Helm Concepts

Internal

• Helm
• Templates
• Configuration
• Dependencies

Overview

Helm is a tool for managing Kubernetes packages called charts. Helm creates new charts from scratch, packages charts into chart archives (tgz) files, interacts with chart repositories where charts are stored, and finally installs and manages the release cycle of the underlying applications into an existing Kubernetes cluster. The installation process takes a chart, applies custom, operational environment-specific configuration and installs the chart into the Kubernetes cluster, creating a release.

Helm allows defining an application as a set of Kubernetes cluster components and provides mechanisms to manage these sets for their entire lifecycle.

Chart

Helm Glossary - Chart

A chart is a Helm package and a package format, the Kubernetes equivalent of a yum RPM file or a Homebrew formula. A chart contains all resource definitions necessary to deploy and run an application inside Kubernetes, arranged in a specific layout. A chart may be used to deploy a simple application, or a full web application stack with multiple dependencies, such as HTTP servers, database, caches and so on. A chart can be unpackaged, in which case it is a folder with files that follow the Helm chart rules and conventions. A chart can also be packaged, as a tar.gz chart archive.

Chart Name

A chart is identified by its name in repositories. Examples: "nginx" or "postgresql". The chart name is sometimes referred to as chart reference. The name is part of the package identification, along with chart version. The name of the chart is also specified in the name field of the Chart.yaml metadata file. Chart names should use lower case letter and numbers, and start with a letter. Hyphens are allowed. Uppercase letter, underscores or dots should not be used in chart names.

The directory that contains a chart must have the same name as the chart.

For more details see Chart Names Best Practices.

Chart Versioning

Chart.yaml carries two version fields: the chart version (version) and the application version (appVersion).

Every chart must have a SemVer 2 version specified in the version field of the Chart.yaml metadata file.

Packages exposed by repositories, as well as packages generated by helm package are identified by their name and their version. For example, the chart archive of an "nginx" package whose version filed is "1.2.3" will be named:

nginx-1.2.3.tgz

Helm tools rely on the version section of the chart archive file name as release marker. 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. For more details on chart version see Chart Version Numbers Best Practices.

The appVersion field is intended to carry an informal application version, not necessarily SemVer 2, and independent from the version field. This field is informational, and has no impact on chart version calculations. It is very common for the chart version and app version to be different:

NAME                      CHART VERSION	APP VERSION DESCRIPTION
stable/postgresql         6.3.2        	11.5.0      Chart for PostgreSQL, an object-relational database manag...

Chart Archive

A chart archive is a chart in a packaged (tgz) format. The chart repositories expose chart archives. The helm package command creates chart archives.

Chart Type

The type designates a chart as 'application' or 'library'. 'application' is the default type. A 'library' or 'helper' chart provides utilities or functions for the chart builder, it has no resource object and therefore is not installable. Example: https://github.com/helm/charts/tree/master/incubator/common. An application chart can be used as a library chart. This is enabled by setting the type to 'library'. The chart will then be rendered as a library chart where all utilities and functions can be leveraged. No resource objects contained by the chart will be rendered.

Chart Installation

Charts are installed into a target Kubernetes cluster and turned into releases with:

helm install

The installation is an asynchronous process, the Helm client does not wait until all resources are running before it exits, this is because charts may need large container images that take long to install in the cluster. Without additional input, a chart will be installed with the default configuration, but custom configuration changes can be applied during the installation process.

The default option for the helm install command is to install from the default repository. However, the chart may come from other sources: a different repository, a local file, an unpacked chart directory or a URL.

Resource Installation and De-Installation Order

Template Installation and De-Installation Order

Chart Layout

This section describes a typical chart directory tree. The top level directory is the chart name, without the version information.

archaeopteryx
 ├─ Chart.yaml
 ├─ values.yaml
 ├─ LICENSE
 ├─ README.md
 ├─ .helmignore
 ├─ charts
 └─ templates
     ├─ NOTES.txt
     ├─ _helpers.tpl
     ├─ deployment.yaml
     ├─ service.yaml
     │ ...
     └─ tests

charts/ Directory

The 'charts' directory contains any charts upon which this chart depends. The content of the directory can either manually managed, or it can be updated dynamically for dependencies declared in Chart.yaml for Helm 3 or requirements.yaml for Helm 2. A dependency can be either a chart archive or an unpacked chart directory. If the name of the dependency, either file or directory, starts with '_' or '.', the dependency will be ignored by the chart loader. For more details on dependencies see:

How Helm Dependencies Work

templates/ Directory

The templates/ Directory

Developing Custom Charts

The development of a custom chart could start with the helm create command, which is a command that creates a standard chart directory layout along with some common files typically used in a chart. The command provides just a basic example and is not meant to cover all Kubernetes resources. While a chart is being developed, the syntactic consistency can be checked with the helm lint command. Then the chart is ready for delivery, it can be packaged with helm package.

Chart Operations

• helm create
• helm lint
• helm package
• helm install
• helm show
• helm get manifest

Release

A release is a running instance of a chart, to which a specific configuration was applied. Whenever a chart is installed, a new release is created. One chart can be installed multiple times into the same cluster, and each can be independently managed and upgraded. Each release has its own release name. It is important to know that releases are namespace-scoped. There could be two releases with the same name, but in two different namespaces.

When a new version of a chart is released, or when the configuration of a release changes, the release can be upgraded with the helm upgrade command. Helm tries to perform the least invasive upgrade, and only upgraded things that changed since the last release. If something fails during the upgrade, a rollback can be performed with the helm rollback command.

At a certain moment in time, a release is in a certain state.

Releases can be uninstalled with the helm uninstall. Helm keeps a history of deleted releases, which can be retrieved with the helm list --deleted.

Release Name

If not specified, release names are generated (e.g. "happy-panda"). To specify a release name, use the helm install <name> ... command. Helm keeps records of deleted releases, so a release name first used by a deleted release cannot be re-used (unless --replace is used). The release name is available in templates as a built-in object:

 {{ .Release.Name }}

Many charts are configured so Kubernetes resources created during a Helm installation carry a "release" label with the value set to the release name.

Release Revision

A release version (revision) is incremental. Every time an install, upgrade or rollback happens, the revision number is incremented by 1, and the first revision is always 1. The revision numbers for a certain release can be inspected with the helm history command. A release revision can be used during a rollback. The release revision is available in templates as a built-in object:

 {{ .Release.Revision }}

Release State

The release state is reported by the helm status command.

A release state is one of:

Release Marker

The package version is included in the release marker.

Resource Association with a Release

Resources created by Helm are associated with their Helm release via annotations:

kind: ...
metadata:
  annotations:
    meta.helm.sh/release-name: release-example
    meta.helm.sh/release-namespace: namespace-example
  labels:
    app.kubernetes.io/managed-by: Helm

Preventing Resource Deletion upon Uninstallation

https://helm.sh/docs/howto/charts_tips_and_tricks/#tell-helm-not-to-uninstall-a-resource

Normally, all resources created by a Helm installation are removed upon uninstallation: Helm uninstalls by default all resources created by the chart for the release being uninstalled. There are cases when we may want to keep resources around: one case is when we want to keep a PersistentVolumeClaim because deleting it would make the a PersistentVolume outside of our control unavailable for binding for the subsequent release.

To indicate to Helm that a resource should be kept on uninstall, use the "helm.sh/resource-policy" annotation in template:

kind: Something
  metadata:
    annotations:
      helm.sh/resource-policy: keep

The "helm.sh/resource-policy" instructs helm to skip deleting the resource on uninstall, upgrade or rollback operations.

Note that this resource becomes orphaned, it will be no longer be managed by Helm. This can lead to problems when using the helm install --replace command on a release that has already been uninstalled, but has kept resources.

Helm does not seem to remove the annotations and labels it placed on the resource, so how does it know not to manage it anymore?

Release Operations

• helm install
• helm get manifest - display the manifest content that was posted to the server as part of the installation process.
• helm list - display currently deployed releases.
• helm history
• helm status
• helm upgrade
• helm rollback
• helm uninstall

The Helm Client

The Helm client is a command-line client for end users. The client is responsible for managing charts. It helps with local chart development and manages the interaction with chart repositories. The interaction with the server consists in installing charts and providing a CLI for the release life cycle (the server manages the releases).

Templates

Helm Templates

Configuration

Configuration

Dependencies

Helm Dependencies

Repository

Helm Repository

Hooks

Hooks

Security

Security Concepts

Plugins

Plugin Concepts

Tests

Helm Chart Tests

Labels

Most charts configure the Kubernetes resources created by them to carry a "release" label, whose value is the release name. This convention could be useful in providing a mechanism to identify these resources using label selectors.

Helm 2 Deprecated Elements

• requirements.yaml
• requirements.lock
• Tiller