How to Monitor Route Synchronization in Kubernetes with the New CCM Metric

Introduction

Kubernetes v1.36 introduces a powerful new alpha metric, route_controller_route_sync_total, in the Cloud Controller Manager (CCM) route controller. This metric counts every route synchronization event with your cloud provider, making it an essential tool for operators who want to validate the CloudControllerManagerWatchBasedRoutesReconciliation feature gate (introduced in v1.35). By comparing the sync rate with the feature gate turned off (fixed-interval loop) versus on (watch-based reconciliation), you can measure the efficiency gains and reduce unnecessary API calls to your infrastructure provider.

This guide will walk you through setting up and using the new metric to A/B test the feature gate, interpret the results, and provide feedback to the Kubernetes community.

What You Need

• A Kubernetes cluster running v1.36 or later with the Cloud Controller Manager deployed.
• Access to the CCM’s metrics endpoint (typically on port 10258 or as configured).
• or similar tool to interact with the cluster.
• Basic familiarity with Kubernetes feature gates and Prometheus metrics.
• Optional: A monitoring system (e.g., Prometheus) to collect and visualize the metric over time.

Step-by-Step Guide

1. Step 1: Verify Cluster Version and CCM Configuration

   Ensure your cluster runs Kubernetes v1.36 or newer. Check the CCM version by running:

   kubectl get pods -n kube-system -l component=cloud-controller-manager -o jsonpath='{.items[0].spec.containers[0].image}'

   Confirm the CCM uses at least the v1.36 image (e.g., registry.k8s.io/cloud-controller-manager:v1.36.0). Also verify the --controllers=route flag is set, as the route controller must be active.

2. Step 2: Access the CCM Metrics Endpoint

   By default, the CCM exposes metrics on port 10258 at the /metrics path. You can access it via port-forwarding:

   kubectl port-forward -n kube-system pod/<ccm-pod-name> 10258:10258

   Then open http://localhost:10258/metrics in a browser or use curl. Look for route_controller_route_sync_total in the output.

3. Step 3: Observe Baseline Metric Behavior (Feature Gate Disabled)

   With the feature gate disabled (the default), the CCM syncs routes every 10 seconds regardless of node changes. Run the following command to watch the metric:

   watch -n 30 'curl -s http://localhost:10258/metrics | grep route_controller_route_sync_total'

   In a stable cluster with no node changes, expect the counter to increase by 3 every 30 seconds (because 30 seconds = three 10-second syncs). For example:

   # HELP route_controller_route_sync_total [ALPHA] Number of times routes have been synced
   # TYPE route_controller_route_sync_total counter
   route_controller_route_sync_total 60

   Record this baseline value for later comparison.

4. Step 4: Enable the Watch‑Based Reconciliation Feature Gate

   Edit the CCM deployment or manifest to add the feature gate:

   containers:
   - name: cloud-controller-manager
     args:
     - --feature-gates=CloudControllerManagerWatchBasedRoutesReconciliation=true

   Apply the change and wait for the CCM pod to restart. Verify the new flag is active by checking the CCM logs:

   kubectl logs -n kube-system <ccm-pod> --tail=10 | grep -i watch

   You should see a message indicating the watch-based reconciliation is enabled.

5. Step 5: Observe the Metric with the Feature Gate Enabled

   Now repeat Step 3. With the feature gate enabled, the counter should only increment when nodes are added, removed, or updated. In a fully stable cluster, you’ll see the counter stay almost constant. For example:

   route_controller_route_sync_total 1

   After 20 minutes with no node changes, the counter remains 1. When a node joins, it increments to 2, and so on. Compare this to the steady increase you saw earlier.

6. Step 6: Compare and Analyze the Results

   Use the data you collected to quantify the reduction in syncs. In a stable production cluster, the difference is dramatic. For instance, without the feature gate, you might see 6 syncs per minute (360 per hour). With watch-based reconciliation, only actual changes trigger syncs—potentially zero or a handful per hour. This reduces pressure on cloud provider APIs and saves quota.

   If your cluster experiences frequent node changes (e.g., auto‑scaling), the reduction may be less pronounced, but the watch‑based approach still avoids unnecessary syncs when nodes are static.

7. Step 7: Provide Feedback and Learn More

   If you tested the feature or have suggestions, share your experience via:

   • The #sig-cloud-provider channel on Kubernetes Slack
   • The KEP‑5237 issue on GitHub
   • The SIG Cloud Provider community page for other channels

   For deeper technical details, refer to the official KEP‑5237 documentation.

Tips for Success

• Enable Prometheus scraping: Integrate the CCM metrics into your Prometheus stack and create a dashboard to track route_controller_route_sync_total over time. This helps identify patterns and anomalies.
• Test in a non‑production cluster first: Since the metric and feature gate are alpha, run thorough tests before enabling in production. Ensure your cloud provider’s CCM implementation supports the watch‑based approach.
• Monitor API quota usage: The primary benefit is reduced load on cloud provider APIs. Use cloud provider dashboards to confirm that API calls drop when the feature gate is enabled.
• Watch for node churn: If your cluster auto‑scales frequently, the reduction may be less visible, but you’ll still benefit from avoided syncs during idle periods.
• Stay updated: As the feature progresses to beta and stable, check the Kubernetes changelog for changes in default behavior or metric naming.