Certified Kubernetes Application Developer (CKAD) Study Guide, 2nd Edition

Chapter 9. Labels and Annotations

The exam curriculum doesn’t explicitly mention the concept of labels; however, it’s an important one for understanding how certain Kubernetes primitives function internally. To avoid confusing labels with annotations, we’ll also discuss the commonalities and differences among those concepts.

Labels are an essential tool for querying, filtering, and sorting Kubernetes objects. Annotations represent descriptive metadata for Kubernetes objects but can’t be used for queries. In this chapter, you will learn how to assign and use both concepts.

Coverage of Curriculum Objectives

The curriculum doesn’t explicitly mention coverage of labels and annotations. It’s important to understand labels because primitives like the Deployment, Service, and NetworkPolicy can’t function without them.

Working with Labels

Kubernetes lets you assign key-value pairs to objects so that you can use them later within a search query. Those key-value pairs are called labels. To draw an analogy, you can think of labels as tags for a blog post.

A label describes a Kubernetes object in distinct terms (e.g., a category like “frontend” or “backend”), but it is not meant for elaborate, multi-word descriptions of its functionality. As part of the specification, Kubernetes limits the length of a label to a maximum of 63 characters and a range of allowed alphanumeric and separator characters.

Figure 9-1 shows the Pods named frontend, backend, and database. Each of the Pods declares ...