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====== 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