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Service selector-based traffic switching

Service selector-based traffic switching is a mechanism used in container orchestration platforms like kubernetes to control the flow of network traffic between different application versions^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md]. By manipulating the label selectors defined in a [[Service]] resource, operators can instantly redirect external traffic from an older version of a deployment to a new one without changing the Service's endpoint configuration (such as its IP or DNS name)^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].

This technique is a foundational element for implementing various deployment strategies, such as Blue/Green Deployment, allowing for distinct separation between versions and facilitating zero-downtime updates^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].

Mechanism

In systems like Kubernetes, a [[Service]] acts as an abstraction layer that defines a stable network endpoint for a set of [[Pods]]^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md]. The Service determines which Pods receive traffic by evaluating a label selector against the labels assigned to those Pods^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].

Traffic switching is accomplished by updating this selector. For example, if version 1 Pods are labeled version: v1 and the Service selector is version: v1, all traffic goes to version 1^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md]. To switch to version 2, the Service configuration is updated so its selector becomes version: v2^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md]. The orchestration platform immediately detects this change and updates the routing rules to forward traffic to the Pods matching the new criteria.

Process

Implementing service selector-based traffic switching typically involves the following workflow^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md]:

  1. Initial State: Deploy the existing version (e.g., v1) of the application. Create a Service configured with a selector that matches v1 labels, ensuring it handles all incoming traffic^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].
  2. Deployment of New Version: Deploy the new version (e.g., v2) of the application with unique labels (e.g., version: v2). Wait for v2 to be fully ready and running alongside v1^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].
  3. Switching: Update the Service's selector to target the new label (e.g., version: v2). This instantly shifts traffic flow from the old set of Pods to the new set^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].
  4. Cleanup: Once traffic has successfully switched to v2 and stability is confirmed, terminate the old v1 resources^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].

Advantages

  • Zero Downtime: Because the new version is fully started before the switch occurs, users experience no interruption in service^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].
  • Stability: This method avoids the risks associated with having multiple versions receiving traffic simultaneously during a gradual rollout, as the switch is atomic^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].

Considerations

  • Resource Overhead: Since the old and new versions must run concurrently during the transition phase, the infrastructure requires enough capacity to handle approximately twice the usual resource load (CPU/Memory) for a short period^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md].

Sources

^[400-devops__06-Kubernetes__k8s-ithelp__Day14__README.md]