====== Health Check ======

**What it is:** A mechanism that tests targets (instances/containers) to see if they are healthy.

**What it’s for:**
  * Route traffic only to healthy targets.
  * Improve availability automatically.

**Key ideas:**
  * Health checks can be HTTP (status codes) or TCP connection checks.
  * Unhealthy targets are removed from routing until healthy again.

**Exam cues:**
  * “stop sending traffic to failing instance” → health checks + ELB/ASG.

**Hard words:**
  * *mechanism* /ˈmekəˌnɪzəm/: cơ chế
  * *status code* /ˈsteɪtəs koʊd/: mã trạng thái (HTTP 200/404…)
  * *removed* /rɪˈmuːvd/: bị loại khỏi