Health checks in Compose in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a health check in Docker Compose?

A health check is a way to test if a container is working correctly by running a command inside it regularly. It helps Docker know if the app inside the container is healthy or not.

beginner

Which key is used to define a health check in a Docker Compose service?

The key healthcheck is used under a service in the docker-compose.yml file to define commands and settings for health checks.

intermediate

What does the test option inside a health check specify?

The test option defines the command Docker runs inside the container to check if it is healthy. For example, test: ["CMD", "curl", "-f", "http://localhost"].

intermediate

What happens if a health check fails multiple times in Docker Compose?

If the health check command fails repeatedly, Docker marks the container as unhealthy. This can be used to trigger restarts or alerts.

intermediate

Name two timing options you can set for a health check in Docker Compose.

You can set interval (how often to run the check) and timeout (how long to wait for the check to finish). Others include retries and start_period.

Which key defines a health check in a Docker Compose service?

A testhealth

B checkhealth

C health

D healthcheck

The correct key to define health checks in Docker Compose is healthcheck.

What does the test option inside a health check do?

A Runs a command inside the container to check health

B Sets the health check interval

C Defines the container restart policy

D Specifies the container image

The test option runs a command inside the container to verify if it is healthy.

If a health check fails repeatedly, Docker marks the container as:

A unhealthy

B stopped

C paused

D running

Repeated health check failures cause Docker to mark the container as unhealthy.

Which of these is NOT a valid health check timing option?

A retries

B timeout

C restart

D interval

restart is not a health check timing option; it is a separate container policy.

How often does Docker run the health check command by default if not specified?

A 10 seconds

B 30 seconds

C 1 minute

D 5 seconds

By default, Docker runs health checks every 30 seconds if interval is not set.

Explain how to add a health check to a service in a Docker Compose file.

Think about the keys under the service in docker-compose.yml

Describe what happens when a container's health check fails multiple times.

Focus on Docker's reaction to health check results