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This document explains the use of Node, Machine, and BareMetalHost objects to represent different facets of implementing a self-hosted and Kubernetes-native hardware provisioning system. It explains the role of each, and why we are using 3 separate resources.


This document details the original design decisions made when the Metal³ project was launched, and places the information in context as an aid to future contributors.


  • Explain Node, Machine, and BareMetalHost and how they relate to Metal³
  • Encourage input on that design, to refine it.


  • Discussion of user interface designs related to the resources.


The Metal³ project is building a hardware provisioning system with a Kubernetes-native API. It includes a provider implementation that fits into the API defined by the cluster-api SIG. It differs from other similar implementations by being self-hosted. Where the AWS or OpenStack providers run within Kubernetes, they allocate resources from a pool outside of the Kubernetes cluster by talking to an API that is also outside of the cluster. Metal³ will run within the cluster and present a Kubernetes API for tracking available resources.

Implementation Details/Notes/Constraints

Data Model

Node objects represent a running instance of kubelet. Their status fields include basic information about the health of the environment in which kubelet is running. A user manipulates the Node to change the cluster, such as draining the node for maintenance. The definition of Node is owned by the Kubernetes core, and offers few opportunities to attach custom data.

Machine objects represent a request for an instance of kubelet. A parallel design is the persistent volume claim and persistent volume in the storage APIs. The Machine CRD is owned by the cluster-api SIG. Machine objects are managed by the cluster-api controller using an actuator which works as a plugin driver for the controller. Machine objects include a "provider spec" field to allow the actuator to store custom data. The lifecycle of a Machine is based on the desired size of the cluster. That is, a Machine object does not typically exist in the database when it does not represent a request to expand the cluster (see caveats below in the Alternatives section.

BareMetalHost objects represent a physical computer, including its hardware inventory and information to access the onboard management controller. The BareMetalHost (or "host") CRD is defined in the baremetal-operator from the Metal³ project. The CRD is made up of completely custom data, needed to fully realize a Kubernetes-native API for a self-hosted baremetal provisioning tool. The lifecycle of a host is based on the availability of hardware, rather than its use as part of the cluster.

Growing the Cluster

The admin user interacts with Metal³ by first registering hosts to be part of the inventory available to the provisioning system. Registration can be a manual process, in which the user uses the UI or API to define a new BareMetalHost object, providing the information needed to access the host's management controller. Eventually registration may be partially automated by having the system create a skeleton host object when it receives a PXE request from an unknown MAC address.

After registration, each host is matched against known hardware profiles (comparing information such as NICs, storage, CPUs, RAM, etc.). Hosts with management controller credentials and that match known profiles are marked as being ready to be used. The admin (or autoscaler) can then create a new Machine object as a request to grow the cluster, either by creating the Machine directly or by incrementing the replica count on a MachineSet.

The actuator in the cluster-api-provider-baremetal repository will match the hardware profile in the provider spec of the Machine object to available hosts with the same profile. When an available host is found, the actuator links it to the Machine, and adds information to the host to tell the baremetal-operator which image to provision to it, which machine configuration to use, etc. After the host is provisioned and rebooted, kubelet connects to the cluster in the usual way, triggering the creation of a Node object.

Shrinking the Cluster

The admin user (or autoscaler) can similarly delete the Machine object, breaking the link to the host. When that happens the host is completely evacuated (going through the standard Kubernetes drain procedure, as well as removing any storage replicas it contains). The host is then powered off and placed back into the available inventory pool.

To completely remove a host, the BareMetalHost object is deleted. Whether this wipes the storage in the host is yet to be determined.

Risks and Mitigations

We risk some confusion by introducing a third entity tied to the "computer" that is part of the cluster. For other providers, the information we are storing in the host record would be stored in some external database (typically behind a cloud API). There will be a few new operations that admin users will perform on hosts that are not typically performed on compute resources managed by other providers; powering a host off to perform hardware maintenance is the main such use case.

Design Details

Component Relationships

Work Items

  • Write and maintain this document



Test Plan


Upgrade / Downgrade Strategy


Version Skew Strategy





Use an external database

We did consider maintaining a separate database for host details, but rejected that approach for a few reasons.

First, unlike with a typical cloud provider, the admin user must manage their inventory somehow. Even if we implement fully automated host discovery, the user must be able to review the information for accuracy, establish the access parameters for the onboard management controllers, etc. This means we must expose the data in some form to the user. We intend to implement Metal³ using an existing provisioning tool, so we could expose that API, but no such tool provides a Kubernetes-native experience, which is the purpose for Metal³ to exist.

The second reason for rejecting the "separate database" approach is that although we are starting by building on top of an existing system that depends on a long-running service to operate, we hope to iterate toward an implementation that does not require a long-running service, instead relying on short-lived Pods, perhaps using the Job API. These are future plans, with the details not completely worked out, but with this vision in mind the initial implementation will strive to completely hide the underlying provisioning service from the end user.

Store our data in the Machine objects

The current implementation and specification for Machine tightly couples it to compute resources participating in the cluster. There is an upstream proposal to disassociate Machine from Kubernetes further, which may allow us to drop the use of a separate CRD. However, it is not yet clear if the proposal has wide support within the community, and we do not want to delay implementation of Metal³ until the decision is finalized.

If that proposal is accepted, we would reevaluate the current design direction and investigate options for converting BareMetalHost resources to be compatible with the new version of Machine objects.