:title: Dual Host Topology for a Deis PaaS Formation
:description: Learn how to build with Deis formations using the Deis command line interface.
:keywords: tutorial, guide, walkthrough, howto, deis, formations

.. _dual-host:

Dual Host
=========
In a dual host topology, the proxy and runtime services of a :ref:`formation` 
run on two :ref:`Nodes <node>`.
This provides improved availability, since the failure of a single node still 
leaves another node with both proxy and runtime services intact.

.. include:: create-formation.txt

Create the Layers
-----------------

.. include:: choose-flavor.txt

Create a Dual Proxy/Runtime Layer
`````````````````````````````````
.. code-block:: console

    $ deis layers:create dev nodes ec2-us-west-2 --proxy=y --runtime=y
    Creating nodes layer... done in 3s

We create a new layer in the "dev" formation called "nodes".
The layer has proxy set to "yes" and runtime set to "yes", which means
the layer will host containers as well as route inbound traffic to them.

.. include:: scale-nodes.txt

Scale Automatically
```````````````````
If you've configured automated provisioning using the Deis :ref:`Provider` API,
you can use the ``deis nodes:scale`` command.

.. code-block:: console

    $ deis nodes:scale dev nodes=2
    Scaling nodes... but first, coffee!
    done in 263s

This will automatically provision 2 new nodes (separate from the controller) 
which will host the entire formation.

.. include:: scale-manually.txt

.. include:: wildcard-dns.txt

.. code-block:: console

    $ dig testing123.deisapp.com +noall +answer
    ...
    testing123.deisapp.com.	45	IN	A	54.245.11.172
    testing123.deisapp.com.	45	IN	A	54.202.163.190