code: function _getByIndex(indexName, indexData, callback) {
      // generate the index
      var index = indexName + '-' +
        crypto.createHash('sha256').update(indexData).digest('hex');
  
      // fetch the value of the index
      client.get(index, function(err, reply) {
        if(err) {
          return callback(err);
        }
        if(!reply) {
          // index does not exist
          return callback(null, null);
        }
  
        // fetch the actual data
        client.get(reply, _deserializeJson(callback));
      });
    }
docs: utility function to get a Redis-based value based on an index