Cassandra cache

The bavarian mapping agency [(Bayerische Vermessungsverwaltung)] (http://vermessung.bayern.de) uses [Apache-Cassandra] (http://cassandra.apache.org/) as tilestore for the tiles used in the [BayernAtlas] (http://geoportal.bayern.de/bayernatlas).

We produce tiles directly into the cache, so we had to add a "read_only"-swich to the cache to avoid storing empty tiles into the cache.