Control plane components (API server, scheduler, controller manager, etcd) in Kubernetes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the role of the Kubernetes API server in the control plane?

The API server acts like the front desk of Kubernetes. It receives all requests from users or components, validates them, and updates the cluster state accordingly. It is the central point for communication.

beginner

What does the Kubernetes scheduler do?

The scheduler decides which worker node will run a new pod. It looks at resource availability and constraints to place pods efficiently.

beginner

Explain the purpose of the controller manager in Kubernetes.

The controller manager runs background tasks that keep the cluster healthy. It watches the cluster state and makes changes to reach the desired state, like starting or stopping pods.

beginner

What is etcd and why is it important in Kubernetes?

etcd is a key-value store that saves all the cluster data. It acts like a reliable notebook where Kubernetes stores the current state and configuration of the cluster.

intermediate

How do the control plane components work together?

The API server receives requests and updates etcd. The scheduler assigns pods to nodes. The controller manager ensures the cluster matches the desired state. etcd stores all data. Together, they keep the cluster running smoothly.

Which component acts as the main communication hub in Kubernetes control plane?

A etcd

B Scheduler

C Controller manager

D API server

The API server is the central point that handles all communication and requests in the Kubernetes control plane.

What does the Kubernetes scheduler primarily do?

A Stores cluster data

B Assigns pods to nodes

C Runs background controllers

D Validates API requests

The scheduler decides which node will run a pod based on resource availability.

Which component ensures the cluster state matches the desired configuration by running controllers?

A Controller manager

B API server

C Scheduler

D etcd

The controller manager runs controllers that watch and adjust the cluster state.

Where does Kubernetes store all its cluster state data persistently?

A Controller manager

B Scheduler

C etcd

D API server

etcd is the persistent key-value store for all cluster data.

Which component validates and processes REST requests in Kubernetes?

A API server

B Controller manager

C Scheduler

D etcd

The API server validates and processes REST API requests from users and components.

Describe the main functions of each Kubernetes control plane component: API server, scheduler, controller manager, and etcd.

Think of the control plane as a team where each member has a specific job to keep the cluster healthy.

Explain how the Kubernetes control plane components work together to manage the cluster.

Imagine a workflow where requests come in, data is saved, pods are assigned, and the system self-corrects.

Practice

1. Which component of the Kubernetes control plane acts as the main entry point for all REST commands and handles communication with the cluster?

easy

A. etcd

B. Scheduler

C. API server

D. Controller manager

Solution

1. Step 1: Understand the role of the API server

   The API server is the front door of the Kubernetes control plane. It receives REST requests and processes them.

2. Step 2: Compare with other components

   The scheduler assigns pods to nodes, the controller manager runs controllers, and etcd stores cluster data. None handle REST communication directly.

3. Final Answer:

   API server -> Option C

4. Quick Check:

   API server = main REST entry point [OK]

Hint: API server handles all REST requests to the cluster [OK]

Common Mistakes:

• Confusing scheduler with API server
• Thinking controller manager handles REST calls
• Assuming etcd processes API requests

2. Which of the following is the correct command to check the status of the Kubernetes scheduler component?

easy

A. kubectl get pods -n kube-system | grep scheduler

B. kubectl get nodes --show-scheduler

C. kubectl describe scheduler

D. kubectl get scheduler-status

Solution

1. Step 1: Identify how to check scheduler status

   The scheduler runs as a pod in the kube-system namespace, so checking pods there helps find it.

2. Step 2: Analyze each command

   kubectl get pods -n kube-system | grep scheduler lists pods in kube-system and filters for scheduler, which is correct. The other commands are invalid kubectl commands or do not exist.

3. Final Answer:

   kubectl get pods -n kube-system | grep scheduler -> Option A

4. Quick Check:

   Scheduler runs as pod, check pods in kube-system [OK]

Hint: Scheduler runs as pod; check pods in kube-system namespace [OK]

Common Mistakes:

• Using invalid kubectl commands
• Trying to describe scheduler directly
• Assuming scheduler is a node property

3. Given the following scenario: The Kubernetes scheduler fails to assign pods to nodes. Which control plane component is most likely responsible for detecting and fixing this issue automatically?

medium

A. API server

B. kubelet

C. etcd

D. Controller manager

Solution

1. Step 1: Understand the roles of components

   The controller manager runs controllers that watch cluster state and fix issues like unscheduled pods.

2. Step 2: Identify responsible component for fixing scheduling issues

   When scheduler fails, controller manager detects pods not assigned and may trigger rescheduling or alerts. API server only handles requests, etcd stores data, kubelet runs on nodes.

3. Final Answer:

   Controller manager -> Option D

4. Quick Check:

   Controller manager fixes cluster state issues [OK]

Hint: Controller manager fixes cluster state problems automatically [OK]

Common Mistakes:

• Confusing API server with controller manager
• Thinking etcd fixes issues
• Assuming kubelet manages scheduling

4. You notice that the etcd cluster is not storing new data correctly. Which of the following is the most likely cause and fix?

medium

A. etcd disk is full; free up disk space or increase storage

B. Scheduler is overloaded; scale scheduler pods

C. API server is down; restart the API server pods

D. Controller manager crashed; restart controller manager

Solution

1. Step 1: Identify symptoms related to etcd

   etcd stores cluster data. If it fails to store new data, disk space or storage issues are common causes.

2. Step 2: Match cause and fix

   API server, scheduler, and controller manager issues do not directly cause etcd storage failure. Disk full on etcd node is the likely cause; freeing space or increasing storage fixes it.

3. Final Answer:

   etcd disk is full; free up disk space or increase storage -> Option A

4. Quick Check:

   etcd storage issues = disk full fix [OK]

Hint: etcd storage issues often mean disk full; check storage first [OK]

Common Mistakes:

• Restarting unrelated components
• Ignoring disk space on etcd nodes
• Assuming scheduler affects etcd storage

5. You want to ensure high availability of the Kubernetes control plane. Which combination of components should you deploy in multiple replicas to avoid a single point of failure?

hard

A. Multiple schedulers and controller managers only

B. Multiple API servers and etcd nodes

C. Multiple etcd nodes only

D. Multiple API servers only

Solution

1. Step 1: Understand high availability needs

   High availability means no single point of failure. Both API servers and etcd store critical data and handle requests.

2. Step 2: Identify components to replicate

   API servers can run multiple replicas behind a load balancer. etcd must run as a cluster with multiple nodes for data safety. Scheduler and controller manager can be run as leader election but are less critical to replicate for HA.

3. Final Answer:

   Multiple API servers and etcd nodes -> Option B

4. Quick Check:

   HA = replicate API servers + etcd nodes [OK]

Hint: Replicate API servers and etcd for control plane high availability [OK]

Common Mistakes:

• Replicating only schedulers or controller managers
• Ignoring etcd clustering
• Assuming API server alone is enough