One of the things I like most about the last enhancements- maybe introduced with FEP6, not sure of that- is the possibility to create an extension of the SOLR index. An extension index is, in a nutshell, an index which complements the data provided by the base index. A enlightening explanation is reported in the following IC page: Extending the WebSphere Commerce search base index schema.
According to the Info Center: "... dynamic information such as product inventory or ratings change more frequently. Therefore, it is put into a separate index, where it can be refreshed at different intervals than the base product index". That's awesome and it looks exactly the basic idea under the concept of Inventory Index (introduced with FEP6).
Above is an extraction of the image reported in the IC page already mentioned before.
Using this feature we can finally think to implement re-index strategies with high frequency, pretty useful for dynamic data. The basic bricks used by the WCS team to build up this new feature are- according to my investigation- a custom Search Component (SolrSearchMultipleQueryComponent) and the possibility to make Join between different indexes.
That combination gives to the WCS implementation of SOLR an added value makes the SOLR native Join feature transparent to the developers. Let's review the main things ...