This topic introduces some of the concepts unique to the cluster management and orchestration features of Docker Engine 1.12.
The cluster management and orchestration features embedded in the Docker Engine are built using SwarmKit. Engines participating in a cluster are running in swarm mode. You enable swarm mode for the Engine by either initializing a swarm or joining an existing swarm.
A swarm is a cluster of Docker Engines where you deploy services. The Docker Engine CLI includes the commands for swarm management, such as adding and removing nodes. The CLI also includes the commands you need to deploy services to the swarm and manage service orchestration.
When you run Docker Engine outside of swarm mode, you execute container commands. When you run the Engine in swarm mode, you orchestrate services.
A node is an instance of the Docker Engine participating in the swarm.
To deploy your application to a swarm, you submit a service definition to a manager node. The manager node dispatches units of work called tasks to worker nodes.
Manager nodes also perform the orchestration and cluster management functions required to maintain the desired state of the swarm. Manager nodes elect a single leader to conduct orchestration tasks.
Worker nodes receive and execute tasks dispatched from manager nodes. By default manager nodes are also worker nodes, but you can configure managers to be manager-only nodes. The agent notifies the manager node of the current state of its assigned tasks so the manager can maintain the desired state.
A service is the definition of the tasks to execute on the worker nodes. It is the central structure of the swarm system and the primary root of user interaction with the swarm.
When you create a service, you specify which container image to use and which commands to execute inside running containers.
In the replicated services model, the swarm manager distributes a specific number of replica tasks among the nodes based upon the scale you set in the desired state.
For global services, the swarm runs one task for the service on every available node in the cluster.
A task carries a Docker container and the commands to run inside the container. It is the atomic scheduling unit of swarm. Manager nodes assign tasks to worker nodes according to the number of replicas set in the service scale. Once a task is assigned to a node, it cannot move to another node. It can only run on the assigned node or fail.
The swarm manager uses ingress load balancing to expose the services you want to make available externally to the swarm. The swarm manager can automatically assign the service a PublishedPort or you can configure a PublishedPort for the service in the 30000-32767 range.
External components, such as cloud load balancers, can access the service on the PublishedPort of any node in the cluster whether or not the node is currently running the task for the service. All nodes in the swarm cluster route ingress connections to a running task instance.
Swarm mode has an internal DNS component that automatically assigns each service in the swarm a DNS entry. The swarm manager uses internal load balancing to distribute requests among services within the cluster based upon the DNS name of the service.