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helm/docs/architecture.md

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The Kubernetes Helm Architecture

This document describes the Helm architecture at a high level.

The Purpose of Helm

Helm is a tool for managing Kubernetes packages called charts. Helm can do the following:

  • Create new charts from scratch
  • Package charts into chart archive (tgz) files
  • Interact with chart repositories where charts are stored
  • Install and uninstall charts into an existing Kubernetes cluster
  • Manage the release cycle of charts that have been installed with Helm

For Helm, there are three important concepts:

  1. The chart is a bundle of information necessary to create an instance of a Kubernetes application.
  2. The config contains configuration information that can be merged into a packaged chart to create a releasable object.
  3. A release is a running instance of a chart, combined with a specific config.

Components

Helm is an executable which is implemented into two distinct parts:

The Helm Client is a command-line client for end users. The client is responsible for the following:

  • Local chart development
  • Managing repositories
  • Managing releases
  • Interfacing with the Helm library
    • Sending charts to be installed
    • Requesting upgrading or uninstalling of existing releases

The Helm Library provides the logic for executing all Helm operations. It interfaces with the Kubernetes API server and provides the following capability:

  • Combining a chart and configuration to build a release
  • Installing charts into Kubernetes, and providing the subsequent release object
  • Upgrading and uninstalling charts by interacting with Kubernetes

The standalone Helm library encapsulates the Helm logic so that it can be leveraged by different clients.

Implementation

The Helm client and library is written in the Go programming language.

The library uses the Kubernetes client library to communicate with Kubernetes. Currently, that library uses REST+JSON. It stores information in Secrets located inside of Kubernetes. It does not need its own database.

Configuration files are, when possible, written in YAML.