Sitecore will point to Solr Cores by their “name”, commonly called an Index. These “pointers” are stored in various Sitecore configuration files, whereby they set the reference to the Solr Core as an “index id”. This typically looks like the following:
<index id="sitecore_fxm_web_index" type="Sitecore.ContentSearch.SolrProvider.SolrSearchIndex, Sitecore.ContentSearch.SolrProvider">
Along with the connectionString (Sitecore 9+ or 8.2 Update 7) or the baseAddress within the Sitecore.ContentSearch.Solr.DefaultIndexConfiguration.config file, is how Sitecore can connect to, and properly communicate with Solr.
It is the combination of the endpoint declared in the connectionString (or baseAddress) and Index ID within the configuration files that makes Solr integration possible, so if you run into communication issues, it will be worthwhile to check these areas.
There may be instances in which you want to change the index name within the Sitecore configuration files. This could be due to several reasons such as hosting two Solr Cores of the same index type, but different name, on the same Solr Server (ex. sitecoreblue_web_index and sitecoregreen_web_index in a Blue/Green deployment scenario). Or, you may have distinct Solr Core names and need to update your Sitecore configuration to ensure connectivity.
Ideally, you should use patch configs instead of changing the values in the config files, but knowing these locations is critical.
The table below provides the Solr Core Name (Index) and its configuration file location within Sitecore 8.2 as an example. Note, that some config files may have more than 1 index listed. For Sitecore 9, you would follow the same methodology for identifying which config files have the Solr index, as these typically include “Solr” in their name.
|Solr Index Name||Sitecore Configuration File Location|