Federated search is an information retrieval technology that allows the simultaneous search of multiple searchable resources. A user makes a single query request which is distributed to the search engines, databases or other query engines participating in the federation.Source: https://en.wikipedia.org/wiki/Federated_search
More: http://www.baileysolutions.co.uk/uploads/Federated_searching.pdf
Xem:
http://www.flax.co.uk/blog/2016/01/25/xjoin-solr-part-1-filtering-using-price-discount-data/
http://www.flax.co.uk/blog/2016/01/29/xjoin-solr-part-2-click-example/