In Chapter 2, the Docker Swarm was introduced. In Chapter 4, Docker Swarm services were introduced. A service consists of zero or more service tasks (replicas), which it schedules on the nodes in a Swarm. The desired state of a service includes the number of tasks that must be run. Scheduling is defined as the process of placing a service task that is required to be run on a node in the Swarm to keep the desired state of a service, as illustrated in Figure 8-1. A service task may only be scheduled on a worker node. A manager node is also a worker node by default.