moby/docs/understanding-docker.md

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# Docker Overview

Docker is an open platform for developing, shipping, and running applications.
Docker is designed to deliver your applications faster. With Docker you can
separate your applications from your infrastructure and treat your
infrastructure like a managed application. Docker helps you ship code faster,
test faster, deploy faster, and shorten the cycle between writing code and
running code.

Docker does this by combining kernel containerization features with workflows
and tooling that help you manage and deploy your applications.

## What is the Docker platform?

At its core, Docker provides a way to run almost any application securely
isolated in a container. The isolation and security allow you to run many
containers simultaneously on your host. The lightweight nature of containers,
which run without the extra load of a hypervisor, means you can get more out of
your hardware.

Surrounding the container is tooling and a platform which can help you in
several ways:

* Get your applications (and supporting components) into Docker containers
* Distribute and ship those containers to your teams for further development
and testing
* Deploy those applications to your production environment,
 whether it is in a local data center or the Cloud

## What is Docker Engine?

Docker Engine is a client-server application with these major components:

* A server which is a type of long-running program called a daemon process.

* A REST API which specifies interfaces that programs can use to talk to the daemon and instruct it what to do.

* A command line interface (CLI) client.

![Docker Engine Components Flow](article-img/engine-components-flow.png)

The CLI makes use of the Docker REST API to control or interact with the Docker daemon through scripting or direct CLI commands. Many other Docker applications make use of the underlying API and CLI.

The daemon creates and manages Docker objects.  Docker objects include images, containers, networks, data volumes, and so forth.

> **Note:** Docker is licensed under the open source Apache 2.0 license.

## What can I use Docker for?

*Faster delivery of your applications*

Docker is perfect for helping you with the development lifecycle. Docker
allows your developers to develop on local containers that contain your
applications and services. It can then integrate into a continuous integration and
deployment workflow.

For example, your developers write code locally and share their development stack via
Docker with their colleagues. When they are ready, they push their code and the
stack they are developing onto a test environment and execute any required
tests. From the testing environment, you can then push the Docker images into
production and deploy your code.

*Deploying and scaling more easily*

Docker's container-based platform allows for highly portable workloads. Docker
containers can run on a developer's local host, on physical or virtual machines
in a data center, or in the Cloud.

Docker's portability and lightweight nature also make dynamically managing
workloads easy. You can use Docker to quickly scale up or tear down applications
and services. Docker's speed means that scaling can be near real time.

*Achieving higher density and running more workloads*

Docker is lightweight and fast. It provides a viable, cost-effective alternative
to hypervisor-based virtual machines. This is especially useful in high density
environments: for example, building your own Cloud or Platform-as-a-Service. But
it is also useful for small and medium deployments where you want to get more
out of the resources you have.

## What is Docker's architecture?
Docker uses a client-server architecture. The Docker *client* talks to the
Docker *daemon*, which does the heavy lifting of building, running, and
distributing your Docker containers. Both the Docker client and the daemon *can*
run on the same system, or you can connect a Docker client to a remote Docker
daemon. The Docker client and daemon communicate via sockets or through a
RESTful API.

![Docker Architecture Diagram](article-img/architecture.svg)

### The Docker daemon
As shown in the diagram above, the Docker daemon runs on a host machine. The
user does not directly interact with the daemon, but instead through the Docker
client.

### The Docker client
The Docker client, in the form of the `docker` binary, is the primary user
interface to Docker. It accepts commands from the user and communicates back and
forth with a Docker daemon.

### Inside Docker
To understand Docker's internals, you need to know about three resources:

* Docker images
* Docker registries
* Docker containers

#### Docker images

A Docker image is a read-only template. For example, an image could contain an Ubuntu
operating system with Apache and your web application installed. Images are used to create
Docker containers. Docker provides a simple way to build new images or update existing
images, or you can download Docker images that other people have already created.
Docker images are the **build** component of Docker.

#### Docker registries
Docker registries hold images. These are public or private stores from which you
upload or download images. The public Docker registry is provided with the
[Docker Hub](http://hub.docker.com). It serves a huge collection of existing
images for your use. These can be images you create yourself or you can use
images that others have previously created. Docker registries are the
**distribution** component of Docker.
For more information, go to [Docker Registry](https://docs.docker.com/registry/overview/) and
[Docker Trusted Registry](https://docs.docker.com/docker-trusted-registry/overview/).

#### Docker containers
Docker containers are similar to a directory. A Docker container holds everything that
is needed for an application to run. Each container is created from a Docker
image. Docker containers can be run, started, stopped, moved, and deleted. Each
container is an isolated and secure application platform. Docker containers are the
 **run** component of Docker.

### How does a Docker image work?
We've already seen that Docker images are read-only templates from which Docker
containers are launched. Each image consists of a series of layers. Docker
makes use of [union file systems](http://en.wikipedia.org/wiki/UnionFS) to
combine these layers into a single image. Union file systems allow files and
directories of separate file systems, known as branches, to be transparently
overlaid, forming a single coherent file system.

One of the reasons Docker is so lightweight is because of these layers. When you
change a Docker image—for example, update an application to a new version— a new layer
gets built. Thus, rather than replacing the whole image or entirely
rebuilding, as you may do with a virtual machine, only that layer is added or
updated. Now you don't need to distribute a whole new image, just the update,
making distributing Docker images faster and simpler.

Every image starts from a base image, for example `ubuntu`, a base Ubuntu image,
or `fedora`, a base Fedora image. You can also use images of your own as the
basis for a new image, for example if you have a base Apache image you could use
this as the base of all your web application images.

> **Note:** [Docker Hub](https://hub.docker.com) is a public registry and stores
images.

Docker images are then built from these base images using a simple, descriptive
set of steps we call *instructions*. Each instruction creates a new layer in our
image. Instructions include actions like:

* Run a command
* Add a file or directory
* Create an environment variable
* What process to run when launching a container from this image

These instructions are stored in a file called a `Dockerfile`. A `Dockerfile` is
a text based script that contains instructions and commands for building the image
from the base image. Docker reads this `Dockerfile` when you request a build of
an image, executes the instructions, and returns a final image.

### How does a Docker registry work?
The Docker registry is the store for your Docker images. Once you build a Docker
image you can *push* it to a public registry such as [Docker Hub](https://hub.docker.com)
or to your own registry running behind your firewall.

Using the Docker client, you can search for already published images and then
pull them down to your Docker host to build containers from them.

[Docker Hub](https://hub.docker.com) provides both public and private storage
for images. Public storage is searchable and can be downloaded by anyone.
Private storage is excluded from search results and only you and your users can
pull images down and use them to build containers. You can [sign up for a storage plan
here](https://www.docker.com/pricing).

### How does a container work?
A container consists of an operating system, user-added files, and meta-data. As
we've seen, each container is built from an image. That image tells Docker
what the container holds, what process to run when the container is launched, and
a variety of other configuration data. The Docker image is read-only. When
Docker runs a container from an image, it adds a read-write layer on top of the
image (using a union file system as we saw earlier) in which your application can
then run.

### What happens when you run a container?
Either by using the `docker` binary or via the API, the Docker client tells the Docker
daemon to run a container.

    $ docker run -i -t ubuntu /bin/bash

The Docker Engine client is launched using the `docker` binary with the `run` option
running a new container. The bare minimum the Docker client needs to tell the
Docker daemon to run the container is:

* What Docker image to build the container from, for example, `ubuntu`
* The command you want to run inside the container when it is launched,
for example,`/bin/bash`

So what happens under the hood when we run this command?

In order, Docker Engine does the following:

- **Pulls the `ubuntu` image:** Docker Engine checks for the presence of the `ubuntu`
image. If the image already exists, then Docker Engine uses it for the new container.
If it doesn't exist locally on the host, then Docker Engine pulls it from
[Docker Hub](https://hub.docker.com).
- **Creates a new container:** Once Docker Engine has the image, it uses it to create a
container.
- **Allocates a filesystem and mounts a read-write _layer_:** The container is created in
the file system and a read-write layer is added to the image.
- **Allocates a network / bridge interface:** Creates a network interface that allows the
Docker container to talk to the local host.
- **Sets up an IP address:** Finds and attaches an available IP address from a pool.
- **Executes a process that you specify:** Runs your application, and;
- **Captures and provides application output:** Connects and logs standard input, outputs
and errors for you to see how your application is running.

You now have a running container! Now you can manage your container, interact with
your application and then, when finished, stop and remove your container.

## The underlying technology
Docker is written in Go and makes use of several kernel features to
deliver the functionality we've seen.

### Namespaces
Docker takes advantage of a technology called `namespaces` to provide the
isolated workspace we call the *container*.  When you run a container, Docker
creates a set of *namespaces* for that container.

This provides a layer of isolation: each aspect of a container runs in its own
namespace and does not have access outside of it.

Some of the namespaces that Docker Engine uses on Linux are:

 - **The `pid` namespace:** Process isolation (PID: Process ID).
 - **The `net` namespace:** Managing network interfaces (NET:
 Networking).
 - **The `ipc` namespace:** Managing access to IPC
 resources (IPC: InterProcess Communication).
 - **The `mnt` namespace:** Managing mount-points (MNT: Mount).
 - **The `uts` namespace:** Isolating kernel and version identifiers. (UTS: Unix
Timesharing System).

### Control groups
Docker Engine on Linux also makes use of another technology called `cgroups` or control groups.
A key to running applications in isolation is to have them only use the
resources you want. This ensures containers are good multi-tenant citizens on a
host. Control groups allow Docker Engine to share available hardware resources to
containers and, if required, set up limits and constraints. For example,
limiting the memory available to a specific container.

### Union file systems
Union file systems, or UnionFS, are file systems that operate by creating layers,
making them very lightweight and fast. Docker Engine uses union file systems to provide
the building blocks for containers. Docker Engine can make use of several union file system variants
including: AUFS, btrfs, vfs, and DeviceMapper.

### Container format
Docker Engine combines these components into a wrapper we call a container format. The
default container format is called `libcontainer`. In the future, Docker may
support other container formats, for example, by integrating with BSD Jails
or Solaris Zones.

## Next steps
Read about [Installing Docker Engine](installation/index.md#installation).
Learn about the [Docker Engine User Guide](userguide/index.md).