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90 lines
3.8 KiB
Markdown
90 lines
3.8 KiB
Markdown
<!--[metadata]>
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+++
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title = "Swarm mode key concepts"
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description = "Introducing key concepts for Docker Engine swarm mode"
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keywords = ["docker, container, cluster, swarm mode"]
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[menu.main]
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identifier="swarm-mode-concepts"
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parent="engine_swarm"
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weight="2"
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<![end-metadata]-->
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# Swarm mode key concepts
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This topic introduces some of the concepts unique to the cluster management and
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orchestration features of Docker Engine 1.12.
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## Swarm
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The cluster management and orchestration features embedded in the Docker Engine
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are built using **SwarmKit**. Engines participating in a cluster are
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running in **swarm mode**. You enable swarm mode for the Engine by either
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initializing a swarm or joining an existing swarm.
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A **swarm** is a cluster of Docker Engines where you deploy
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[services](#Services-and-tasks). The Docker Engine CLI includes the commands for
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swarm management, such as adding and removing nodes. The CLI also includes the
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commands you need to deploy services to the swarm and manage service
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orchestration.
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When you run Docker Engine outside of swarm mode, you execute container
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commands. When you run the Engine in swarm mode, you orchestrate services.
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## Node
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A **node** is an instance of the Docker Engine participating in the swarm.
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To deploy your application to a swarm, you submit a service definition to a
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**manager node**. The manager node dispatches units of work called
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[tasks](#Services-and-tasks) to worker nodes.
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Manager nodes also perform the orchestration and cluster management functions
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required to maintain the desired state of the swarm. Manager nodes elect a single leader to conduct orchestration tasks.
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**Worker nodes** receive and execute tasks dispatched from manager nodes. By
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default manager nodes are also worker nodes, but you can configure managers to
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be manager-only nodes. The agent notifies the manager node of the current
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state of its assigned tasks so the manager can maintain the desired state.
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## Services and tasks
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A **service** is the definition of the tasks to execute on the worker nodes. It
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is the central structure of the swarm system and the primary root of user
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interaction with the swarm.
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When you create a service, you specify which container image to use and which
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commands to execute inside running containers.
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In the **replicated services** model, the swarm manager distributes a specific
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number of replica tasks among the nodes based upon the scale you set in the
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desired state.
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For **global services**, the swarm runs one task for the service on every
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available node in the cluster.
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A **task** carries a Docker container and the commands to run inside the
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container. It is the atomic scheduling unit of swarm. Manager nodes assign tasks
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to worker nodes according to the number of replicas set in the service scale.
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Once a task is assigned to a node, it cannot move to another node. It can only
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run on the assigned node or fail.
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## Load balancing
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The swarm manager uses **ingress load balancing** to expose the services you
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want to make available externally to the swarm. The swarm manager can
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automatically assign the service a **PublishedPort** or you can configure a
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PublishedPort for the service in the 30000-32767 range.
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External components, such as cloud load balancers, can access the service on the
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PublishedPort of any node in the cluster whether or not the node is currently
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running the task for the service. All nodes in the swarm route ingress
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connections to a running task instance.
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Swarm mode has an internal DNS component that automatically assigns each service
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in the swarm a DNS entry. The swarm manager uses **internal load balancing** to
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distribute requests among services within the cluster based upon the DNS name of
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the service.
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## What's next?
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* Read the [swarm mode overview](index.md).
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* Get started with the [swarm mode tutorial](swarm-tutorial/index.md).
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