valuefrom

ValueFrom is a mechanism in Kubernetes that allows a container’s environment variables to be populated from sources other than hard-coded literals. Defined on an EnvVar as valueFrom, the value is resolved when the container starts, enabling dynamic configuration without embedding values in the pod spec.

ValueFrom can reference several sources. ConfigMapKeyRef points to a specific key within a ConfigMap, and SecretKeyRef points to a key within a Secret. FieldRef exposes downward API fields from the pod or node, such as metadata.name, metadata.namespace, status.podIP, or other recorded fields. ResourceFieldRef exposes container resource values, such as limits or requests for CPU, memory, or ephemeral storage, with an optional divisor to scale the value. Each source supports an optional flag to indicate that the reference is not required; if the referenced key or field is missing and optional is true, the environment variable will be empty or omitted rather than causing the pod to fail.

Usage patterns include injecting configuration from ConfigMaps or Secrets, exposing pod metadata to application logic, or deriving values from resource requests and limits to adapt behavior at startup. Values sourced via valueFrom are resolved once at container startup and do not automatically update if the underlying sources change; a pod restart is typically required to refresh the value.

In summary, valueFrom provides a flexible, declarative approach to configuring containers by deriving environment values from ConfigMaps, Secrets, pod fields, or resource specifications, with control over optionality and startup behavior.