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# The Kubernetes Helm Architecture
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This document describes the Helm architecture at a high level.
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## The Purpose of Helm
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Helm is a tool for managing Kubernetes packages called _charts_. Helm
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can do the following:
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- Create new charts from scratch
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- Package charts into chart archive (tgz) files
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- Interact with chart repositories where charts are stored
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- Install and uninstall charts into an existing Kubernetes cluster
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- Manage the releases of charts that have been installed with Helm
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For Helm, there are three important concepts:
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1. The _chart_ is a bundle of information necessary to create an
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instance of a Kubernetes application.
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2. The _config_ contains configuration information that can be merged
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into a packaged chart to create a releasable object.
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3. A _release_ is a running instance of a _chart_, combined with a
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specific _config_.
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Following the formula made famous by the [12 Factor App](http://12factor.net/), _chart + config
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= release_.
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## Components
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Helm has two major components:
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**The Helm Client** is a command-line client for end users. The client
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is responsible for the following domains:
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- Local chart development
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- Managing repositories
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- Interacting with the Tiller server
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- Sending charts to be installed
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- Asking for information about releases
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- Requesting upgrading or uninstalling of existing releases
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**The Tiller Server** is an in-cluster server that interacts with the
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Helm client, and interfaces with the Kubernetes API server. The server
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is responsible for the following:
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- Listing for incomming requests from the Helm client
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- Combining a chart and configuration to build a release
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- Installing charts into Kubernetes, and then tracking the subsequent
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release
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- Upgrading and uninstalling charts by interacting with Kubernetes
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In a nutshell, the client is responsible for managing charts, and the
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server is responsible for managing releases.
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## Implementation
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The Helm client is written in the Go programming language, and uses the
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gRPC protocol suite to interact with the Tiller server.
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The Tiller server is also written in Go. It provides a gRPC server to
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connect with the client, and it uses the Kubernetes client library to
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communicate with Kubernetes. Currently, that library uses REST+JSON.
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The Tiller server stores information in ConfigMaps located inside of
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Kubernetes. It does not need its own database.
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### Structure of the Code
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The individual programs are located in `cmd/`. Shared libraries are
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stored in `pkg/`. The raw ProtoBuf files are stored in `_proto/hapi`
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(where `hapi` stands for the Helm Application Programming Interface).
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The Go files generated from the `proto` definitions are stored in
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`pkg/proto`.
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Docker images are built by cross-compiling Linux binaries and then
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building a Docker image from the files in `rootfs`.
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The `scripts/` directory contains a number of utility scripts, including
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`local-cluster.sh`, which can start a full Kubernetes instance inside of
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a Docker container.
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Go dependencies are managed with
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[Glide](https://github.com/Masterminds/glide) and stored in the
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`vendor/` directory.
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