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Flesh out swarm key concepts (#4634)

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Misty Stanley-Jones 2017-10-20 11:09:23 -07:00 committed by GitHub
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@ -10,19 +10,42 @@ orchestration features of Docker Engine 1.12.
## What is a swarm?
The cluster management and orchestration features embedded in the Docker Engine
are built using **SwarmKit**. Docker engines participating in a cluster are
running in **swarm mode**. You enable swarm mode for an engine by either
initializing a swarm or joining an existing swarm.
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** is a cluster of Docker engines, or _nodes_, where you deploy
[services](key-concepts.md#services-and-tasks). The Docker Engine CLI and API
include commands to manage swarm nodes (e.g., add or remove nodes), and deploy
and orchestrate services across the swarm.
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](key-concepts.md#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 managed by a swarm
manager, as opposed to a standalone container.
When you run Docker without using swarm mode, you execute container
commands. When you run the Docker in swarm mode, you orchestrate services. You can run swarm services and standalone containers on the same Docker instances.
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update the configuration, stop the service tasks with the out of date
configuration, and create new ones matching the desired configuration.
## What is a node?
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](/compose/) to define and run
containers, you can define and run swarm service
[stacks](/get-started/part5.md).
Keep reading for details about concepts relating 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.