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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 releases 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.

Following the formula made famous by the 12 Factor App, chart + config = release.

Components

Helm has two major components:

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

  • Local chart development
  • Managing repositories
  • Interacting with the Tiller server
    • Sending charts to be installed
    • Asking for information about releases
    • Requesting upgrading or uninstalling of existing releases

The Tiller Server is an in-cluster server that interacts with the Helm client, and interfaces with the Kubernetes API server. The server is responsible for the following:

  • Listing for incomming requests from the Helm client
  • Combining a chart and configuration to build a release
  • Installing charts into Kubernetes, and then tracking the subsequent release
  • Upgrading and uninstalling charts by interacting with Kubernetes

In a nutshell, the client is responsible for managing charts, and the server is responsible for managing releases.

Implementation

The Helm client is written in the Go programming language, and uses the gRPC protocol suite to interact with the Tiller server.

The Tiller server is also written in Go. It provides a gRPC server to connect with the client, and it uses the Kubernetes client library to communicate with Kubernetes. Currently, that library uses REST+JSON.

The Tiller server stores information in ConfigMaps located inside of Kubernetes. It does not need its own database.

Structure of the Code

The individual programs are located in cmd/. Shared libraries are stored in pkg/. The raw ProtoBuf files are stored in _proto/hapi (where hapi stands for the Helm Application Programming Interface). The Go files generated from the proto definitions are stored in pkg/proto.

Docker images are built by cross-compiling Linux binaries and then building a Docker image from the files in rootfs.

The scripts/ directory contains a number of utility scripts. Most of these are used by the CI/CD pipeline.

Go dependencies are managed with Glide and stored in the vendor/ directory.