You can run Docker on a single PC, which is what I've done so far in this book, and it's how you would work with Docker in development and basic test environments. In more advanced test environments and in production, a single server isn't suitable. For high availability and to give you the flexibility to scale your solutions, you need multiple servers running as a single cluster. Docker has cluster support built into the platform, and you can join several Docker hosts together using the swarm mode.
All the concepts you've learned so far: images, containers, registries, networks, volumes, and services--still apply in the swarm mode. The swarm mode is an orchestration layer. It presents the same API as the standalone Docker engine, with additional functions to manage aspects of distributed computing. When you run a service in the swarm mode, Docker determines which hosts to run the containers on; it manages secure communication...