For the installation of apache solr integration module in Drupal we need to install solr.
The must do thing is we need to change the solr schema.xml and configure.xml files with the files in apache solr integration module.
can any body explain the reason behind this change.
The schema.xml and solrconfig.xml files packaged with Solr are pre-configured for an example installation which coincides with the tutorial on the Solr Tutorial website. You'll want to replace these files with those provided with the Apache Solr Drupal module in order to properly index the fields from content extracted off of your drupal system.
If you have any specific questions related to the differences between the example configs and drupal module configs, please post as a follow-up question. Thanks!