Init containers in Kubernetes - Step-by-Step Execution

Process Flow - Init containers

Pod starts

↓

Init Container 1 runs

↓

Init Container 1 completes?

No→Wait

Yes↓

Init Container 2 runs

↓

Init Container 2 completes?

No→Wait

Yes↓

Main containers start

↓

Pod running

Init containers run one by one before main containers start. Each must finish successfully before the next begins.

Execution Sample

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  initContainers:
  - name: init-setup
    image: busybox
    command: ['sh', '-c', 'echo Init container running; sleep 3']
  containers:
  - name: main-app
    image: nginx

This pod runs an init container that prints a message and waits 3 seconds before starting the main nginx container.

Process Table

Step	Container	Action	Status	Next Step
1	init-setup	Start init container	Running	Wait for completion
2	init-setup	Complete command 'echo Init container running; sleep 3'	Succeeded	Start main-app container
3	main-app	Start main container nginx	Running	Pod is running
4	main-app	Container running normally	Running	Pod stays running

💡 Init container completed successfully, main container started, pod is running

Status Tracker

Container	Initial State	After Step 1	After Step 2	Final State
init-setup	Not started	Running	Succeeded	Completed
main-app	Not started	Not started	Running	Running

Key Moments - 2 Insights

Why does the main container not start immediately?

The execution_table shows the init container must complete successfully (Step 2) before the main container starts (Step 3). This ensures setup tasks finish first.

What happens if an init container fails?

If an init container fails, the pod will keep retrying that init container and will not start the main containers, as shown by the 'Wait for completion' action in Step 1.

Visual Quiz - 3 Questions

Look at the execution table, what is the status of 'init-setup' at Step 2?

A Succeeded

B Running

C Failed

D Not started

💡 Hint

Check the 'Status' column for Step 2 in the execution_table.

At which step does the main container start running?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for 'Start main container nginx' in the 'Action' column.

If the init container took longer to complete, which step would be delayed?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Main container starts only after init container succeeds (Step 3).

Concept Snapshot

Init containers run before main containers in a pod.
They run sequentially and must complete successfully.
Main containers start only after all init containers finish.
Used for setup tasks like config or waiting for services.
If an init container fails, pod startup pauses until fixed.

Full Transcript

Init containers are special containers in Kubernetes pods that run before the main containers start. They run one after another, and each must finish successfully before the next begins. This ensures setup tasks like configuration or waiting for resources complete first. In the example, the init container runs a simple command and waits 3 seconds. Only after it finishes does the main nginx container start. If an init container fails, the pod will keep retrying it and will not start the main containers until it succeeds.