Using labels for service routing in Kubernetes - Step-by-Step Execution

Process Flow - Using labels for service routing

Create Pods with labels

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Define Service selector

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Service routes traffic

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Traffic reaches matching Pods

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Pods respond to requests

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END

Pods are created with labels. The Service uses selectors to find Pods with matching labels. Traffic sent to the Service is routed to those Pods.

Execution Sample

Kubernetes

apiVersion: v1
kind: Pod
metadata:
  labels:
    app: myapp
---
apiVersion: v1
kind: Service
metadata:
  name: myservice
spec:
  selector:
    app: myapp
  ports:
  - port: 80
    targetPort: 80

Defines Pods with label 'app: myapp' and a Service selecting Pods with that label to route traffic on port 80.

Process Table

Step	Action	Resource	Labels/Selector	Result
1	Create Pod	Pod 'pod1'	app: myapp	Pod created with label app=myapp
2	Create Pod	Pod 'pod2'	app: myapp	Pod created with label app=myapp
3	Create Service	Service 'myservice'	selector app=myapp	Service created selecting pods with app=myapp
4	Send traffic to Service	Service 'myservice'	selector app=myapp	Traffic routed to pod1 and pod2
5	Pods respond	Pods 'pod1' and 'pod2'	app=myapp	Pods handle requests
6	Send traffic to Service	Service 'myservice'	selector app=myapp	Traffic routed only to pods matching label
7	Create Pod	Pod 'pod3'	app: otherapp	Pod created with label app=otherapp (not selected)
8	Send traffic to Service	Service 'myservice'	selector app=myapp	Traffic NOT routed to pod3
9	END			Routing only to pods with matching labels

💡 Pods without matching labels are excluded from Service routing

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 7	Final
Pods with label app=myapp	0	1 (pod1)	2 (pod1, pod2)	2 (pod1, pod2)	2 (pod1, pod2)
Pods with label app=otherapp	0	0	0	1 (pod3)	1 (pod3)
Service selector	none	none	app=myapp	app=myapp	app=myapp
Traffic routed pods	none	none	pod1, pod2	pod1, pod2	pod1, pod2

Key Moments - 3 Insights

Why doesn't the Service route traffic to pod3 even though it exists?

Because pod3 has label app=otherapp which does not match the Service selector app=myapp (see execution_table step 8). Only pods with matching labels receive traffic.

What happens if a Pod is created without any labels?

The Service selector won't find it, so traffic won't be routed to that Pod. Labels are required for Service routing (see execution_table steps 1-3).

Can a Service route traffic to Pods with multiple different labels?

No, the Service selector matches Pods with the exact label set specified. To route to multiple label sets, multiple Services or label selectors with multiple keys are needed.

Visual Quiz - 3 Questions

Look at the execution table at step 4. Which Pods receive traffic from the Service?

A pod1 and pod2

B pod3 only

C pod1, pod2, and pod3

D No Pods receive traffic

💡 Hint

Check the 'Result' column at step 4 in the execution_table.

At which step does a Pod with a non-matching label get created?

A Step 2

B Step 5

C Step 7

D Step 3

💡 Hint

Look for the Pod with label app=otherapp in the execution_table.

If the Service selector changed to app=otherapp, which Pods would receive traffic?

A pod1 and pod2

B pod3 only

C All Pods

D No Pods

💡 Hint

Refer to variable_tracker for Pods with label app=otherapp.

Concept Snapshot

Pods have labels as tags.
Services use selectors to find Pods by labels.
Traffic sent to Service goes only to matching Pods.
Pods without matching labels do not get traffic.
Labels enable flexible routing in Kubernetes.

Full Transcript

In Kubernetes, Pods are created with labels, which are like name tags. A Service uses a selector to find Pods with specific labels. When you send traffic to the Service, it routes the requests only to Pods whose labels match the selector. Pods without matching labels do not receive traffic. This method allows you to control which Pods handle which requests by labeling them accordingly.