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ConfigMaps
A ConfigMap is an API object used to store non-confidential data in key-value pairs. Pods can consume ConfigMaps as environment variables, command-line arguments, or as configuration files in a volume.
A ConfigMap allows you to decouple environment-specific configuration from your container images, so that your applications are easily portable.
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 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.
A ConfigMap is not designed to hold large chunks of data. The data stored in a ConfigMap cannot exceed 1 MiB. If you need to store settings that are larger than this limit, you may want to consider mounting a volume or use a separate database or file service.
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 data
and binaryData
fields. These fields accept key-value pairs as their values. Both the data
field and the binaryData
are optional. The data
field is designed to
contain UTF-8 strings while the binaryData
field is designed to
contain binary data as base64-encoded strings.
The name of a ConfigMap must be a valid DNS subdomain name.
Each key under the data
or the binaryData
field must consist of
alphanumeric characters, -
, _
or .
. The keys stored in data
must not
overlap with the keys in the binaryData
field.
Starting from v1.19, you can add an immutable
field to a ConfigMap
definition to create an immutable ConfigMap.
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 namespace.
spec
of a static Pod cannot refer to a ConfigMap
or any other API objects.
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:
- Inside a container command and args
- Environment variables for a container
- Add a file in read-only volume, for the application to read
- 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 kubelet 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: alpine
command: ["sleep", "3600"]
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.
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 addons or operators 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:
- Create a ConfigMap or use an existing one. Multiple Pods can reference the same ConfigMap.
- 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. - Add a
.spec.containers[].volumeMounts[]
to each container that needs the ConfigMap. Specify.spec.containers[].volumeMounts[].readOnly = true
and.spec.containers[].volumeMounts[].mountPath
to an unused directory name where you would like the ConfigMap to appear. - Modify your image or command line so that the program looks for files in
that directory. Each key in the ConfigMap
data
map becomes the filename undermountPath
.
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 ConfigMap currently consumed in a volume is updated, projected keys are eventually updated as well.
The kubelet checks whether the mounted ConfigMap 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 by 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
Kubernetes v1.21 [stable]
The Kubernetes 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 ConfigMaps marked as immutable.
You can create an immutable ConfigMap by setting the immutable
field to true
.
For example:
apiVersion: v1
kind: ConfigMap
metadata:
...
data:
...
immutable: true
Once a ConfigMap is marked as immutable, it is not possible to revert this change
nor to mutate the contents of the data
or the binaryData
field. You can
only delete and recreate the ConfigMap. Because existing Pods maintain a mount point
to the deleted ConfigMap, it is recommended to recreate these pods.
What's next
- Read about Secrets.
- Read Configure a Pod to Use a ConfigMap.
- Read about changing a ConfigMap (or any other Kubernetes object)
- Read The Twelve-Factor App to understand the motivation for separating code from configuration.