I have always felt that architecturally it makes sense to keep your search index as close to your data as possible, therefore I love how Basho have integrated Solr with Riak 2.0. Here is a great overview of their "Yokozuna" search solution by Basho's Eric Redmond:

As I find myself hitting feature shortcomings in Redis ^{1 2}, I am looking harder at Riak and in particular the forthcoming 2.0 release.³

References