Oct 11, 2023 • Mikolaj Gasior

Context

In a Kubernetes cluster, there’s a ConfigMap resource that holds configuration data. Whenever it gets updated, an application endpoint (URL) needs to be triggered to refresh its settings dynamically.

Implementation

To watch for changes, we can build a custom Kubernetes controller. Fortunately, this is straightforward in Golang using the foundations provided by the sample-controller from the Kubernetes project.

I’ve put together a working implementation, available in my repository: configmap-controller-example.

The controller listens for Kubernetes events, specifically updates to ConfigMaps. It filters changes based on a specific annotation and, when detected, triggers an HTTP request to a predefined endpoint.

For instance, a ConfigMap can include an annotation like:

myctlr/onchange-url: 'https://neverssl.com'

To make the controller act on it, simply start it with:

--annotation=myctlr/onchange-url

Source code

You can find the complete source code in configmap-controller-example repository. The repo includes sample YAML manifests, a Dockerfile, and a README with setup instructions.

Kubernetes SDK

For a deep dive into how Kubernetes client libraries power this controller, check out this detailed guide, which includes an insightful architecture diagram.