Introduction to service constraints

Not all cluster members run all the services in a deployment. Some may run the services running business logic, some may run management software, and some may run logging or auditing software, and so on. Hence, it's imperative that cluster management software provides mechanisms to control service deployment so they run only on the members satisfying their properties. We will study the mechanisms provided by CoreOS to control the deployment.

CoreOS uses the fleet service to schedule the services on the members with constraints. Unit file configuration options help to target a service on a particular member or members meeting configured properties. In due course, we will also learn to integrate the fleet service into the cloud-config file and auto start a custom service inside a docker container.