Desired replicas vs actual replicas in Kubernetes - Visual Side-by-Side Comparison

Process Flow - Desired replicas vs actual replicas

User sets desired replicas

↓

Kubernetes Controller Manager

↓

Check current actual replicas

↓

Actual < Desired

↓

Create pods

↓

Update actual replicas count

↓

Match desired replicas?

No→Repeat check

Yes↓

Stable state

This flow shows how Kubernetes compares desired replicas with actual pods and adjusts by creating or deleting pods until they match.

Execution Sample

Kubernetes

kubectl scale deployment myapp --replicas=3
kubectl get deployment myapp

Set desired replicas to 3 and check actual replicas running in the deployment.

Process Table

Step	Desired Replicas	Actual Replicas	Action Taken	Result
1	3	1	Create 2 pods	Actual replicas increase to 3
2	3	3	No action	Desired matches actual, stable state
3	2	3	Delete 1 pod	Actual replicas decrease to 2
4	2	2	No action	Desired matches actual, stable state

💡 Execution stops when actual replicas equal desired replicas, indicating stable deployment state.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
Desired Replicas	3	3	3	2	2
Actual Replicas	1	3	3	2	2

Key Moments - 2 Insights

Why does Kubernetes create or delete pods instead of instantly matching desired replicas?

Kubernetes adjusts pods gradually to ensure stability and avoid sudden disruptions, as shown in steps 1 and 3 where pods are created or deleted step-by-step.

What happens if actual replicas are already equal to desired replicas?

No action is taken and the system remains stable, as seen in steps 2 and 4 where desired equals actual and no changes occur.

Visual Quiz - 3 Questions

Look at the execution table, what is the actual replicas count after step 1?

A 3

B 1

C 2

D 0

💡 Hint

Refer to the 'Actual Replicas' column in row for step 1.

At which step does Kubernetes delete a pod to match desired replicas?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Action Taken' column for the step where pods are deleted.

If desired replicas were set to 4 at start, how would the actual replicas change after step 1?

A Increase to 3

B Increase to 4

C Decrease to 1

D Stay at 1

💡 Hint

Look at how actual replicas adjust to match desired replicas in the variable_tracker.

Concept Snapshot

Desired replicas are the number of pods you want running.
Actual replicas are the pods currently running.
Kubernetes controller compares these and creates or deletes pods to match.
This ensures your app runs the right number of instances.
Stable state is when desired equals actual replicas.

Full Transcript

In Kubernetes, you set the desired number of pod replicas for your app. The system checks how many pods are actually running. If fewer pods run than desired, Kubernetes creates more pods. If more pods run than desired, it deletes some. This process repeats until the actual number matches the desired number. When they match, the deployment is stable. This ensures your app has the right capacity to handle traffic.