Docker Swarm is a group of physical or virtual machines that run the Docker application and have already been configured in order to join in a cluster. After the machines are clustered, you can continue to run the Docker commands that you use. Unlike earlier, these tasks will now be carried out by the cluster of machines. The numerous activities carried out by the cluster are controlled by a swarm manager and the machines in the cluster are called nodes.
There are two types of nodes in Docker Swarm, namely, manager node and worker node. The manager nodes handle tasks of the cluster manager like maintaining the cluster state, scheduling services, and serving swarm mode HTTP API endpoints while the worker nodes are mainly used to execute the containers. The worker nodes do not take part in making scheduling decisions or serving the swarm mode HTTP API.
Docker Swarm has two types of services that are global and replicated. In the case of replicated service, you have to specify the number of similar tasks that you need to run. On the other hand, global service runs a single task on each node.
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You should also check out this video to learn in-depth about Docker: