Health Checks

Definition

An instruction added to a Dockerfile or Compose file that tells Docker how to test if the application running inside the container is actually functioning properly, not just whether the container process is running.

Explain Like I'm New

Checking the pulse of the app. Just because a web server container is turned 'On' doesn't mean the website isn't returning 500 Crash Errors. A health check actively pings the website every 30 seconds. If the site fails to load, Docker marks the container as 'Unhealthy'.

Real World Example

In a `docker-compose.yml`, you configure a health check to run `curl -f http://localhost/ || exit 1` every 30 seconds. A load balancer watches this status. If the container becomes 'Unhealthy', the load balancer automatically stops sending user traffic to it.

Common Use Cases

•Zero-downtime deployments
•Automated container restarts
•Load balancer routing

Interview Questions

basic

What does it mean if a container status says 'Up 5 hours (unhealthy)'?

intermediate

Why is relying purely on the container's 'Running' state dangerous in production?

Flash Cards

Question

What does unhealthy mean?

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Answer

It means the container process is physically turned on and running, but the internal application failed the defined Health Check test (e.g., the web server is completely frozen and not answering HTTP requests).

Question

Why is 'Running' state dangerous?

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Answer

A Java application might enter an infinite loop (deadlock). The container is technically still 'Running' because the Java process hasn't crashed, but the application is entirely broken. Without Health Checks, the orchestrator thinks everything is fine and routes users to a broken app.