Init container pattern in Docker - Step-by-Step Execution

Process Flow - Init container pattern

Pod starts

↓

Init Container runs

↓

Init Container completes successfully?

No→Pod waits

Yes↓

Main Container starts

↓

Pod runs normally

The pod starts by running the init container first. Only after it finishes successfully does the main container start.

Execution Sample

Docker

apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  initContainers:
  - name: init-myservice
    image: busybox
    command: ['sh', '-c', 'echo Init task; 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	Action	Init Container State	Main Container State	Pod State
1	Pod starts	Not started	Not started	Starting
2	Init container starts running	Running	Not started	Waiting for init container
3	Init container prints message and sleeps	Running	Not started	Waiting for init container
4	Init container completes successfully	Completed	Not started	Init container done
5	Main container starts	Completed	Running	Running
6	Main container runs nginx server	Completed	Running	Running
7	Pod runs normally	Completed	Running	Running

💡 Init container completes successfully, so main container starts and pod runs normally.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	Final
Init Container State	Not started	Running	Completed	Completed	Completed
Main Container State	Not started	Not started	Not started	Running	Running
Pod State	Starting	Waiting for init container	Init container done	Running	Running

Key Moments - 3 Insights

Why does the main container not start immediately when the pod starts?

Because the pod waits for the init container to finish first, as shown in execution_table step 2 and 4 where the main container remains 'Not started' until the init container completes.

What happens if the init container fails?

The pod will keep waiting and not start the main container. This is implied in the flow where if the init container does not complete successfully, the pod waits.

Can the init container run multiple times?

No, the init container runs once before the main container starts, as shown in the execution_table where it runs and completes before the main container starts.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of the main container at Step 3?

A Not started

B Running

C Completed

D Failed

💡 Hint

Check the 'Main Container State' column at Step 3 in the execution_table.

At which step does the pod start running the main container?

A Step 2

B Step 4

C Step 5

D Step 7

💡 Hint

Look at the 'Main Container State' column to see when it changes to 'Running'.

If the init container never completes, what will happen to the pod state?

A Pod runs normally

B Pod waits indefinitely

C Pod starts main container anyway

D Pod crashes immediately

💡 Hint

Refer to the concept_flow where the pod waits if the init container does not complete.

Concept Snapshot

Init container pattern in Kubernetes pods:
- Pod starts with init container first
- Init container runs setup tasks
- Main container waits until init container finishes
- If init container fails, pod waits
- Ensures main container starts only after setup is done

Full Transcript

The init container pattern means a pod runs a special container first to prepare things. The pod starts, then the init container runs. The main container waits until the init container finishes successfully. If the init container fails, the pod keeps waiting and does not start the main container. This ensures the main container only runs after the setup is complete. In the example, the init container prints a message and sleeps for 3 seconds before finishing. Then the main nginx container starts and runs normally.