add how nodes work to how swarm works guide

Signed-off-by: Charles Smith <charles.smith@docker.com>

docs/swarm/how-swarm-mode-works/menu.md

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+<!--[metadata]>
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+title = "How swarm mode works"
+description = "How the components of swarm mode work"
+keywords = ["cluster, swarm"]
+advisory = "rc"
+[menu.main]
+identifier="how-swarm-works"
+parent="engine_swarm"
+weight=11
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+<![end-metadata]-->
+
+#  How swarm mode works guide
+
+## TOC
+
+* [How nodes work](nodes.md)

docs/swarm/how-swarm-mode-works/nodes.md

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+<!--[metadata]>
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+aliases = [
+"/engine/swarm/how-swarm-mode-works/"
+]
+title = "How nodes work"
+description = "How swarm nodes work"
+keywords = ["docker, container, cluster, swarm mode, node"]
+advisory = "rc"
+[menu.main]
+identifier="how-nodes-work"
+parent="how-swarm-works"
+weight="3"
++++
+<![end-metadata]-->
+
+# How nodes work
+
+Docker Engine 1.12 introduces swarm mode that enables you to create a
+cluster of one or more Docker Engines called a swarm. A swarm consists
+of one or more nodes: physical or virtual machines running Docker
+Engine 1.12 or later in swarm mode.
+
+There are two types of nodes: [**managers**](#manager-nodes) and
+[**workers**](#worker-nodes).
+
+![Swarm mode cluster](../images/swarm-diagram.png)
+
+If you haven't already, read through the [swarm mode overview](../index.md) and [key concepts](../key-concepts.md).
+
+## Manager nodes
+
+Manager nodes handle cluster management tasks:
+
+* maintaining cluster state
+* scheduling services
+* serving swarm mode [HTTP API endpoints](../../reference/api/index.md)
+
+Using a [Raft](https://raft.github.io/raft.pdf) implementation, the managers
+maintain a consistent internal state of the entire swarm and all the services
+running on it. For testing purposes it is OK to run a swarm with a single
+manager. If the manager in a single-manager swarm fails, your services will
+continue to run, but you will need to create a new cluster to recover.
+
+To take advantage of swarm mode's fault-tolerance features, Docker recommends
+you implement an odd number of nodes nodes according to your organization's
+high-availability requirements. When you have multiple managers you can recover
+from the failure of a manager node without downtime.
+
+* A three-manager swarm tolerates a maximum loss of one manager.
+* A five-manager swarm tolerates a maximum simultaneous loss two
+manager nodes.
+* An `N` manager cluster will tolerate the loss of at most
+`(N-1)/2` managers.
+* Docker recommends a maximum of seven manager nodes for a swarm.
+
+    >**Important Note**: Adding more managers does NOT mean increased
+    scalability or higher performance. In general, the opposite is true.
+
+## Worker nodes
+
+Worker nodes are also instances of Docker Engine whose sole purpose is to
+execute containers. Worker nodes don't participate in the Raft distributed
+state, make in scheduling decisions, or serve the swarm mode HTTP API.
+
+You can create a swarm of one manager node, but you cannot have a worker node
+without at least one manager node. By default, all managers are also workers.
+In a single manager node cluster, you can run commands like `docker service
+create` and the scheduler will place all tasks on the local Engine.
+
+To prevent the scheduler from placing tasks on a manager node in a multi-node
+swarm, set the availability for the manager node to `Drain`. The scheduler
+gracefully stops tasks on nodes in `Drain` mode and schedules the tasks on an
+`Active` node. The scheduler does not assign new tasks to nodes with `Drain`
+availability.
+
+Refer to the [`docker node update`](../../reference/commandline/node_update.md)
+command line reference to see how to change node availability.
+
+## Changing roles
+
+You can promote a worker node to be a manager by running `docker node promote`.
+For example, you may want to promote a worker node when you
+take a manager node offline for maintenance. See [node promote](../../reference/commandline/node_promote.md).
+
+You can also demote a manager node to a worker node. See
+[node demote](../../reference/commandline/node_demote.md).
+
+<!-- TODO For when How services work guide is ready
+## What's Next
+
+* Read about how swarm mode [services](services.md) work.
+-->