Desired state vs actual state reconciliation in Kubernetes - Visual Side-by-Side Comparison

Process Flow - Desired state vs actual state reconciliation

User defines desired state

↓

Kubernetes stores desired state

↓

Controller checks actual state

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Compare actual state with desired state

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Do nothing

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Update actual state

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Loop back to check actual state

This flow shows how Kubernetes controllers continuously compare the desired state with the actual state and make changes to keep the system as the user wants.

Execution Sample

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  name: my-pod
spec:
  containers:
  - name: nginx
    image: nginx:latest

This YAML defines the desired state of a Pod named 'my-pod' running an nginx container.

Process Table

Step	Action	Desired State	Actual State	Comparison Result	Controller Action
1	User applies Pod YAML	Pod 'my-pod' with nginx container	No Pod exists	No match	Create Pod 'my-pod'
2	Controller checks cluster	Pod 'my-pod' with nginx container	Pod 'my-pod' starting	Partial match	Wait for Pod to be ready
3	Pod starts running	Pod 'my-pod' with nginx container	Pod 'my-pod' running	Match	No action needed
4	Pod crashes unexpectedly	Pod 'my-pod' with nginx container	Pod 'my-pod' crashed	No match	Restart Pod
5	Pod restarts and runs	Pod 'my-pod' with nginx container	Pod 'my-pod' running	Match	No action needed
6	User deletes Pod YAML	No Pod desired	Pod 'my-pod' running	No match	Delete Pod 'my-pod'
7	Pod deleted	No Pod desired	No Pod exists	Match	No action needed

💡 Desired state matches actual state; reconciliation complete.

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6	Final
Desired State	No Pod	Pod 'my-pod' with nginx	Pod 'my-pod' with nginx	Pod 'my-pod' with nginx	No Pod	No Pod
Actual State	No Pod	Pod 'my-pod' starting	Pod 'my-pod' running	Pod 'my-pod' crashed	Pod 'my-pod' running	No Pod

Key Moments - 3 Insights

Why does the controller restart the Pod after it crashes instead of creating a new one?

The controller sees the Pod exists but is not in the desired running state (Step 4). It tries to fix the actual state by restarting the existing Pod to match the desired state, not by creating a new Pod.

What happens if the desired state is deleted but the Pod still runs?

The controller detects the mismatch (Step 6) and deletes the running Pod to match the desired state of no Pod.

Why does the controller sometimes wait instead of acting immediately?

When the Pod is starting (Step 2), the actual state partially matches the desired state, so the controller waits for the Pod to become ready before taking further action.

Visual Quiz - 3 Questions

Look at the execution table, what is the actual state at Step 3?

A No Pod exists

B Pod 'my-pod' crashed

C Pod 'my-pod' running

D Pod 'my-pod' starting

💡 Hint

Check the 'Actual State' column for Step 3 in the execution table.

At which step does the controller delete the Pod to match the desired state?

A Step 4

B Step 6

C Step 2

D Step 1

💡 Hint

Look for the step where desired state is 'No Pod' but actual state is 'Pod running'.

If the Pod never crashes, which steps would be skipped?

A Steps 4 and 5

B Steps 2 and 3

C Steps 6 and 7

D Steps 1 and 2

💡 Hint

Check the steps related to Pod crashing and restarting in the execution table.

Concept Snapshot

Desired state is what you want the system to be.
Actual state is what is currently running.
Kubernetes controllers compare both.
If they differ, controllers act to fix actual state.
This loop keeps your system stable and as you defined.

Full Transcript

In Kubernetes, you tell the system what you want by defining the desired state, like a Pod running nginx. The system checks what is actually running, called the actual state. If the actual state does not match the desired state, Kubernetes controllers take action to fix it. For example, if the Pod crashes, the controller restarts it. If you delete the Pod definition, the controller deletes the running Pod. This continuous check and fix process is called reconciliation, and it keeps your cluster running as you want.