Merge pull request kubernetes#122 from smarterclayton/kubectl_extension

Proposal: kubectl extension

kubectl-extension.md

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+<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
+
+If you are using a released version of Kubernetes, you should
+refer to the docs that go with that version.
+
+<!-- TAG RELEASE_LINK, added by the munger automatically -->
+<strong>
+The latest release of this document can be found
+[here](http://releases.k8s.io/release-1.3/docs/proposals/kubectl-extension.md).
+
+Documentation for other releases can be found at
+[releases.k8s.io](http://releases.k8s.io).
+</strong>
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+# Kubectl Extension
+
+Abstract
+--------
+
+Allow `kubectl` to be extended to include other commands that can provide new functionality without recompiling Kubectl
+
+
+Motivation and Background
+-------------------------
+
+Kubernetes is designed to be a composable and extensible system, with the ability to add new APIs and features via Third Party Resources
+or API federation, by making the server provide functionality that eases writing generic clients, and by supporting other authentication
+systems. Given that `kubectl` is the primary method for interacting with the server, some new extensions are difficult to make usable
+for end users without recompiling that command. In addition, it is difficult to prototype new functionality for kubectl outside of the
+Kubernetes source tree.
+
+Ecosystem tools like OpenShift, Deis, and Helm add additional workflow around kubectl targeted at the end user. It is beneficial
+to encourage workflows to develop around Kubernetes without requiring them to be part of Kubernetes to both the end user community
+and the Kubernetes developer community.
+
+There are many tools that currently offer CLI extension for the same reasons - [Git](https://www.kernel.org/pub/software/scm/git/docs/howto/new-command.html) and
+[Heroku](https://devcenter.heroku.com/articles/developing-cli-plug-ins#creating-the-package) are two relevant examples in the space.
+
+
+Proposal
+--------
+
+Define a system for `kubectl` that allows new subcommands and subcommand trees to be added by placing an executable in a specific
+location on disk, like Git.  Allow third parties to extend kubectl by placing their extensions in that directory. Ensure that help
+and other logic correctly includes those extensions.
+
+A kubectl command extension would be an executable located in `EXEC_PATH` (an arbitrary directory to be defined that follows similar
+conventions in Linux) with a name pattern like `kubectl-COMMAND[-SUBCOMMAND[...]]` with one or many sub parts. The presence of
+a command extension overrides any built in command.
+
+A key requirement is that the lookup be fast (since it would be invoked on every execution of `kubectl`) and so some true extension
+behavior (such as complex inference of commands) may not be supported in order to reduce the complexity of the lookup.
+
+Kubectl would lazily include the appropriate commands in preference to the internal command structure if detected (a user asking for
+`kubectl a b c` would *first* check for `kubectl-a-b-c`, `kubectl-a-b`, or `kubectl-a` before loading the internal command).
+
+All kubectl command extensions MUST:
+
+* Support the `-h` and `--help` flags to display a help page
+* Respect the semantics of KUBECONFIG lookup (to be further specified)
+
+All kubectl command extensions SHOULD:
+
+* Follow the display and output conventions of normal kubectl commands.
+