In Kubernetes, ConfigMaps provide a way to decouple configuration data from containerized applications, allowing you to modify application behavior without rebuilding or redeploying the container images. There are two primary types of ConfigMaps you can use to manage application configurations: Static ConfigMaps and Dynamic ConfigMaps.

In this post, we’ll explore the differences between static and dynamic ConfigMaps and how to effectively use them in your Kubernetes environment.

What Is a ConfigMap?

A ConfigMap is a key-value store that Kubernetes uses to hold configuration data for applications running inside Pods. It allows you to externalize configuration from the container image, making it easy to update application settings without the need for changing the application code or rebuilding the image.

ConfigMaps are commonly used to:

• Store environment variables
• Define application settings
• Hold configuration files or scripts

Static ConfigMaps

A Static ConfigMap is a configuration that is loaded once when the Pod is created. It remains unchanged throughout the lifetime of the Pod unless manually updated and reloaded through a redeployment. Static ConfigMaps are typically used in scenarios where the configuration is not expected to change frequently.

Example of a Static ConfigMap

apiVersion: v1
kind: ConfigMap
metadata:
  name: static-config
data:
  config.properties: |
    APP_ENV=production
    DATABASE_URL=jdbc:mysql://db:3306/appdb    

In this example, the static configuration includes environment variables for an application. Once this ConfigMap is created, the configuration will not change unless you manually update it and restart the Pod.

Use Cases for Static ConfigMaps

• Environment variables: Settings that do not change often, such as production or testing configurations.
• Application defaults: Defaults that are set once and don’t need frequent adjustments, such as database connections or API keys.

Dynamic ConfigMaps

A Dynamic ConfigMap refers to a configuration that is updated during the runtime of a Pod without requiring a full redeployment. When the ConfigMap is updated, Pods can react to the new configuration without restarting.

This approach is useful for applications that need to adapt to changing configurations or settings, such as feature toggles, thresholds, or dynamically updated configurations.

Example of a Dynamic ConfigMap

apiVersion: v1
kind: ConfigMap
metadata:
  name: dynamic-config
data:
  feature-flags.properties: |
    FEATURE_X_ENABLED=true
    MAX_CONNECTIONS=100    

How to Enable Dynamic ConfigMap Reloading

To make ConfigMaps dynamic, you can set up a sidecar container or use a config reloader like Reloader or configmap-reload. These utilities monitor changes in ConfigMaps and automatically trigger reloads in the application.

Here’s how to use configmap-reload in a Kubernetes Pod:

apiVersion: v1
kind: Pod
metadata:
  name: dynamic-app
spec:
  containers:
  - name: my-app
    image: my-app:latest
    volumeMounts:
    - name: config-volume
      mountPath: /etc/config
  - name: configmap-reload
    image: jimmidyson/configmap-reload:v0.5.0
    args:
    - --volume-dir=/etc/config
  volumes:
  - name: config-volume
    configMap:
      name: dynamic-config

In this configuration:

• The configmap-reload sidecar container monitors the /etc/config directory for changes. When a change is detected, it triggers the main application to reload the updated configuration without restarting the Pod.

Use Cases for Dynamic ConfigMaps

• Feature toggles: Turning features on or off without redeploying the application.
• Thresholds or limits: Adjusting performance parameters such as connection limits, memory thresholds, or request timeouts.
• Runtime configuration updates: Any configuration that may need to change based on the operational needs of the application.

Key Differences Between Static and Dynamic ConfigMaps

Best Practices for Using ConfigMaps in Kubernetes

1. Use Static ConfigMaps for Stable Configurations: If your application settings are not expected to change frequently, use static ConfigMaps to simplify management.
2. Use Dynamic ConfigMaps for Real-Time Updates: For applications that need real-time configuration updates, implement dynamic ConfigMaps with a sidecar reloader.
3. Monitor ConfigMap Changes: Implement monitoring and alerting to track changes in ConfigMaps, ensuring that your applications respond correctly to updated settings.
4. Limit Sensitive Data in ConfigMaps: Avoid placing sensitive data like passwords or tokens in ConfigMaps. Instead, use Secrets to manage sensitive information securely.

Conclusion

Choosing between Static and Dynamic ConfigMaps depends on the nature of your application and its configuration needs. Static ConfigMaps work well for stable settings that don’t change often, while dynamic ConfigMaps are ideal for applications that need to adapt to changing configurations in real time. By using the right type of ConfigMap and following best practices, you can optimize the performance and flexibility of your Kubernetes applications.