Control plane components (API server, scheduler, controller manager, etcd) in Kubernetes - Step-by-Step Execution

Process Flow - Control plane components (API server, scheduler, controller manager, etcd)

User/API Client

↓

API Server

↓

etcd (State Storage)

↓

Controller Manager

The user sends requests to the API server, which stores state in etcd. The controller manager and scheduler watch the API server to manage cluster state and assign work.

Execution Sample

Kubernetes

kubectl run mypod --image=nginx
# API server receives request
# Scheduler assigns pods to nodes
# Controller manager ensures desired state
# etcd stores cluster state

This sequence shows how a pod request flows through control plane components to schedule and maintain pods.

Process Table

Step	Component	Action	Input	Output/Effect
1	User/API Client	Sends request	kubectl run mypod --image=nginx	Request sent to API server
2	API Server	Receives request	kubectl run mypod --image=nginx	Validates and processes request
3	API Server	Reads/Writes state	Request data	Writes pending pod to etcd
4	Scheduler	Watches API server	Pod pending scheduling	Assigns pod to a node
5	Controller Manager	Watches API server	Pod assigned to node	Ensures pod is running as desired
6	etcd	Stores cluster state	Pod info updates	Persistent cluster state updated
7	API Server	Responds to user	Pod info from etcd	Returns pod info to kubectl
8	User/API Client	Receives response	Pod info	Displays pod info to user

💡 Request completed and pod state maintained by control plane components

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
Pod Request State	None	Received by API server	Assigned to node by Scheduler	Stored in etcd	Confirmed running by Controller Manager
Cluster State in etcd	Initial cluster state	No change	No change	Updated with pod info	Updated with pod running status

Key Moments - 3 Insights

Why does the scheduler assign pods after the API server receives the request?

Because the API server validates and stores the pod request first (see execution_table step 3), the scheduler can then watch for new pods needing assignment (step 4).

How does etcd help keep the cluster state consistent?

etcd stores the persistent cluster state (step 6). All components read and write to etcd via the API server, ensuring a single source of truth.

What role does the controller manager play after the scheduler assigns a pod?

The controller manager ensures the pod is running as desired by monitoring the API server state and making adjustments if needed (step 5).

Visual Quiz - 3 Questions

Look at the execution table, at which step does the scheduler assign a pod to a node?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Check the 'Component' and 'Action' columns in execution_table row 4.

According to the variable tracker, what is the state of the pod request after step 6?

A Stored in etcd

B Assigned to node by Scheduler

C Received by API server

D Confirmed running by Controller Manager

💡 Hint

Look at 'Pod Request State' column 'After Step 6' in variable_tracker.

If the API server did not update etcd, what would happen to the cluster state?

A Cluster state would remain consistent

B Cluster state would not update with pod info

C Scheduler would assign pods twice

D Controller manager would ignore pods

💡 Hint

Refer to execution_table step 6 and variable_tracker 'Cluster State in etcd'.

Concept Snapshot

Control plane components manage Kubernetes cluster state.
API server handles requests and communicates with etcd.
Scheduler assigns pods to nodes.
Controller manager ensures desired state.
etcd stores persistent cluster data.

Full Transcript

In Kubernetes, the control plane components work together to manage the cluster. The user sends a request to the API server, which validates and processes it. The API server reads and writes cluster state to etcd, the persistent storage. The scheduler watches for pods needing assignment and assigns them to nodes. The controller manager monitors the cluster state and ensures pods run as expected. This flow keeps the cluster consistent and responsive to user commands.

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