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ConfigMap: Inline vs File-Based Configuration

category: Kubernetes
tags: configmap, yaml, volumes, kubectl

Main Topic 1

Understanding the difference between defining Kubernetes ConfigMaps inline vs from external files is important for managing application configurations cleanly and declaratively.

Subtopic A: Inline ConfigMap (application.yml: |)

  • Config embedded directly - File content (like application.yml) is included directly in the YAML manifest using multiline string syntax (|).
  • All-in-one manifest - Easy to version control and deploy as part of GitOps or CI/CD pipelines.
  • Use this when configs are small and you want to manage everything in one place.
  • Ideal for templated environments or tools like Helm.

Subtopic B: File-Based ConfigMap (--from-file)

  • Reads file from disk - You create a ConfigMap from a real file using kubectl create configmap --from-file=...
  • No manual copying - The file is uploaded to the Kubernetes API server; no need to copy it to the node or container.
  • Cleaner for large configs - Better separation of concerns when you already have config files.

Main Topic 2

Code Example (if applicable)

# Inline
apiVersion: v1
kind: ConfigMap
metadata:
  name: backend-config
data:
  application.yml: |
    server:
      port: 8080

# File-based
kubectl create configmap backend-config --from-file=application.yml

Commands (if applicable)

kubectl apply -f configmap-inline.yml
kubectl create configmap backend-config --from-file=application.yml

Key Concepts Summary

  • Inline config - Embed file contents directly into the ConfigMap YAML.
  • File-based config - Reference external files and upload them using the CLI.
  • K8s stores config - Kubernetes stores the ConfigMap in the API server, not on nodes.
  • File gets mounted - The config file is injected into the container as a real file at runtime.

Best Practices / Tips

  1. Use inline for small or templated configs - Great for GitOps and human-readability.
  2. Use file-based for larger files - Keeps your manifests clean and leverages existing config files.
  3. Mount ConfigMaps as files - When the app expects a file (e.g., Spring Boot's application.yml), mount the config as a volume.

Common Issues / Troubleshooting

Problem 1

  • Symptom: Spring Boot app fails to find application.yml
  • Cause: ConfigMap wasn't mounted correctly or wrong path used
  • Solution: Mount the file into /config and set --spring.config.location=file:/config/application.yml

Problem 2

  • Symptom: ConfigMap value is too large or unmanageable in YAML
  • Solution: Use --from-file method to externalize the content cleanly

References / Further Reading

Last updated: 2025-08-26 20:00 UTC