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Charts

Helm uses a packaging format called charts. A chart is a collection of files that describe a related set of Kubernetes resources. A single chart might be used to deploy something simple, like a memcached pod, or something complex, like a full web app stack with HTTP servers, databases, caches, and so on.

Charts are created as files laid out in a particular directory tree, then they can be packaged into versioned archives to be deployed.

This document explains the chart format, and provides basic guidance for building charts with Helm.

The Chart File Structure

A chart is organized as a collection of files inside of a directory. The directory name is the name of the chart (without versioning information). Thus, a chart describing Wordpress would be stored in the wordpress/ directory.

Inside of this directory, Helm will expect a structure that matches this:

wordpress/
  Chart.yaml        # A YAML file containing information about the chart
  LICENSE           # OPTIONAL: A plain text file containing the license for the chart
  README.md         # OPTIONAL: A human-readable README file
  values.yaml       # The default configuration values for this chart
  charts/           # OPTIONAL: A directory containing any charts upon which this chart depends.
  templates/        # OPTIONAL: A directory of templates that, when combined with values,
                    # will generate valid Kubernetes manifest files.

Helm will silently strip out any other files.

The Chart.yaml File

The Chart.yaml file is required for a chart. It contains the following fields:

name: The name of the chart (required)
version: A SemVer 2 version (required)
description: A single-sentence description of this project (optional)
keywords:
  - A list of keywords about this project (optional)
home: The URL of this project's home page (optional)
sources:
  - A list of URLs to source code for this project (optional)
maintainers: # (optional)
  - name: The maintainer's name (required for each maintainer)
    email: The maintainer's email (optional for each maintainer)
engine: gotpl # The name of the template engine (optional, defaults to gotpl)

If you are familiar with the Chart.yaml file format for Helm Classic, you will notice that fields specifying dependencies have been removed. That is because the new Chart format expresses dependencies using the charts/ directory.

Other fields will be silently ignored.

Charts and Versioning

Every chart must have a version number. A version must follow the SemVer 2 standard. Unlike Helm Classic, Kubernetes Helm uses version numbers as release markers. Packages in repositories are identified by name plus version.

For example, an nginx chart whose version field is set to version: 1.2.3 will be named:

nginix-1.2.3.tgz

More complex SemVer 2 names are also supported, such as version: 1.2.3-alpha.1+ef365. But non-SemVer names are explicitly disallowed by the system.

NOTE: Whereas Helm Classic and Deployment Manager were both very GitHub oriented when it came to charts, Kubernetes Helm does not rely upon or require GitHub or even Git. Consequently, it does not use Git SHAs for versioning at all.

The version field inside of the Chart.yaml is used by many of the Helm tools, including the CLI and the Tiller server. When generating a package, the helm package command will use the version that it finds in the Chart.yaml as a token in the package name. The system assumes that the version number in the chart package name matches the version number in the Chart.yaml. Failure to meet this assumption will cause an error.

Chart Dependencies

In Helm, one chart may depend on any number of other charts. These dependencies are expressed explicitly by copying the dependency charts into the charts/ directory.

Note: The dependencies: section of the Chart.yaml from Helm Classic has been completely removed.

For example, if the Wordpress chart depends on the Apache chart, the Apache chart (of the correct version) is supplied in the Wordpress chart's charts/ directory:

wordpress:
  Chart.yaml
  # ...
  charts/
    apache/
      Chart.yaml
      # ...
    mysql/
      Chart.yaml
      # ...

The example above shows how the Wordpress chart expresses its dependency on Apache and MySQL by including those charts inside of its charts/ directory.

TIP: To drop a dependency into your charts/ directory, use the helm fetch command.

Templates and Values

By default, Helm Chart templates are written in the Go template language, with the addition of 50 or so add-on template functions. (In the future, Helm may support other template languages, like Python Jinja.)

All template files are stored in a chart's templates/ folder. When Helm renders the charts, it will pass every file in that directory through the template engine.

Values for the templates are supplied two ways:

Chart developers may supply a file called values.yaml inside of a chart. This file can contain default values.
Chart users may supply a YAML file that contains values. This can be provided on the command line with helm install.

When a user supplies custom values, these values will override the values in the chart's values.yaml file.

Template Files

Template files follow the standard conventions for writing Go templates. An example template file might look something like this:

apiVersion: v1
kind: ReplicationController
metadata:
  name: deis-database
  namespace: deis
  labels:
    heritage: deis
spec:
  replicas: 1
  selector:
    app: deis-database
  template:
    metadata:
      labels:
        app: deis-database
    spec:
      serviceAccount: deis-database
      containers:
        - name: deis-database
          image: {{.imageRegistry}}/postgres:{{.dockerTag}}
          imagePullPolicy: {{.pullPolicy}}
          ports:
            - containerPort: 5432
          env:
            - name: DATABASE_STORAGE
              value: {{default "minio" .storage}}

The above example, based loosely on https://github.com/deis/charts, is a template for a Kubernetes replication controller. It can use the following four template values:

imageRegistry: The source registry for the Docker image.
dockerTag: The tag for the docker image.
pullPolicy: The Kubernetes pull policy.
storage: The storage backend, whose default is set to "minio"

All of these values are defined by the template author. Helm does not require or dictate parameters.

Predefined Values

The following values are pre-defined, are available to every template, and cannot be overridden. As with all values, the names are case sensitive.

Release.Name: The name of the release (not the chart)
Release.Time: The time the chart release was last updated. This will match the Last Released time on a Release object.
Release.Namespace: The namespace the chart was released to.
Release.Service: The service that conducted the release. Usually this is Tiller.
Chart: The contents of the Chart.yaml. Thus, the chart version is obtainable as Chart.Version and the maintainers are in Chart.Maintainers.

NOTE: Any unknown Chart.yaml fields will be dropped. They will not be accessible inside of the Chart object. Thus, Chart.yaml cannot be used to pass arbitrarily structured data into the template.

Values files

Considering the template in the previous section, a values.yaml file that supplies the necessary values would look like this:

imageRegistry = "quay.io/deis"
dockerTag = "latest"
pullPolicy = "alwaysPull"
storage = "s3"

A values file is formatted in YAML. A chart may include a default values.yaml file. The Helm install command allows a user to override values by supplying additional YAML values:

$ helm install --values=myvals.yaml wordpress

When values are passed in this way, they will be merged into the default values file. For example, consider a myvals.yaml file that looks like this:

storage = "gcs"

When this is merged with the values.yaml in the chart, the resulting generated content will be:

imageRegistry = "quay.io/deis"
dockerTag = "latest"
pullPolicy = "alwaysPull"
storage = "gcs"

Note that only the last field was overridden.

NOTE: The default values file included inside of a chart must be named values.yaml. But files specified on the command line can be named anything.

Scope, Dependencies, and Values

Values files can declare values for the top-level chart, as well as for any of the charts that are included in that chart's charts/ directory. Or, to phrase it differently, a values file can supply values to the chart as well as to any of its dependencies. For example, the demonstration Wordpress chart above has both mysql and apache as dependencies. The values file could supply values to all of these components:

title = "My Wordpress Site" # Sent to the Wordpress template

[mysql]
max_connections = 100 # Sent to MySQL
password = "secret"

[apache]
port = 8080 # Passed to Apache

Charts at a higher level have access to all of the variables defined beneath. So the wordpress chart can access mysql.password. But lower level charts cannot access things in parent charts, so MySQL will not be able to access the title property. Nor, for that matter, can it access apache.port.

Values are namespaced, but namespaces are pruned. So for the Wordpress chart, it can access the MySQL password field as mysql.password. But for the MySQL chart, the scope of the values has been reduced and the namespace prefix removed, so it will see the password field simply as password.

References

When it comes to writing templates and values files, there are several standard references that will help you out.

Using Helm to Manage Charts

The helm tool has several commands for working with charts.

It can create a new chart for you:

$ helm create mychart
Created mychart/

Once you have edited a chart, helm can package it into a chart archive for you:

$ helm package mychart
Archived mychart-0.1.-.tgz

You can also use helm to help you find issues with your chart's formatting or information:

$ helm lint mychart
No issues found

Chart Repositories

A chart repository is an HTTP server that houses one or more packaged charts. While helm can be used to manage local chart directories, when it comes to sharing charts, the preferred mechanism is a chart repository.

Any HTTP server that can serve YAML files and tar files and can answer GET requests can be used as a repository server.

Helm comes with built-in package server for developer testing (helm serve). The Helm team has tested other servers, including Google Cloud Storage with website mode enabled, and S3 with website mode enabled.

A repository is characterized primarily by the presence of a special file called index.yaml that has a list of all of the packages supplied by the repository, together with metadata that allows retrieving and verifying those packages.

On the client side, repositories are managed with the helm repo commands. However, Helm does not provide tools for uploading charts to remote repository servers. This is because doing so would add substantial requirements to an implementing server, and thus raise the barrier for setting up a repository.

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