The Metal³ project (pronounced: Metal Kubed) exists to provide components that allow you to do bare metal host management for Kubernetes. Metal³ works as a Kubernetes application, meaning it runs on Kubernetes and is managed through Kubernetes interfaces.

If you are looking for documentation about how to use Metal³, please check the user-guide.

Metal3 Component Overview

It is helpful to understand the high level architecture of of the Machine API Integration. Click on each step to learn more about that particular component.

The first component is the Bare Metal Actuator, which is an implementation of the Machine Actuator interface defined by the cluster-api project. This actuator reacts to changes to Machine objects and acts as a client of the BareMetalHost custom resources managed by the Bare Metal

The architecture also includes a new Bare Metal Operator, which includes the following:

A Controller for a new Custom Resource, BareMetalHost. This custom resource represents an inventory of known (configured or automatically discovered) bare metal hosts. When a Machine is created the Bare Metal Actuator will claim one of these hosts to be provisioned as a new Kubernetes node. In response to BareMetalHost updates, the controller will perform bare metal host provisioning actions as necessary to reach the desired state. The creation of the BareMetalHost inventory can be done in two ways:

  1. Manually via creating BareMetalHost objects.
  2. Optionally, automatically created via a bare metal host discovery process. For more information about Operators, see the operator-sdk.

The operator manages a set of tools for controlling the power on the host, monitoring the host status, and provisioning images to the host. These tools run inside the pod with the operator, and do not require any configuration by the user.


  1. Enroll nodes by creating BareMetalHost resources. This would either be manually or done by a component doing node discovery and introspection.

    See the documentation in the baremetal-operator repository for details.

  2. Use the machine API to allocate a machine.

    See the documentation in the cluster-api-provider-metal3 repository for details.

  3. The new Machine is associated with an available BareMetalHost, which triggers provisioning of that host to join the cluster. This association is done by the Actuator when it sets the MachineRef field on the BareMetalHost.

Design Documents

The design documents for Metal3 are all publicly available. Refer to the metal3-io/metal3-docs github repository for details.

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