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helm/docs/plugins.md

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The Helm Plugins Guide

Helm 2.1.0 introduced the concept of a client-side Helm plugin. A plugin is a tool that can be accessed through the helm CLI, but which is not part of the built-in Helm codebase.

Existing plugins can be found on related section or by searching Github.

This guide explains how to use and create plugins.

An Overview

Helm plugins are add-on tools that integrate seamlessly with Helm. They provide a way to extend the core feature set of Helm, but without requiring every new feature to be written in Go and added to the core tool.

Helm plugins have the following features:

  • They can be added and removed from a Helm installation without impacting the core Helm tool.
  • They can be written in any programming language.
  • They integrate with Helm, and will show up in helm help and other places.

Helm plugins live in $(helm home)/plugins.

The Helm plugin model is partially modeled on Git's plugin model. To that end, you may sometimes hear helm referred to as the porcelain layer, with plugins being the plumbing. This is a shorthand way of suggesting that Helm provides the user experience and top level processing logic, while the plugins do the "detail work" of performing a desired action.

Installing a Plugin

Plugins are installed using the $ helm plugin install <path|url> command. You can pass in a path to a plugin on your local file system or a url of a remote VCS repo. The helm plugin install command clones or copies the plugin at the path/url given into $ (helm home)/plugins

$ helm plugin install https://github.com/technosophos/helm-template

If you have a plugin tar distribution, simply untar the plugin into the $(helm home)/plugins directory.

You can also install tarball plugins directly from url by issuing helm plugin install http://domain/path/to/plugin.tar.gz

Building Plugins

In many ways, a plugin is similar to a chart. Each plugin has a top-level directory, and then a plugin.yaml file.

$(helm home)/plugins/
  |- keybase/
      |
      |- plugin.yaml
      |- keybase.sh

In the example above, the keybase plugin is contained inside of a directory named keybase. It has two files: plugin.yaml (required) and an executable script, keybase.sh (optional).

The core of a plugin is a simple YAML file named plugin.yaml. Here is a plugin YAML for a plugin that adds support for Keybase operations:

name: "keybase"
version: "0.1.0"
usage: "Integrate Keybase.io tools with Helm"
description: |-
  This plugin provides Keybase services to Helm.
ignoreFlags: false
useTunnel: false
command: "$HELM_PLUGIN_DIR/keybase.sh"

The name is the name of the plugin. When Helm executes it plugin, this is the name it will use (e.g. helm NAME will invoke this plugin).

name should match the directory name. In our example above, that means the plugin with name: keybase should be contained in a directory named keybase.

Restrictions on name:

  • name cannot duplicate one of the existing helm top-level commands.
  • name must be restricted to the characters ASCII a-z, A-Z, 0-9, _ and -.

version is the SemVer 2 version of the plugin. usage and description are both used to generate the help text of a command.

The ignoreFlags switch tells Helm to not pass flags to the plugin. So if a plugin is called with helm myplugin --foo and ignoreFlags: true, then --foo is silently discarded.

The useTunnel switch indicates that the plugin needs a tunnel to Tiller. This should be set to true anytime a plugin talks to Tiller. It will cause Helm to open a tunnel, and then set $TILLER_HOST to the right local address for that tunnel. But don't worry: if Helm detects that a tunnel is not necessary because Tiller is running locally, it will not create the tunnel.

Finally, and most importantly, command is the command that this plugin will execute when it is called. Environment variables are interpolated before the plugin is executed. The pattern above illustrates the preferred way to indicate where the plugin program lives.

There are some strategies for working with plugin commands:

  • If a plugin includes an executable, the executable for a command: should be packaged in the plugin directory.
  • The command: line will have any environment variables expanded before execution. $HELM_PLUGIN_DIR will point to the plugin directory.
  • The command itself is not executed in a shell. So you can't oneline a shell script.
  • Helm injects lots of configuration into environment variables. Take a look at the environment to see what information is available.
  • Helm makes no assumptions about the language of the plugin. You can write it in whatever you prefer.
  • Commands are responsible for implementing specific help text for -h and --help. Helm will use usage and description for helm help and helm help myplugin, but will not handle helm myplugin --help.

Downloader Plugins

By default, Helm is able to fetch Charts using HTTP/S. As of Helm 2.4.0, plugins can have a special capability to download Charts from arbitrary sources.

Plugins shall declare this special capability in the plugin.yaml file (top level):

downloaders:
- command: "bin/mydownloader"
  protocols:
  - "myprotocol"
  - "myprotocols"

If such plugin is installed, Helm can interact with the repository using the specified protocol scheme by invoking the command. The special repository shall be added similarly to the regular ones: helm repo add favorite myprotocol://example.com/ The rules for the special repos are the same to the regular ones: Helm must be able to download the index.yaml file in order to discover and cache the list of available Charts.

The defined command will be invoked with the following scheme: command certFile keyFile caFile full-URL. The SSL credentials are coming from the repo definition, stored in $HELM_HOME/repository/repositories.yaml. Downloader plugin is expected to dump the raw content to stdout and report errors on stderr.

The downloader command also supports sub-commands or arguments, allowing you to specify for example bin/mydownloader subcommand -d in the plugin.yaml. This is useful if you want to use the same executable for the main plugin command and the downloader command, but with a different sub-command for each.

Environment Variables

When Helm executes a plugin, it passes the outer environment to the plugin, and also injects some additional environment variables.

Variables like KUBECONFIG are set for the plugin if they are set in the outer environment.

The following variables are guaranteed to be set:

  • HELM_PLUGIN: The path to the plugins directory
  • HELM_PLUGIN_NAME: The name of the plugin, as invoked by helm. So helm myplug will have the short name myplug.
  • HELM_PLUGIN_DIR: The directory that contains the plugin.
  • HELM_BIN: The path to the helm command (as executed by the user).
  • HELM_HOME: The path to the Helm home.
  • HELM_PATH_*: Paths to important Helm files and directories are stored in environment variables prefixed by HELM_PATH.
  • TILLER_HOST: The domain:port to Tiller. If a tunnel is created, this will point to the local endpoint for the tunnel. Otherwise, it will point to $HELM_HOST, --host, or the default host (according to Helm's rules of precedence).

While HELM_HOST may be set, there is no guarantee that it will point to the correct Tiller instance. This is done to allow plugin developer to access HELM_HOST in its raw state when the plugin itself needs to manually configure a connection.

A Note on useTunnel

If a plugin specifies useTunnel: true, Helm will do the following (in order):

  1. Parse global flags and the environment
  2. Create the tunnel
  3. Set TILLER_HOST
  4. Execute the plugin
  5. Close the tunnel

The tunnel is removed as soon as the command returns. So, for example, a command cannot background a process and assume that process will be able to use the tunnel.

A Note on Flag Parsing

When executing a plugin, Helm will parse global flags for its own use. Some of these flags are not passed on to the plugin.

  • --debug: If this is specified, $HELM_DEBUG is set to 1
  • --home: This is converted to $HELM_HOME
  • --host: This is converted to $HELM_HOST
  • --kube-context: This is simply dropped. If your plugin uses useTunnel, this is used to set up the tunnel for you.

Plugins should display help text and then exit for -h and --help. In all other cases, plugins may use flags as appropriate.