How to define a healthcheck for a Docker container
Today I learned that you can define a health check for a Docker container, directly on the Dockerfile.
I was already familiar with the liveness and readiness checks of Kubernetes, but somehow I had not yet bumped into the HEALTHCHECK statement for the Dockerfile.
Example for Nginx, let’s say you have some HTML content in a content directory,
then you can write a Dockerfile like this:
FROM nginx
COPY content /usr/share/nginx/html
HEALTHCHECK --interval=5s --timeout=3s \
CMD curl -f http://localhost/?healthcheck=1 || exit 1
If you build that image and then start a container with it:
docker build . -t simple-nginx-docker # Build the image
docker run --rm -p 8000:80 simple-nginx-docker # Run a container
Then, you’ll see:
- the incoming health check requests arriving in the container logs as per the configuration, every 5 seconds:
127.0.0.1 - - [01/Nov/2023:18:42:49 +0000] "GET /?healthcheck=1 HTTP/1.1" 200 411 "-" "curl/7.88.1" "-"
- a health status in the output of
docker ps, like these:
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
0f7b75e705c1 simple-nginx-docker "/docker-entrypoint.…" 4 seconds ago Up 3 seconds (health: starting) 0.0.0.0:8000->80/tcp, :::8000->80/tcp frosty_dewdney
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
0f7b75e705c1 simple-nginx-docker "/docker-entrypoint.…" 9 seconds ago Up 8 seconds (healthy) 0.0.0.0:8000->80/tcp, :::8000->80/tcp frosty_dewdney
Good to know: to debug a failing health check command, run docker inspect CONTAINER_ID