Chapter 2: Inside the Control Room
In Chapter 1, we learned about the main parts of Kubernetes — the Cluster, Control Plane, Worker Nodes, Pods, and Containers.
Now it is time to open the doors of the Control Office and look inside. The Control Plane is not one thing. It is four components working together, and each Worker Node also has internal components that make everything run.
The Four Components of the Control Plane
Part 1: The Front Door
Kubernetes Concept: API Server
The API Server is the front door of the entire Kubernetes system. Every request — from you, from other components, from automated systems — goes through the API Server.
Nothing in Kubernetes happens without going through the API Server.
BMTC Analogy: The Customer Service Counter at Majestic.
Want to ask about a route? Go to the counter. Want to submit a complaint? Go to the counter. Want to request a new bus? Go to the counter. Internal departments sending updates? They go through the counter.
No one bypasses the counter. Everything is recorded, validated, and processed at the counter.
# Check the API Server health
kubectl get componentstatuses
# List all API resources available
kubectl api-resources
# Explain a specific resource
kubectl explain pod
Part 2: The Master Database
Kubernetes Concept: etcd
etcd is the database of Kubernetes. It stores everything:
- What Pods should exist
- What Pods currently exist
- All configurations
- All the state of the cluster
If etcd is lost without backup, the cluster loses all knowledge of itself. This is why etcd is treated with extreme care.
BMTC Analogy: The Central BMTC Master Database.
Every route, every bus, every depot location, every driver record, every schedule, every maintenance log. Everything is here. If this database was wiped clean, BMTC would not know how many buses it has, what routes exist, or which depots are operational.
Part 3: The Operations Manager
Kubernetes Concept: Scheduler
The Scheduler watches for new Pods that have been created but not yet assigned to a Worker Node. It then assigns them to the best available Node based on resource availability, constraints, and policies.
BMTC Analogy: The Operations Manager at the Central Office.
When a new bus needs to be dispatched, the Operations Manager checks: Which depot has available bays? Which depot has fuel? Which depot is closest to the route? Then they say: "Send the bus from Yeshwanthpur Depot."
Part 4: The Senior Operations Officer
Kubernetes Concept: Controller Manager
The Controller Manager runs a set of control loops. A control loop is a simple process that runs forever:
- Check the current situation
- Compare it to the desired situation
- Take action to fix any difference
- Repeat
BMTC Analogy: A Senior Operations Officer who walks through the entire system all day, every day. They check every route, every depot, every bus. If something is wrong — a bus broke down, a depot is overloaded, a route has no buses — they trigger the right action. They never stop checking.
CONTROL LOOP EXAMPLE:
Desired State: 5 buses on Route 500D
Current State: 4 buses on Route 500D (one broke down)
Action: Create 1 new Pod (dispatch 1 replacement bus)
Result: 5 buses on Route 500D ✓
Part 5: The External Coordination Officer
Kubernetes Concept: Cloud Controller Manager
When Kubernetes runs on a cloud provider (AWS, GCP, Azure), there are external systems to coordinate with:
- Cloud load balancers
- Cloud storage
- Cloud networking
- Cloud node management
The Cloud Controller Manager handles this communication with external systems.
BMTC Analogy: The External Coordination Officer who interfaces with Bengaluru Traffic Police (route approvals), BBMP (road conditions), GPS providers (real-time tracking), and fuel suppliers (automated ordering). These are outside BMTC's direct control. This officer is the bridge.
Part 6: Inside a Worker Node
Now let us look inside a Worker Node (Depot). Three key components live on every Worker Node.
Kubelet — The Depot Manager
The Kubelet runs on every Worker Node. It receives instructions from the Control Plane and ensures the right Pods are running on its Node.
BMTC Analogy: The Depot Manager. The Control Office sends orders. The Depot Manager receives them and makes sure the right buses are dispatched, running, and healthy.
# Check which Node a Pod is running on
kubectl get pods -o wide
# Get detailed info about a Node
kubectl describe node <node-name>
Container Runtime — The Mechanic
The Container Runtime is the software that actually starts and runs containers. It does the low-level work of pulling images and starting processes.
BMTC Analogy: The Mechanic who physically starts the bus engine, checks the oil, and gets the bus ready to roll. The Depot Manager (Kubelet) gives the order. The Mechanic (Container Runtime) executes it.
Kube-proxy — The Traffic Marshaller
Kube-proxy runs on every Node and handles networking. It maintains network rules so that traffic to a Service gets forwarded to the right Pod.
BMTC Analogy: The depot's traffic marshaller who directs arriving passenger vehicles to the right bus bay. "Route 500D passengers? Bay 5. Airport Express? Bay 12."
The Complete Request Flow
Let us trace what happens when you run: kubectl apply -f deployment.yaml
🛠️ Try It Yourself
# View cluster info
kubectl cluster-info
# List all nodes in the cluster
kubectl get nodes
# View component health
kubectl get componentstatuses
# Get detailed node information
kubectl describe nodes
# View events in the cluster
kubectl get events --sort-by='.lastTimestamp'
Chapter 2 Summary
| Term | BMTC Meaning | Kubernetes Meaning |
|---|---|---|
| API Server | Customer Service Counter | Front door for all requests |
| etcd | Central BMTC Database | Stores all cluster state |
| Scheduler | Operations Manager | Assigns Pods to Nodes |
| Controller Manager | Senior Operations Officer | Runs control loops to maintain desired state |
| Cloud Controller Manager | External Coordination Officer | Interfaces with cloud providers |
| Kubelet | Depot Manager | Manages Pods on each Worker Node |
| Container Runtime | Mechanic | Starts and runs containers |
| Kube-proxy | Traffic Marshaller | Handles networking on each Node |