A description of the architecture from bottom to top is as follows:

• We have some physical or virtual machines. These machines could be on a public or private cloud. Some of them may be a physical box running inside a firewall or an organization. They may be mixed together as a hybrid infrastructure.
• The next layer is the Operating System and, of course, the kernels. We need an OS with a modern kernel that supports container isolation, such as Linux, or that is at least compatible with runC. Windows or Windows Server 2016 has its own Hyper-V based isolation that is compatible with Docker.
• The next layer in the architecture is the Container Runtime (System-Level). We emphasize that it is the system-level container runtime as it is not for running FaaS functions directly. This layer is responsible for provisioning the cluster.
• Next is the optional container orchestration engine, or Container Orchestrator, layer. This layer is Docker Swarm or Kubernetes. We use Docker Swarm in this book, but you may find that some FaaS platforms presented in this book do not use any kind of orchestration. Basically, just Docker alone with container networking is enough for a FaaS platform to get up and running effectively.