What are annotations in Kubernetes (k8s)? What is their purpose?

In Kubernetes, annotations are metadata that can be added to objects in the cluster, such as pods, services, and deployments. Annotations are key-value pairs that are stored in the object’s metadata and are not used by the Kubernetes control plane.

Annotations are intended to be used to store information that is relevant to the object, but that is not used by the control plane to manage the object. They can be used to store information such as:

  • Notes or comments about the object
  • Configuration details or settings that are not used by the control plane
  • Metrics or statistics about the object
  • Custom labels or tags that are not used by the control plane

Annotations are a flexible and convenient way to store additional information about objects in the cluster that is not used by the control plane. They can be used to store information that is relevant to the object, but that does not fit in the object’s standard metadata.

Examples

Storing notes or comments about an object:

metadata:
  annotations:
    notes: "This pod is used for testing purposes only."

Storing configuration details or settings for an object:

metadata:
  annotations:
    config: |
      {
        "max_connections": 100,
        "log_level": "debug"
      }

Storing metrics or statistics about an object:

metadata:
  annotations:
    metrics: |
      {
        "cpu_usage": 0.5,
        "memory_usage": 0.8
      }

Storing custom labels or tags for an object:

metadata:
  annotations:
    tags: "testing,dev"

Annotations can be accessed and modified using the Kubernetes API or the kubectl command-line tool. They can also be used in conjunction with other features in Kubernetes, such as labels and selectors, to filter and group objects in the cluster.


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