Kubernetes: Up and Running, 3rd Edition

Chapter 6. Labels and Annotations

Kubernetes was made to grow with you as your application scales in both size and complexity. Labels and annotations are fundamental concepts in Kubernetes that let you work in sets of things that map to how you think about your application. You can organize, mark, and cross-index all of your resources to represent the groups that make the most sense for your application.

Labels are key/value pairs that can be attached to Kubernetes objects such as Pods and ReplicaSets. They can be arbitrary and are useful for attaching identifying information to Kubernetes objects. Labels provide the foundation for grouping objects.

Annotations, on the other hand, provide a storage mechanism that resembles labels: key/value pairs designed to hold nonidentifying information that tools and libraries can leverage. Unlike labels, annotations are not meant for querying, filtering, or otherwise differentiating Pods from each other.

Labels

Labels provide identifying metadata for objects. These are fundamental qualities of the object that will be used for grouping, viewing, and operating. The motivations for labels grew out of Google’s experience in running large and complex applications. A couple of lessons emerged from this experience:

• Production abhors a singleton. When deploying software, users often start with a single instance. However, as the application matures, these singletons often multiply and become sets of objects. With this in mind, Kubernetes uses labels ...