World D2: Recipes

HEALTHCHECK: is it alive?

You will learn: to let Docker check that your app still answers, not just that it runs.

Checked with Docker Engine 29 on 31 Oct 2026

The problem

docker ps says your box is Up. But every page shows an error. Is it healthy?

Watch

~/diary $ cat broken/DockerfileFROM nginx:1.30-alpineCOPY index.html /usr/share/nginx/html/HEALTHCHECK --interval=5s --timeout=3s \  CMD curl -fs http://localhost/health || exit 1~/diary $ docker build -t diary:broken broken[+] Building 0.1s (7/7) FINISHED                                  docker:default => [internal] load build definition from Dockerfile                        0.0s => => transferring dockerfile: 189B                                        0.0s => [internal] load metadata for docker.io/library/nginx:1.30-alpine        0.0s => [internal] load .dockerignore                                           0.0s => => transferring context: 2B                                             0.0s => [1/2] FROM docker.io/library/nginx:1.30-alpine@sha256:0e52bd054126cdc0  0.0s => => resolve docker.io/library/nginx:1.30-alpine@sha256:0e52bd054126cdc0  0.0s => [internal] load build context                                           0.0s => => transferring context: 232B                                           0.0s => [2/2] COPY index.html /usr/share/nginx/html/                            0.0s => exporting to image                                                      0.1s => => exporting layers                                                     0.0s => => exporting manifest sha256:ebb36d6cd5640cbf03395a313040768dbb51a7aff  0.0s => => exporting config sha256:bbe5baeedf01d763862cfb832c17f0d5b47aec4f6b7  0.0s => => exporting attestation manifest sha256:1b63be82f7c2cf434f5bc40c06a58  0.0s => => exporting manifest list sha256:da97c645dd67971a5aa4403eb00b1a1e8c04  0.0s => => naming to docker.io/library/diary:broken                             0.0s => => unpacking to docker.io/library/diary:broken                          0.0s~/diary $ docker run -d --name web diary:brokena38f59ca5adda6bba9144b3c77a385d92916137599ce5190200680fc2b636181~/diary $ sleep 20~/diary $ docker psCONTAINER ID   IMAGE          COMMAND                  CREATED          STATUS                      PORTS     NAMESa38f59ca5add   diary:broken   "/docker-entrypoint.…"   26 seconds ago   Up 26 seconds (unhealthy)   80/tcp    web

Your laptop`~/diary`

No open hatches

Lunchboxes (containers)

No containers yet

Frozen packs (images)

nginx:1.30-alpine8 layers · 93.5MB

A knocks every 5 seconds. Here it asks for /health, which does not exist. After 3 misses in a row, the status says unhealthy.

Try it

Build a check that knocks on the home page every 10 seconds.

Tap the pieces in the right order.

…

Try it

This recipe checks the home page. Build it, run it as web, wait a little, then look.

Your laptop`~/diary`

No open hatches

Lunchboxes (containers)

No containers yet

Frozen packs (images)

nginx:1.30-alpine8 layers · 93.5MB

Hint

The first knock comes after 5 seconds. sleep 6 waits.

Predict

web turns unhealthy. What does plain Docker do about it?

You did it!

You can now add a HEALTHCHECK and read the health status.

Go deeper (optional)

Defaults: every 30s, 30s timeout, 3 retries. --start-period gives a slow app time to wake up. Compose can wait for service_healthy (World D4). Kubernetes ignores HEALTHCHECK and uses its own probes (Part 3). Check / or a small /health page, never anything slow.

Real mission

Add a HEALTHCHECK to your nginx recipe.
Build and run it, then wait 30 seconds.
Read the status.
docker ps

0 of 3 steps done · Your ticks are saved.

HEALTHCHECK: is it alive?

Your laptop~/diary

Lunchboxes (containers)

Frozen packs (images)

Your laptop~/diary

Lunchboxes (containers)

Frozen packs (images)

Your laptop`~/diary`

Your laptop`~/diary`