Swarm Mode Key Concepts: Swarm, Nodes, Services, and Tasks

--- description: Introducing key concepts for Docker Engine swarm mode keywords: docker, container, cluster, swarm mode, docker engine title: Swarm mode key concepts --- This topic introduces some of the concepts unique to the cluster management and orchestration features of Docker Engine 1.12. ## What is a swarm? The cluster management and orchestration features embedded in Docker Engine are built using [swarmkit](https://github.com/docker/swarmkit/). Swarmkit is a separate project which implements Docker's orchestration layer and is used directly within Docker. A swarm consists of multiple Docker hosts which run in Swarm mode and act as managers, to manage membership and delegation, and workers, which run [swarm services](#services-and-tasks). A given Docker host can be a manager, a worker, or perform both roles. When you create a service, you define its optimal state - number of replicas, network and storage resources available to it, ports the service exposes to the outside world, and more. Docker works to maintain that desired state. For instance, if a worker node becomes unavailable, Docker schedules that node's tasks on other nodes. A task is a running container which is part of a swarm service and is managed by a swarm manager, as opposed to a standalone container. One of the key advantages of swarm services over standalone containers is that you can modify a service's configuration, including the networks and volumes it is connected to, without the need to manually restart the service. Docker will update the configuration, stop the service tasks with out of date configuration, and create new ones matching the desired configuration. When Docker is running in Swarm mode, you can still run standalone containers on any of the Docker hosts participating in the swarm, as well as swarm services. A key difference between standalone containers and swarm services is that only swarm managers can manage a swarm, while standalone containers can be started on any daemon. Docker daemons can participate in a swarm as managers, workers, or both. In the same way that you can use [Docker Compose](/manuals/compose/_index.md) to define and run containers, you can define and run [Swarm service](services.md) stacks. Keep reading for details about concepts related to Docker swarm services, including nodes, services, tasks, and load balancing. ## Nodes A node is an instance of the Docker engine participating in the swarm. You can also think of this as a Docker node. You can run one or more nodes on a single physical computer or cloud server, but production swarm deployments typically include Docker nodes distributed across multiple physical and cloud machines. To deploy your application to a swarm, you submit a service definition to a

This section introduces key concepts for Docker Engine swarm mode, including what a swarm is, the roles of manager and worker nodes, and the concept of services and tasks. It highlights the advantages of swarm services over standalone containers, such as the ability to modify configurations without manual restarts. It also defines nodes as instances of the Docker engine participating in the swarm.