Merge pull request #32400 from sftim/20220321_document_downward_api_concept

Revise downward API documentation

content/en/docs/reference/glossary/downward-api.md

@@ -0,0 +1,28 @@
+---
+title: Downward API
+id: downward-api
+date: 2022-03-21
+short_description: >
+  A mechanism to expose Pod and container field values to code running in a container.
+aka:
+full_link: /docs/concepts/workloads/pods/downward-api/
+tags:
+- architecture
+---
+Kubernetes' mechanism to expose Pod and container field values to code running in a container.
+<!--more-->
+It is sometimes useful for a container to have information about itself, without
+needing to make changes to the container code that directly couple it to Kubernetes.
+
+The Kubernetes downward API allows containers to consume information about themselves
+or their context in a Kubernetes cluster. Applications in containers can have
+access to that information, without the application needing to act as a client of
+the Kubernetes API.
+
+There are two ways to expose Pod and container fields to a running container:
+
+- using [environment variables](/docs/tasks/inject-data-application/environment-variable-expose-pod-information/)
+- using [a `downwardAPI` volume](/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/)
+
+Together, these two ways of exposing Pod and container fields are called the _downward API_.
+