code: function _createIndex(indexName, indexData, value, ex, cb) {
      var callback = cb;
      var expiry = ex;
      if(!callback) {
        expiry = null;
        callback = ex;
      }
  
      // generate the index value
      var index = indexName + '-' +
        crypto.createHash('sha256').update(indexData).digest('hex');
  
      client.set(index, value, function(err, reply) {
        if(err) {
          return callback(err);
        }
        if(expiry) {
          return client.expire(index, expiry, function(err) {
            callback(err, reply);
          });
        }
        callback(err, reply);
      });
    }
docs: utility function to create a Redis-based index to a particular value