Pod-to-Pod communication in Kubernetes - Step-by-Step Execution

Process Flow - Pod-to-Pod communication

Pod A sends request

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Kubernetes Network Plugin

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Pod B receives request

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Pod B sends response

↓

Kubernetes Network Plugin

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Pod A receives response

Pod A sends a request through the Kubernetes network plugin to Pod B, which receives it and sends a response back through the same network path.

Execution Sample

Kubernetes

kubectl exec pod-a -- curl http://pod-b:80
# Pod A sends HTTP request to Pod B's IP or DNS

Pod A uses curl to send an HTTP request to Pod B using its service name or IP address.

Process Table

Step	Action	Source Pod	Destination Pod	Network Plugin Role	Result
1	Pod A initiates HTTP request	pod-a	pod-b	Routes request to pod-b IP	Request sent
2	Network plugin routes packet	pod-a	pod-b	Forwards packet to pod-b network namespace	Packet delivered
3	Pod B receives request	pod-a	pod-b	N/A	Pod B processes request
4	Pod B sends HTTP response	pod-b	pod-a	Routes response back to pod-a	Response sent
5	Network plugin routes response	pod-b	pod-a	Forwards packet to pod-a network namespace	Response delivered
6	Pod A receives response	pod-b	pod-a	N/A	Pod A receives data
7	Connection closed	pod-a	pod-b	N/A	Communication ends

💡 Communication ends after Pod A receives response and connection closes

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 6	Final
Request Status	Not sent	Sent	Received by Pod B	Response received by Pod A	Closed
Response Status	Not sent	N/A	Not sent	Sent by Pod B	Received

Key Moments - 3 Insights

How does Pod A know where to send the request?

Pod A uses the DNS name or IP address of Pod B, which Kubernetes manages via services or direct pod IPs, as shown in execution_table step 1.

What role does the network plugin play in communication?

The network plugin routes packets between pods across nodes or within the same node, forwarding requests and responses as shown in steps 2 and 5.

Why does communication end after step 6?

After Pod A receives the response, the connection is closed to free resources, ending communication as noted in step 7.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Pod B process the request?

A Step 5

B Step 1

C Step 3

D Step 7

💡 Hint

Check the 'Result' column for when Pod B processes the request.

According to variable_tracker, what is the 'Request Status' after step 3?

A Not sent

B Received by Pod B

C Sent

D Closed

💡 Hint

Look at the 'Request Status' row under 'After Step 3' column.

If the network plugin fails at step 2, what happens to the request?

A Request is lost and Pod B never receives it

B Pod A receives an immediate response

C Request is delivered to Pod B

D Pod B sends a response anyway

💡 Hint

Step 2 shows the network plugin forwarding the packet; failure means no delivery.

Concept Snapshot

Pod-to-Pod communication in Kubernetes:
- Pods communicate via IP or DNS names
- Network plugin routes packets between pods
- Requests and responses flow through network namespaces
- Communication ends when connection closes
- Services can abstract pod IPs for stable access

Full Transcript

Pod-to-Pod communication in Kubernetes works by one pod sending a request to another pod's IP or DNS name. The Kubernetes network plugin routes the network packets between pods, even if they are on different nodes. Pod B receives the request, processes it, and sends a response back through the network plugin to Pod A. After Pod A receives the response, the connection closes, ending the communication. This process relies on Kubernetes networking and service discovery to enable pods to find and talk to each other.

Practice

1.

How do pods inside a Kubernetes cluster typically communicate with each other?

easy

A. Using external DNS names only

B. Using pod IP addresses or service names

C. Through direct access to the host machine's network

D. By sharing the same storage volume

Solution

1. Step 1: Understand pod networking basics

   Pods have their own IP addresses inside the cluster and can communicate directly using these IPs.

2. Step 2: Recognize the role of services

   Services provide stable DNS names to reach pods even if pod IPs change.

3. Final Answer:

   Using pod IP addresses or service names -> Option B

4. Quick Check:

   Pod communication = IP or service name [OK]

Hint: Pods use IPs or services to talk inside cluster [OK]

Common Mistakes:

• Thinking pods use external DNS only
• Assuming pods share host network by default
• Believing storage volumes enable communication

2.

Which of the following is the correct way to define a Kubernetes Service to expose pods for pod-to-pod communication?

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: myapp
  ports:
  - protocol: TCP
    port: 80
    targetPort: 8080

easy

A. The service exposes port 8080 externally but does not select pods

B. The selector matches pods with label app: myapp but ports are reversed

C. The selector matches pods with label app: myapp and forwards port 80 to 8080

D. The service definition is missing the type field, so it won't work

Solution

1. Step 1: Check the selector field

   The selector app: myapp correctly matches pods with that label.

2. Step 2: Verify ports mapping

   Port 80 on the service forwards traffic to port 8080 on the pods, which is correct.

3. Final Answer:

   The selector matches pods with label app: myapp and forwards port 80 to 8080 -> Option C

4. Quick Check:

   Selector and ports correct = The selector matches pods with label app: myapp and forwards port 80 to 8080 [OK]

Hint: Selector matches pod labels; ports map service to pod [OK]

Common Mistakes:

• Confusing port and targetPort roles
• Thinking 'type' is mandatory for basic service
• Ignoring selector means no pods matched

3.

Given two pods in the same namespace, pod-a and pod-b, where pod-b runs a web server on port 80, what is the expected output of this command run inside pod-a?

curl http://pod-b:80

Assuming pod-b is reachable by its pod IP but not by name.

medium

A. Connection refused or host not found error

B. Successful HTTP response from pod-b's web server

C. Timeout error because pods cannot communicate

D. DNS resolution error for pod-b

Solution

1. Step 1: Understand pod DNS naming

   Pods do not have DNS names by default; only services have stable DNS names.

2. Step 2: Analyze the command

   Trying to curl pod-b by name will fail DNS resolution because pod names are not DNS entries.

3. Final Answer:

   DNS resolution error for pod-b -> Option D

4. Quick Check:

   Pod names not DNS = DNS resolution error [OK]

Hint: Pod names are not DNS names; use service names [OK]

Common Mistakes:

• Assuming pod names resolve in DNS
• Expecting direct pod name communication
• Ignoring need for service abstraction

4.

You have a service named backend exposing pods on port 8080. A pod tries to connect using curl http://backend:80 but gets a connection refused error. What is the likely cause?

medium

A. The service port is 8080, but curl uses port 80

B. The service selector does not match any pods

C. The pod network is not enabled

D. The service type is NodePort instead of ClusterIP

Solution

1. Step 1: Check service port configuration

   The service exposes port 8080, but the curl command targets port 80, causing connection refusal.

2. Step 2: Understand port mapping importance

   Clients must use the service port, not the pod's internal port, to connect successfully.

3. Final Answer:

   The service port is 8080, but curl uses port 80 -> Option A

4. Quick Check:

   Port mismatch causes connection refused [OK]

Hint: Match curl port to service port exactly [OK]

Common Mistakes:

• Ignoring port mismatch between service and client
• Assuming network is disabled without checking
• Confusing service types with connectivity issues

5.

You want to ensure stable pod-to-pod communication in a Kubernetes cluster where pods restart frequently and get new IPs. Which approach best solves this problem?

hard

A. Use a Kubernetes Service to provide a stable DNS name and load balancing

B. Use pod IP addresses directly in application configuration

C. Configure pods to share the same static IP address

D. Use host networking mode for all pods

Solution

1. Step 1: Identify the problem with pod IPs

   Pods get new IPs on restart, so direct IP communication is unstable.

2. Step 2: Use Kubernetes Service for stability

   Services provide a stable DNS name and load balancing to reach pods regardless of IP changes.

3. Step 3: Evaluate other options

   Static IPs for pods are not practical; host networking breaks isolation; direct IP use is unreliable.

4. Final Answer:

   Use a Kubernetes Service to provide a stable DNS name and load balancing -> Option A

5. Quick Check:

   Service = stable pod communication [OK]

Hint: Use services for stable pod communication despite IP changes [OK]

Common Mistakes:

• Hardcoding pod IPs in configs
• Trying to assign static IPs to pods
• Using host networking unnecessarily