Today we are improving the reclamation process with the addition of a two-minute warning, formally known as a Spot Instance Termination Notice.
不過目前不是 trigger，而是透過 polling 的方式提供：
The Termination Notice is accessible to code running on the instance via the instance’s metadata at
http://169.254.169.254/latest/meta-data/spot/termination-time. This field will become available when the instance has been marked for termination (step 3, above), and will contain the time when a shutdown signal will be sent to the instance’s operating system.
建議的 polling 時間是五秒鐘一次：
We recommend that interested applications poll for the termination notice at five-second intervals.