A container image for testing various app states in orchestration. It aims to misbehave in simple ways based on its config so you can ensure your orchestration responds how you want/expect. You can set this Node.js web app to:
- Have a slow startup
- Fail startup
- Exit while running, with error code
- Enable or disable Docker healthchecks
- Fail healthchecks
- Slow healthchecks
- Responds with a different 20x HTTP status code for different image tags (versions)
You can configure all these things at setup/runtime via environment variables, and some of them while the container is running, via specific routes (URLs).
Image tags marked healthcheck will have a HEALTHCHECK set in Dockerfile.
HTTP Codes are different per image tag version to make container update identification easier when using httping or similar tool.
Here's a list of images stored on Docker Hub and how they are different.
|/healthz HTTP Code
|Identical to v1
|Fails on start
|Identical to healthcheck
|Healthcheck returns 500
Startup config options via environment variables
PORT- int, port to listen on, defaults to 80
VERSION- string, doesn't change the functionality, but used to create different images for update testing
FAIL_STARTUP- true/false, exit(1) right at start
DELAY_STARTUP- int, set to milliseconds the app will wait before listening on PORT
DELAY_HEALTHCHECK- int, set to milliseconds the app will wait before responding on /healthz
HAPPYHEALTHCHECK- true/false, false will cause /healthz to return 500, defaults to true on v1/v2, false on v3
ENABLE_LOGGER- true/false, defaults to true, logs all HTTP requests to stdout
Useful routes (URLs)
/- Returns a random image of Serenity crew.
/healthz- Returns JSON of environment variables. Returns 500 if
HAPPYHEALTHCHECKis false. Returns a 20x if true. Actual status code matches the version of app running. This is useful for using tools like httping to test connectivity while doing rolling or blue/green updates and being able to see which is responding.
- 201 - v1
- 202 - v2
- 203 - v3
/fail- Returns 500 and exits the Node.js process with an error code.
/togglehealthcheck= By default the healthcheck URL returns 20x. Hitting this URL will cause them to start returning 500. Hitting it again will return to 20x.
Examples with Docker Run
Show slow startup
Start the v1 image with a slow startup of 5 seconds (Node.js will wait 5s before listening on the port). This is useful to simulate apps that take more time to startup, and also simulate distributed environments where not all things start in "proper" order.
docker run -p 80:80 --env DELAY_STARTUP=5000 bretfisher/browncoat:v1
You could use httping or
curl to show
how the first 5 seconds of that container running, it doesn't repond to HTTP requests.
Fail on app startup
Next, start the v2 image and cause it to exit with an error code. This will simulate
what would happen if you didn't test proper running of a container in CI and something
was set wrong when you deployed container. Do you systems catch this type
of quick failure that may not show up in unit tests? Note:
does this startup failure by default.
docker run -p 80:80 --env FAIL_STARTUP=true bretfisher/browncoat:v2
docker run and
docker compose don't react to failed healthchecks like
orchestrators, but they do provide healthcheck status.
Examples with Swarm
Create the initial Service
Create a three-container service that doesn't have a Dockerfile healthcheck built-in.
docker service create --name browncoat -p 80:80 --replicas 3 bretfisher/browncoat
Next, install httping on your host to monitor the health endpoint
/healthz with something like
httping -i .1 -GsY localhost/healthz. You can also run it from a container:
docker run --rm bretfisher/httping -i .1 -GsY <hostIP>/healthz. Notice you get a 201 response code. v2
will have a 202 response code to make it easier to identity how updates are distributing connections.
Now we can update the service with a new image.
docker service update --image bretfisher/browncoat:v2 browncoat
Notice some connections will fail. This is normal without a healthcheck.
Docker has no awareness of if you're container is truly "ready" for connections and
starts sending them to new containers before app is listening. This gets worse as your
Now let's update to another new image. This time you should have zero connection failures. The new image has HEALTHCHECK command in it, so Swarm uses it to wait for ready state.
docker service update --image bretfisher/browncoat:healthcheck browncoat
Note: this YAML should also work with
docker compose but it won't replace services, it'll only indicate
the health status with
docker compose ps. This is useful for using the
to have one service startup wait for another service to be healthy.
Examples with Kubernetes
Kubernetes ignores Dockerfile
HEALTHCHECK commands, so you'll need to add them
to your Pod spec. In Kubernetes we have two main checks: livenessProbe and readinessProbe.
A little mnemonic is livenessProbe to check if container is live,
and readinessProbe to check if container is ready to serve traffic
But For more info you can check the
Create the initial Kubernetes Deployment
The below creates a Deployment with 5 pods without livenessProbe or readinessProbe and a Service to expose the pods.
kubectl apply -f kubernetes-examples/browncoat-v1.yaml
Causing a rolling update outages due to a lack of probes
Now let's update that Deployment with the default rolling update style.
We will change the container image to
bretfisher/browncoat:v2 and add the
DELAY_STARTUP of 5s to simulate a small delay and cause some connection failures
during the rolling update. Probes would have helped prevent connections
from being directed to the new Pod until it was ready. Without probes,
Kubernetes is blind and will start directing Service traffic to the new Pod as
soon as it started.
To follow the rollout use is command in a different window
kubectl rollout status deploy/browncoat
To monitor the Service connections, run httping in a different window
kubectl run httping -it --image bretfisher/httping --rm=true -- browncoat/healthz
Now apply the new image and env var
kubectl apply -f kubernetes-examples/browncoat-v2.yaml
Add readinessProbe and monitor a new
Now we will add a readinessProbe to check if the endpoint
healthz returns a
successful response and will change to the image v1 to see a different response code (
kubectl apply -f kubernetes-examples/browncoat-v1-withProbe.yaml
Express for easier http serveringThis is a Node.js app using
and Stoppable for better graceful shutdowns.