Desired replicas vs actual replicas in Kubernetes - Quick Revision & Key Differences

Recall & Review

beginner

What does the term 'desired replicas' mean in Kubernetes?

Desired replicas refer to the number of pod instances you want running for a specific application or service, as defined in the deployment configuration.

beginner

What are 'actual replicas' in Kubernetes?

Actual replicas are the number of pod instances currently running in the cluster for a deployment or replica set.

intermediate

How does Kubernetes ensure the actual replicas match the desired replicas?

Kubernetes uses a controller that constantly monitors the actual replicas and creates or deletes pods to match the desired replicas automatically.

beginner

What command shows the desired and actual replicas of a deployment?

The command kubectl get deployment <deployment-name> shows desired replicas in the 'READY' column (e.g., 3/3) and actual available replicas under 'AVAILABLE'.

intermediate

What might cause the actual replicas to be less than the desired replicas?

Possible causes include insufficient resources, pod startup failures, or node issues preventing pods from running.

In Kubernetes, what does the 'desired replicas' number represent?

A The number of pods currently running

B The number of containers per pod

C The number of nodes in the cluster

D The number of pods you want running

Desired replicas are the number of pod instances you want running as set in the deployment.

Which Kubernetes component adjusts pods to match desired replicas?

A Controller Manager

B kubectl

C API Server

D Scheduler

The Controller Manager monitors and adjusts pods to match the desired replicas.

What kubectl command shows the desired and actual replicas of a deployment?

A kubectl get pods

B kubectl describe node

C kubectl get deployment <name>

D kubectl logs <pod>

kubectl get deployment shows desired and actual replicas.

If actual replicas are less than desired replicas, what could be a reason?

A Too many nodes available

B Pod startup failures

C kubectl is not installed

D Deployment is paused

Pod startup failures can cause fewer actual replicas than desired.

What does Kubernetes do if actual replicas exceed desired replicas?

A Deletes extra pods to match desired replicas

B Creates more pods

C Does nothing

D Restarts the cluster

Kubernetes deletes extra pods to keep actual replicas equal to desired replicas.

Explain the difference between desired replicas and actual replicas in Kubernetes.

Think about what you want vs what you have running.

Describe what happens when the actual replicas do not match the desired replicas in a Kubernetes deployment.

Consider Kubernetes as a system that fixes differences automatically.

Practice

1. What does the desired replicas value represent in a Kubernetes Deployment?

easy

A. The number of pods currently running

B. The number of containers inside a pod

C. The number of nodes in the cluster

D. The number of pod copies Kubernetes should run

Solution

1. Step 1: Understand the role of desired replicas

   Desired replicas specify how many copies of a pod Kubernetes should maintain to run your app.

2. Step 2: Differentiate from actual replicas

   Actual replicas show how many pods are currently running, which may differ temporarily.

3. Final Answer:

   The number of pod copies Kubernetes should run -> Option D

4. Quick Check:

   Desired replicas = target pod count [OK]

Hint: Desired replicas = target pods Kubernetes tries to run [OK]

Common Mistakes:

• Confusing desired replicas with actual running pods
• Thinking desired replicas count containers, not pods
• Mixing desired replicas with cluster node count

2. Which of the following is the correct YAML snippet to set desired replicas to 3 in a Deployment?

easy

A. replicaCount: 3

B. desiredReplicas: 3

C. replicas: 3

D. pods: 3

Solution

1. Step 1: Identify the correct field name in Deployment spec

   The correct field to set desired replicas is replicas under the spec section.

2. Step 2: Verify syntax correctness

   Using replicas: 3 is valid YAML and recognized by Kubernetes.

3. Final Answer:

   replicas: 3 -> Option C

4. Quick Check:

   Use 'replicas' to set desired pod count [OK]

Hint: Use 'replicas' key in Deployment spec to set desired pods [OK]

Common Mistakes:

• Using incorrect field names like desiredReplicas or replicaCount
• Placing replicas outside the spec section
• Using plural 'pods' instead of 'replicas'

3. You run kubectl get deployment myapp and see:
DESIRED=4, CURRENT=3, UP-TO-DATE=3, AVAILABLE=3
What does this mean?

medium

A. One pod has crashed and won't restart

B. Kubernetes is still creating the 4th pod

C. All pods are running as expected

D. Deployment is scaled down to 3 pods

Solution

1. Step 1: Compare desired and current replicas

   Desired is 4 but current is 3, so Kubernetes aims for 4 pods but only 3 are running now.

2. Step 2: Interpret the status fields

   Up-to-date and available are 3, meaning 3 pods are ready and updated; the 4th pod is likely starting.

3. Final Answer:

   Kubernetes is still creating the 4th pod -> Option B

4. Quick Check:

   Desired > Current means pods are still launching [OK]

Hint: Desired > Current means pods are starting or recovering [OK]

Common Mistakes:

• Assuming pods crashed without checking events
• Thinking deployment scaled down when desired is higher
• Confusing available with desired replicas

4. You notice your Deployment's desired replicas is 5 but actual replicas stays at 3. Which is the most likely cause?

medium

A. Pods are stuck in Pending state due to resource limits

B. Deployment YAML has syntax errors

C. kubectl command was typed incorrectly

D. Actual replicas always match desired replicas instantly

Solution

1. Step 1: Understand why actual replicas lag behind desired

   If actual replicas are less than desired, pods may be stuck starting, often due to resource constraints.

2. Step 2: Eliminate other options

   Syntax errors prevent deployment creation, incorrect kubectl commands won't show this state, and actual replicas do not update instantly.

3. Final Answer:

   Pods are stuck in Pending state due to resource limits -> Option A

4. Quick Check:

   Pending pods cause actual < desired replicas [OK]

Hint: Check pod status if actual replicas < desired replicas [OK]

Common Mistakes:

• Assuming syntax errors cause replica mismatch
• Expecting instant replica count update
• Ignoring pod scheduling issues

5. You want to ensure your Deployment always has 3 pods running. You set replicas: 3 but sometimes kubectl get pods shows only 2 running pods. What should you do to fix this?

hard

A. Check pod events and logs to find why one pod is not running and fix the issue

B. Increase replicas to 4 to force Kubernetes to create more pods

C. Delete the Deployment and recreate it with replicas set to 3

D. Ignore it; actual replicas will eventually match desired without intervention

Solution

1. Step 1: Investigate why pods are missing

   Check pod events and logs to identify issues like crashes, scheduling failures, or resource limits causing pods not to run.

2. Step 2: Fix the root cause

   Resolve the identified problem (e.g., fix image, increase resources) so Kubernetes can maintain 3 running pods as desired.

3. Final Answer:

   Check pod events and logs to find why one pod is not running and fix the issue -> Option A

4. Quick Check:

   Fix pod issues to match actual to desired replicas [OK]

Hint: Always check pod status and logs if replicas don't match [OK]

Common Mistakes:

• Scaling replicas blindly without fixing pod issues
• Deleting Deployment unnecessarily
• Assuming Kubernetes fixes pod issues automatically