Container health checks in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a container health check in Docker?

A container health check is a command or script that Docker runs inside a container to check if the container is working properly. It helps to know if the app inside the container is healthy or needs attention.

beginner

Which Dockerfile instruction is used to define a health check?

The HEALTHCHECK instruction is used in a Dockerfile to specify how Docker should test the container's health by running a command inside it.

beginner

What does the HEALTHCHECK CMD command do?

It runs a command inside the container at regular intervals. If the command succeeds, the container is healthy. If it fails, Docker marks the container as unhealthy.

beginner

What are the possible health statuses Docker can show for a container?

Docker can show three health statuses: starting (health check is running but not ready), healthy (container passed the health check), and unhealthy (container failed the health check).

intermediate

How can health checks improve container management?

Health checks help automatically detect problems inside containers. This allows tools to restart unhealthy containers or alert users, making apps more reliable and easier to maintain.

Which Dockerfile instruction defines a container health check?

A CMD

B HEALTHCHECK

C RUN

D EXPOSE

The HEALTHCHECK instruction tells Docker how to test the container's health.

What does Docker do if a health check command fails?

A Marks the container as unhealthy

B Stops the container immediately

C Ignores the failure

D Restarts the Docker daemon

Docker marks the container as unhealthy if the health check command fails.

How often does Docker run the health check command by default?

A Every 30 seconds

B Every 5 minutes

C Every 1 second

D Every hour

By default, Docker runs the health check every 30 seconds.

What is the purpose of the HEALTHCHECK CMD in a Dockerfile?

A To start the container

B To copy files into the container

C To expose ports

D To specify the command Docker runs to check container health

HEALTHCHECK CMD defines the command Docker uses to test if the container is healthy.

Which health status means the container is still starting and health checks are not yet conclusive?

A unhealthy

B healthy

C starting

D stopped

The 'starting' status means the container is initializing and health checks are in progress.

Explain how to add a health check to a Docker container using a Dockerfile.

Think about the Dockerfile instruction and what it does.

Why are container health checks important in managing Docker containers?

Consider how health checks help keep apps running smoothly.