr configuration.</p>
  
  <pre><code>Caused by: org.apache.solr.common.SolrException: Unable to use updateLog: _version_field must exist in schema, using indexed="true" stored="true" and multiValued="false" (_version_ does not exist)
          at org.apache.solr.core.SolrCore.&lt;init&gt;(SolrCore.java:806)
          at org.apache.solr.core.SolrCore.&lt;init&gt;(SolrCore.java:619)
          at org.apache.solr.core.CoreContainer.createFromLocal(CoreContainer.java:1021)
          at org.apache.solr.core.CoreContainer.create(CoreContainer.java:1051)
          ... 10 more
  Caused by: org.apache.solr.common.SolrException: Unable to use updateLog: _version_field must exist in schema, using indexed="true" stored="true" and multiValued="false" (_version_ does not exist)
          at org.apache.solr.update.UpdateLog.init(UpdateLog.java:245)
          at org.apache.solr.update.UpdateHandler.initLog(UpdateHandler.java:84)
          at org.apache.solr.update.UpdateHandler.&lt;init&gt;(UpdateHandler.java:134)
          at org.apache.solr.update.DirectUpdateHandler2.&lt;init&gt;(DirectUpdateHandler2.java:95)
          at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
          at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
          at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
          at java.lang.reflect.Constructor.newInstance(Constructor.java:532)
          at org.apache.solr.core.SolrCore.createInstance(SolrCore.java:526)
          at org.apache.solr.core.SolrCore.createUpdateHandler(SolrCore.java:597)
          at org.apache.solr.core.SolrCore.&lt;init&gt;(SolrCore.java:790)
          ... 13 more
  Caused by: org.apache.solr.common.SolrException: _version_field must exist in schema, using indexed="true" stored="true" and multiValued="false" (_version_ does not exist)
          at org.apache.solr.update.VersionInfo.getAndCheckVersionField(VersionInfo.java:57)
          at org.apache.solr.update.VersionInfo.&lt;init&gt;(VersionInfo.java:83)
          at org.apache.solr.update.UpdateLog.init(UpdateLog.java:242)
          ... 23 more
  </code></pre>
  
  <p>I wanted to know that what is <code>_version_field</code>, and why its must required ?</p>
  
  <p>Can anybody suggest me on this??</p>