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Configure components via environment variables and use ConfigMaps for configuration parameters

In Kubernetes any configuration of an application component which varies between environments must be passed into the relevant containers from outside. This can happen either

  1. by directly passing values as environment variables (such values can be provided explicitly or fetched from ConfigMaps or Secrets) or
  2. by using file-based ConfigMaps to mount config files in the relevant locations of a containers file system

Rule of thumb

It should never be necessary to open a shell in a container in order to set some configuration options before an application component is started.

Examples

Two NGINX deployments. One obtains its config via a ConfigMap mounted as a volume. The other one obtains parts of its config via environment variables. In the latter case, one environment variable is set explicitly, another one is read from a ConfigMap.

Use the following command to apply the deployments

kubectl apply -f examples/

Check the pods and remember their IP addresses

kubectl get pods -o wide

Make requests the nginx instances and check the outputs

kubectl run curl --image=curlimages/curl -i --tty --rm --restart=Never -- <Pod 1 IP>

kubectl run curl --image=curlimages/curl -i --tty --rm --restart=Never -- <Pod 2 IP>

Use the following command to delete the deployments

kubectl delete -f examples/