ASG + ALB Health Checks (ELB health checks)

What it is: Auto Scaling can use load balancer health checks (ALB target health) as part of its health evaluation.

What it’s for:

• Replace instances that are unhealthy at the application level (not just EC2 status checks).

Key ideas:

• By default ASG ignores ELB health checks unless you enable them.
• When enabled and the load balancer reports an instance `Unhealthy`, ASG marks it unhealthy and replaces it.
• If deregistration delay (connection draining) is enabled, ASG can wait for in-flight requests before terminating.

Hard words:

• *deregistration delay* /diːˌredʒɪˈstreɪʃn dɪˈleɪ/: thời gian chờ hủy đăng ký (chờ request chạy xong)
• *in-flight* /ɪn flaɪt/: đang xử lý (chưa xong)
• *periodic* /ˌpɪriˈɑːdɪk/: định kỳ