BlueGreenDeployment 的目的不計成本想辦法把上線的 downtime 壓到最低，而且當出問題時 rollback 的時間壓到最低的方法：
One of the challenges with automating deployment is the cut-over itself, taking software from the final stage of testing to live production. You usually need to do this quickly in order to minimize downtime.
Blue-green deployment also gives you a rapid way to rollback - if anything goes wrong you switch the router back to your blue environment.
其實就是直接跑兩個環境 (所以成本比較高)，一套跑舊的一套跑新的，然後在前面的 load balancer 切換：
The blue-green deployment approach does this by ensuring you have two production environments, as identical as possible.