A Helm package that contains information sufficient for installing a set of Kubernetes resources into a Kubernetes cluster.
Charts contain a
Chart.yaml file as well as templates, default values
values.yaml), and dependencies.
Charts are developed in a well-defined directory structure, and then packaged into an archive format called a chart archive.
A chart archive is a tarred and gzipped (and optionally signed) chart.
Chart Dependency (Subcharts)
Charts may depend upon other charts. There are two ways a dependency may occur:
- Soft dependency: A chart may simply not function without another chart being installed in a cluster. Helm does not provide tooling for this case. In this case, dependencies may be managed separately.
- Hard dependency: A chart may contain (inside of its
charts/directory) another chart upon which it depends. In this case, installing the chart will install all of its dependencies. In this case, a chart and its dependencies are managed as a collection.
When a chart is packaged (via
helm package) all of its hard dependencies are
bundled with it.
Charts are versioned according to the SemVer 2 spec. A version number is required on every chart.
Information about a chart is stored in a special file called
chart must have this file.
Helm (and helm)
Helm is the package manager for Kubernetes. As an operating system package manager makes it easy to install tools on an OS, Helm makes it easy to install applications and resources into Kubernetes clusters.
While Helm is the name of the project, the command line client is also named
helm. By convention, when speaking of the project, Helm is capitalized. When
speaking of the client, helm is in lowercase.
Helm Configuration Files (XDG)
Helm stores its configuration files in XDG directories. These directories are
created the first time
helm is run.
Kube Config (KUBECONFIG)
The Helm client learns about Kubernetes clusters by using files in the Kube
config file format. By default, Helm attempts to find this file in the place
kubectl creates it (
To lint a chart is to validate that it follows the conventions and
requirements of the Helm chart standard. Helm provides tools to do this, notably
helm lint command.
Provenance (Provenance file)
Helm charts may be accompanied by a provenance file which provides information about where the chart came from and what it contains.
Provenance files are one part of the Helm security story. A provenance contains a cryptographic hash of the chart archive file, the Chart.yaml data, and a signature block (an OpenPGP "clearsign" block). When coupled with a keychain, this provides chart users with the ability to:
- Validate that a chart was signed by a trusted party
- Validate that the chart file has not been tampered with
- Validate the contents of a chart metadata (
- Quickly match a chart to its provenance data
Provenance files have the
.prov extension, and can be served from a chart
repository server or any other HTTP server.
When a chart is installed, the Helm library creates a release to track that installation.
A single chart may be installed many times into the same cluster, and create
many different releases. For example, one can install three PostgreSQL databases
helm install three times with a different release name.
Release Number (Release Version)
A single release can be updated multiple times. A sequential counter is used to
track releases as they change. After a first
helm install, a release will have
release number 1. Each time a release is upgraded or rolled back, the release
number will be incremented.
A release can be upgraded to a newer chart or configuration. But since release
history is stored, a release can also be rolled back to a previous release
number. This is done with the
helm rollback command.
Importantly, a rolled back release will receive a new release number.
|rollback 1||release 4 (but running the same config as release 1)|
The above table illustrates how release numbers increment across install, upgrade, and rollback.
Helm Library (or SDK)
The Helm Library (or SDK) refers to the Go code that interacts directly with the Kubernetes API server to install, upgrade, query, and remove Kubernetes resources. It can be imported into a project to use Helm as a client library instead of a CLI.
Repository (Repo, Chart Repository)
Helm charts may be stored on dedicated HTTP servers called chart repositories (repositories, or just repos).
A chart repository server is a simple HTTP server that can serve an
file that describes a batch of charts, and provides information on where each
chart can be downloaded from. (Many chart repositories serve the charts as well
A Helm client can point to zero or more chart repositories. By default, Helm
clients are not configured with any chart repositories. Chart repositories can
be added at any time using the
helm repo add command.
Values (Values Files, values.yaml)
Values provide a way to override template defaults with your own information.
Helm Charts are "parameterized", which means the chart developer may expose
configuration that can be overridden at installation time. For example, a chart
may expose a
username field that allows setting a user name for a service.
These exposed variables are called values in Helm parlance.
Values can be set during
helm install and
helm upgrade operations, either by
passing them in directly, or by using a