* @param $exp Integer: (optional) Expiration time. This can be a number of seconds
* to cache for (up to 30 days inclusive). Any timespans of 30 days + 1 second or
* longer must be the timestamp of the time at which the mapping should expire. It
* @param $exp Integer: (optional) Expiration time. This can be a number of seconds
* to cache for (up to 30 days inclusive). Any timespans of 30 days + 1 second or
* longer must be the timestamp of the time at which the mapping should expire. It