| title: | Concepts |
|---|---|
| description: | Concepts of the Deis application platform, which deploys and scales twelve factor apps. Learn about formations, layers, build, release, run & backing services. |
| keywords: | deis, formations, layers, build, release, run, backing services |
Deis is an application platform that deploys and scales Twelve Factor apps using formations of Chef nodes, Docker containers and Nginx proxies.
A :ref:`formation` is a set of infrastructure used to host applications. Each formation includes :ref:`Layers <layer>` of :ref:`Nodes <node>` that provide different services to the formation.
:ref:`Layers <layer>` are homogeneous groups of :ref:`Nodes <node>` that perform work on behalf of a formation. Each node in a layer has the same :ref:`Flavor` and configuration, allowing them to be scaled easily. Formations have two primary types of layers.
Runtime layers host :ref:`Containers <container>` for a formation. Nodes in a runtime layer use a Chef Databag to deploy containers for each :ref:`application` in the formation.
Proxy layers expose :ref:`Applications <application>` to the outside world. Nodes in a proxy layer use a Chef Databag to configure routing of inbound requests to :ref:`Containers <container>` hosted on runtime layers.
An :ref:`application` lives on a :ref:`formation` where it uses :ref:`Containers <container>` to process requests and run background jobs for a deployed git repository. Developers use :ref:`Applications <application>` to push code, change config, scale containers, view logs, or run admin commands -- regardless of the formation's underlying infrastructure.
Deis enforces strict separation between Build and Run stages following the Twelve Factor model.
The :ref:`Controller` includes a gitreceive hook that receives incoming git push requests over SSH and builds applications inside ephemeral Docker containers. Tarballs of the /app directory are extracted into a slug and exposed on the Controller using an Nginx static file server. The slug is later downloaded by the runtime layer and bind-mounted into a Docker container for execution.
During the release stage, a :ref:`build` is combined with :ref:`config` to create a new numbered :ref:`release`. The release stage is triggered any time a new build is created or config is changed, making it easy to rollback code and configuration.
The run stage updates Chef databags and converges all nodes in the formation. The databag specifies the current application releases, the placement of containers across the runtime layer, and the configuration of the proxy layer. SSH is used to converge nodes in runtime layers followed by nodes in proxy layers, making zero downtime deployment possible.
In keeping with Twelve Factor methodology, backing services like databases, queues and storage are decoupled and attached using environment variables. This allows applications to use backing services provided by other applications, or external/third-party services accessible over the network. The use of environment variables makes it easy to swap backing services when necessary.