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Welcome to this lesson on troubleshooting a peculiar behavior in Kubernetes deployments. In this guide, we will explore why our blue service sometimes returns responses from the green application. Let’s dive in.

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Observing the Problem

When inspecting the cluster, both the blue and green deployments are running as expected. The following command output confirms this:
The blue service is expected to serve a blue screen, while the green service serves a green screen using a basic web server. When accessing the green service in your browser, each refresh may hit a different replica due to load balancing across multiple pods. However, upon accessing the blue service endpoint repeatedly, you may notice an unexpected mix of responses that sometimes include a green background.

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Investigating Service Definitions

To diagnose the issue, let’s review the service definitions.

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Current Cluster State

The initial output of our cluster services is as follows:

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Listing Service Definition Files

The two service configuration files in the directory are:

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blue-svc.yaml

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green-svc.yaml

The green service’s selector uses both version: v1 and app: green, isolating the green pods properly. The blue service, however, only uses version: v1 as its selector. Since both blue and green pods share the version: v1 label, the blue service unintentionally selects green pods too, leading to mixed responses.

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Understanding Label Selectors and Endpoints

Kubernetes services route traffic to all pods matching their label selectors. To verify the selected pods for label version=v1, run:
Checking the endpoints for each service further clarifies the issue:
Notice that the blue service includes endpoints for both blue and green pods, whereas the green service correctly targets only green pods.
Use the command “k edit svc blue-service” to inspect and modify the blue service configuration in real time.

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Fixing the Issue

To ensure that the blue service routes traffic only to blue pods, update the blue service selector by adding a unique label. Modify the selector to include both version: v1 and app: blue. After making this change, the blue service will exclusively target blue pods. Once updated, verify the endpoints again:
This confirmation shows that the blue service now exclusively routes requests to the correct blue pods, resolving the intermittent misrouting issue.

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Final Thoughts

This troubleshooting exercise highlights the critical importance of using unique labels in Kubernetes deployments. Overlapping selectors can cause unexpected behavior and intermittent failures that are challenging to diagnose in large-scale environments. Understanding how label selectors, service endpoints, and load balancing interact is vital for maintaining reliable application deployments in Kubernetes.
The image illustrates a Kubernetes deployment setup called "Schrödinger’s Deployment," featuring two services with version selectors, a load balancer, and multiple pods. It also indicates a 90% success rate for responses.
Even if the application appears to work correctly most of the time, such misconfigurations can lead to intermittent issues that are hard to catch during routine monitoring.
The image is a graphic about "Troubleshooting Configuration," featuring an icon of a document with a gear and code symbol, and a note about label issues being overlooked during monitoring.
Having a robust troubleshooting process, including verifying service endpoints and label selectors, is essential for diagnosing and resolving configuration issues in Kubernetes.
The image lists three key considerations: Endpoints Application, Service Load Balancing, and Label Selector Matching, each represented by a distinct icon.
Happy troubleshooting, and see you in the next lesson!

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