ConfigMaps

A ConfigMap is an API object used to store non-confidential data in key-value pairs. PodsA Pod represents a set of running containers in your cluster. can consume ConfigMaps as environment variables, command-line arguments, or as configuration files in a volumeA directory containing data, accessible to the containers in a pod. .

A ConfigMap allows you to decouple environment-specific configuration from your container imagesStored instance of a container that holds a set of software needed to run an application. , so that your applications are easily portable.

Caution: ConfigMap does not provide secrecy or encryption. If the data you want to store are confidential, use a SecretStores sensitive information, such as passwords, OAuth tokens, and ssh keys. rather than a ConfigMap, or use additional (third party) tools to keep your data private.

Motivation

Use a ConfigMap for setting configuration data separately from application code.

For example, imagine that you are developing an application that you can run on your own computer (for development) and in the cloud (to handle real traffic). You write the code to look in an environment variable named DATABASE_HOST. Locally, you set that variable to localhost. In the cloud, you set it to refer to a Kubernetes ServiceA way to expose an application running on a set of Pods as a network service. that exposes the database component to your cluster.

This lets you fetch a container image running in the cloud and debug the exact same code locally if needed.

ConfigMap object

A ConfigMap is an API object that lets you store configuration for other objects to use. Unlike most Kubernetes objects that have a spec, a ConfigMap has a data section to store items (keys) and their values.

The name of a ConfigMap must be a valid DNS subdomain name.

ConfigMaps and Pods

You can write a Pod spec that refers to a ConfigMap and configures the container(s) in that Pod based on the data in the ConfigMap. The Pod and the ConfigMap must be in the same namespaceAn abstraction used by Kubernetes to support multiple virtual clusters on the same physical cluster. .

Here's an example ConfigMap that has some keys with single values, and other keys where the value looks like a fragment of a configuration format.

apiVersion: v1
kind: ConfigMap
metadata:
  name: game-demo
data:
  # property-like keys; each key maps to a simple value
  player_initial_lives: "3"
  ui_properties_file_name: "user-interface.properties"
  #
  # file-like keys
  game.properties: |
    enemy.types=aliens,monsters
    player.maximum-lives=5
  user-interface.properties: |
    color.good=purple
    color.bad=yellow
    allow.textmode=true

There are four different ways that you can use a ConfigMap to configure a container inside a Pod:

  1. Command line arguments to the entrypoint of a container
  2. Environment variables for a container
  3. Add a file in read-only volume, for the application to read
  4. Write code to run inside the Pod that uses the Kubernetes API to read a ConfigMap

These different methods lend themselves to different ways of modeling the data being consumed. For the first three methods, the kubeletAn agent that runs on each node in the cluster. It makes sure that containers are running in a pod. uses the data from the ConfigMap when it launches container(s) for a Pod.

The fourth method means you have to write code to read the ConfigMap and its data. However, because you're using the Kubernetes API directly, your application can subscribe to get updates whenever the ConfigMap changes, and react when that happens. By accessing the Kubernetes API directly, this technique also lets you access a ConfigMap in a different namespace.

Here's an example Pod that uses values from game-demo to configure a Pod:

apiVersion: v1
kind: Pod
metadata:
  name: configmap-demo-pod
spec:
  containers:
    - name: demo
      image: game.example/demo-game
      env:
        # Define the environment variable
        - name: PLAYER_INITIAL_LIVES # Notice that the case is different here
                                     # from the key name in the ConfigMap.
          valueFrom:
            configMapKeyRef:
              name: game-demo           # The ConfigMap this value comes from.
              key: player_initial_lives # The key to fetch.
        - name: UI_PROPERTIES_FILE_NAME
          valueFrom:
            configMapKeyRef:
              name: game-demo
              key: ui_properties_file_name
      volumeMounts:
      - name: config
        mountPath: "/config"
        readOnly: true
  volumes:
    # You set volumes at the Pod level, then mount them into containers inside that Pod
    - name: config
      configMap:
        # Provide the name of the ConfigMap you want to mount.
        name: game-demo
        # An array of keys from the ConfigMap to create as files
        items:
        - key: "game.properties"
          path: "game.properties"
        - key: "user-interface.properties"
          path: "user-interface.properties"

A ConfigMap doesn't differentiate between single line property values and multi-line file-like values. What matters is how Pods and other objects consume those values.

For this example, defining a volume and mounting it inside the demo container as /config creates two files, /config/game.properties and /config/user-interface.properties, even though there are four keys in the ConfigMap. This is because the Pod definition specifies an items array in the volumes section. If you omit the items array entirely, every key in the ConfigMap becomes a file with the same name as the key, and you get 4 files.

Using ConfigMaps

ConfigMaps can be mounted as data volumes. ConfigMaps can also be used by other parts of the system, without being directly exposed to the Pod. For example, ConfigMaps can hold data that other parts of the system should use for configuration.

Note:

The most common way to use ConfigMaps is to configure settings for containers running in a Pod in the same namespace. You can also use a ConfigMap separately.

For example, you might encounter addonsResources that extend the functionality of Kubernetes. or operatorsA specialized controller used to manage a custom resource that adjust their behavior based on a ConfigMap.

Using ConfigMaps as files from a Pod

To consume a ConfigMap in a volume in a Pod:

  1. Create a config map or use an existing one. Multiple Pods can reference the same config map.
  2. Modify your Pod definition to add a volume under .spec.volumes[]. Name the volume anything, and have a .spec.volumes[].configMap.name field set to reference your ConfigMap object.
  3. Add a .spec.containers[].volumeMounts[] to each container that needs the config map. Specify .spec.containers[].volumeMounts[].readOnly = true and .spec.containers[].volumeMounts[].mountPath to an unused directory name where you would like the config map to appear.
  4. Modify your image or command line so that the program looks for files in that directory. Each key in the config map data map becomes the filename under mountPath.

This is an example of a Pod that mounts a ConfigMap in a volume:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - name: mypod
    image: redis
    volumeMounts:
    - name: foo
      mountPath: "/etc/foo"
      readOnly: true
  volumes:
  - name: foo
    configMap:
      name: myconfigmap

Each ConfigMap you want to use needs to be referred to in .spec.volumes.

If there are multiple containers in the Pod, then each container needs its own volumeMounts block, but only one .spec.volumes is needed per ConfigMap.

Mounted ConfigMaps are updated automatically

When a config map currently consumed in a volume is updated, projected keys are eventually updated as well. The kubelet checks whether the mounted config map is fresh on every periodic sync. However, the kubelet uses its local cache for getting the current value of the ConfigMap. The type of the cache is configurable using the ConfigMapAndSecretChangeDetectionStrategy field in the KubeletConfiguration struct. A ConfigMap can be either propagated by watch (default), ttl-based, or simply redirecting all requests directly to the API server. As a result, the total delay from the moment when the ConfigMap is updated to the moment when new keys are projected to the Pod can be as long as the kubelet sync period + cache propagation delay, where the cache propagation delay depends on the chosen cache type (it equals to watch propagation delay, ttl of cache, or zero correspondingly).

ConfigMaps consumed as environment variables are not updated automatically and require a pod restart.

Immutable ConfigMaps

FEATURE STATE: Kubernetes v1.19 [beta]

The Kubernetes beta feature Immutable Secrets and ConfigMaps provides an option to set individual Secrets and ConfigMaps as immutable. For clusters that extensively use ConfigMaps (at least tens of thousands of unique ConfigMap to Pod mounts), preventing changes to their data has the following advantages:

  • protects you from accidental (or unwanted) updates that could cause applications outages
  • improves performance of your cluster by significantly reducing load on kube-apiserver, by closing watches for config maps marked as immutable.

This feature is controlled by the ImmutableEphemeralVolumes feature gate, which is enabled by default since v1.19. You can create an immutable ConfigMap by setting the immutable field to true. For example,

apiVersion: v1
kind: ConfigMap
metadata:
  ...
data:
  ...
immutable: true
Note: Once a ConfigMap or Secret is marked as immutable, it is not possible to revert this change nor to mutate the contents of the data field. You can only delete and recreate the ConfigMap. Existing Pods maintain a mount point to the deleted ConfigMap - it is recommended to recreate these pods.

What's next

Last modified September 13, 2020 at 5:40 AM PST: Fix for 23732 (#23822) (7e22ae5ad)