Kubernetes Init Container

Definition:

• specialized containers that run successfully before app containers in a Kubernetes Pod
• Init containers can contain utilities or setup scripts not present in an app image.
• Use cases:
  1. Database readiness: Verify connection or schema migration before app startup.
  2. External API health checks: Ensure critical APIs are reachable.
  3. File or directory initialization: Create required directories or files for the app.
  4. Fetching secrets/configurations: Download secrets from external systems.
  5. Cleaning up temporary files: Ensure temporary files from previous runs are removed.

Understand init containers

• Run before the main app containers are started (sequentially)
• Init containers:
  • always run to completion
  • each init container must complete successfully before the next one starts
• If a Pod’s init container fails, kubelet daemon repeatedly restarts until it succeed (without restartPolicy: Always, if set it become Kubernetes Sidecar Container)
  • can set restartPolicy to Never

Differences from regular Container

Differences from Kubernetes Sidecar Container

• initContainers do not run continuously
• donot support lifecycle, livenessProbe, readinessProbe, or startupProbe

Using init containers

Detailed behavior