The What and the Why of the Cluster API


Throughout the evolution of software tools there exists a tension between generalization and partial specialization. A tool’s broader adoption is a form of natural selection, where its evolution is predicated on filling a given need, or role, better than its competition. This premise is imbued in the central tenets of Unix philosophy:

  • Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new features.
  • Expect the output of every program to become the input to another, as yet unknown, program.

The domain of configuration management tooling is rife with examples of not heeding this lesson (i.e. Terraform, Puppet, Chef, Ansible, Juju, Saltstack, etc.), where expanse in generality has given way to partial specialization of different tools, causing fragmentation of an ecosystem. This pattern has not gone unnoticed by those in the Kubernetes cluster lifecycle special interest group, or SIG, whose objective is to simplify the creation, configuration, upgrade, downgrade, and teardown of Kubernetes clusters and their components. Therefore, one of the primary design principles for any subproject that the SIG endorses is: Where possible, tools should be composable to solve a higher order set of problems.

In this post, we will outline the history and motivations behind the creation of the Cluster API as a specialized toolset to bring declarative, Kubernetes-style APIs to cluster creation, configuration, and management in the Kubernetes ecosystem. The primary function of Cluster API is not meant to supplant existing tools, but to serve as a partial specialization that can be used in a composable fashion with those tools.

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