When working with Kubernetes, you may have encountered Endpoints resources, which define how traffic is routed to pods selected by a service. Within these resources, two key fields often spark questions: Addresses and NotReadyAddresses. Understanding the distinction between these fields is crucial for troubleshooting service connectivity and readiness issues in your Kubernetes cluster.
What Are Kubernetes Endpoints?
In Kubernetes, an Endpoint represents the IPs and ports of the pods selected by a service. These are dynamically updated based on the state of the pods and their readiness. Kubernetes uses this information to decide which pods should handle traffic.
The Endpoints resource includes two fields:
Addresses:
- Lists the IPs of pods that are ready to receive traffic.
- Pods in this list have successfully passed their readiness checks, if defined.
NotReadyAddresses:
- Lists the IPs of pods that are not ready to receive traffic.
- Pods in this list may have failed their readiness checks, are still initializing, or are otherwise considered not ready by Kubernetes.
Traffic Routing Behavior
One of the most common questions is: Does Kubernetes send requests to pods in NotReadyAddresses?
The short answer is no. Kubernetes exclusively routes traffic to pods listed under Addresses. Here’s a detailed breakdown:
- Pods in
Addresses- Actively participate in service load balancing.
- These pods are considered healthy and ready to serve requests.
- Pods in
NotReadyAddresses- Are excluded from service traffic routing.
- These pods remain in the list until they transition to a ready state by passing their readiness checks.
Common Scenarios
Scenario 1: Service Without Readiness Gates
When a pod does not define a ReadinessProbe, Kubernetes assumes the pod is ready as soon as it is running. In this case, all pods are listed under Addresses, and NotReadyAddresses remains empty.
Scenario 2: Pods with Readiness Probes
If a ReadinessProbe is configured, pods may fail readiness checks for various reasons—for example, during startup or when they encounter application-level issues. Such pods are moved to the NotReadyAddresses list until they pass the readiness probe.
Scenario 3: Headless Services
For headless services (those with ClusterIP: None), Kubernetes does not provide load balancing. Instead, it publishes DNS records for all pod IPs, including those in NotReadyAddresses. It’s up to the client to decide whether to use these endpoints and handle readiness on its own.
Scenario 4: StatefulSets
In StatefulSets, all pods—ready or not—may be critical for the application’s functionality. For example, a database cluster might need to synchronize data with not-ready pods. NotReadyAddresses can be used to discover such pods for specific use cases.
Why NotReadyAddresses Matters
The presence of NotReadyAddresses provides valuable insights into your cluster’s health and readiness state. Here’s why they’re essential:
- Troubleshooting:
Pods inNotReadyAddressesindicate readiness issues. Investigate and resolve these issues to ensure seamless service delivery. - Monitoring:
Tools like Prometheus and Grafana can alert you to pods in theNotReadyAddresseslist, helping you proactively address problems. - Advanced Use Cases:
For debugging or special configurations, you might need to interact with not-ready pods explicitly.
Best Practices
- Configure Readiness Probes:
Define meaningful readiness probes for your pods. These should accurately reflect whether your application is ready to serve traffic.
- Monitor
NotReadyAddresses:
Regularly monitor yourEndpointsresources for entries inNotReadyAddresses. This can indicate potential issues with your pods or cluster.
- Understand Your Application’s Needs:
For stateful applications, consider scenarios where interacting with not-ready pods may be necessary.
- Debug Readiness Issues:
Usekubectl describe podto inspect why a pod is failing readiness checks. Logs and events can often provide the clues you need.
Wrapping Up
Kubernetes’s distinction between Addresses and NotReadyAddresses is a powerful mechanism for managing traffic and maintaining application reliability. By understanding and leveraging these fields, you can ensure your services route traffic only to healthy pods while keeping tabs on those that need attention.
Next time you troubleshoot a service issue or design a robust Kubernetes application, remember to check the Endpoints resource for these crucial details. It’s a small yet impactful step toward maintaining a healthy and resilient cluster.