Merge pull request kubernetes#88126 from johnbelamaric/behaviors-readme

Add a README describing behaviors

Author: k8s-ci-robot

test/conformance/behaviors/README.md

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+# Behaviors Defining Conformance
+
+The conformance program is intended to answer the question "What is
+Kubernetes?". That is, what features, functions, and APIs are needed in order to
+call something "Kubernetes". Since v1.9, this has been defined as passing a
+specific set of e2e tests.  As part of the [conformance behavior KEP](https://git.k8s.io/enhancements/keps/sig-architecture/20190412-conformance-behaviors.md),
+this instead is moved to an explicit list of "behaviors", which are captured in
+this directory tree. The e2e tests are used to validate whether specific
+behaviors are met, but the definition of conformance is based upon that list of
+approved behaviors. This allows separate reviewers for behaviors and tests,
+provides a description of conformance separate from long, complex tests with
+code, and enables the definition of conformance to encompass behaviors for which
+tests have not yet been written.  All of this begs the question, though, "what is
+a behavior?".
+
+In behavior driven development, it is sometimes defined as the "business logic"
+expected from the software. That is, it is a sequence of reactions to some
+stimulus: an API call, a component failure, a network request on the data plane,
+or some other action. We can classify these reactions into a few different types
+in Kubernetes:
+
+1. Transient runtime / communication state
+1. Cluster state changes observable via the control plane
+1. Cluster state changes observable via the data plane
+
+Another way to think about this is that a behavior is the combination of the
+question and answer to "What happens when...".
+
+A behavior will consist of:
+
+* A description of the initial conditions and stimulus
+* A description of the resulting reactions
+* If necessary, a sequencing (ordering) of those reactions
+
+All this is still pretty vague, so it is helpful to enumerate some of the
+characteristics expected of behavior descriptions for conformance in particular.
+
+ - Behaviors should be defined at the user-visible level. Things happening
+   behind the scenes that are not visible to the user via the API or actual data
+   plane execution do not need to be part of conformance.
+ - Behaviors should describe portable features. That is, they should be
+   expected to work as described across vendor systems.
+ - Behaviors should be defined so they are minimally constraining; if a detail is
+   not needed for portability, it should be left out.
+ - Behaviors should not be tied to specific implementations of components, or even
+   to the existence of specific components. If a vendor chooses to rewrite a
+   component or replace it with something else, they should still pass conformance
+   simply by meeting the expected behaviors.
+ - Ordered sequencing of the reactions should be avoided unless absolutely
+   necessary. For example, it is core to the `initContainers` functionality that
+   they run before the main containers, so in that case ordering is required.
+
+<!--
+ TODO Examples: include example of a good behavior and a poorly written behavior
+-->