docker/docs/understanding-docker.md

Docker Overview

Docker is an open platform for developing, shipping, and running applications. Docker enables you to separate your applications from your infrastructure so you can deliver software quickly. With Docker, you can manage your infrastructure in the same ways you manage your applications. By taking advantage of Docker's methodoligies for shipping, testing, and deploying code quickly, you can significantly reduce the delay between writing code and running it in production.

What is the Docker platform?

Docker provides the ability to package and run an application in a loosely isolated environment called a container. The isolation and security allow you to run many containers simultaneously on a given host. Because of the lightweight nature of containers, which run without the extra load of a hypervisor, you can run more containers on a given hardware combination than if you were using virtual machines.

Docker provides tooling and a platform to manage the lifecycle of your containers:

• Encapsulate 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.

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

The daemon creates and manages Docker objects, such as images, containers, networks, and data volumes.

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

What can I use Docker for?

Fast, consistent delivery of your applications

Docker can streamline the development lifecycle by allowing developers to work in standardized environments using local containers which provide your applications and services. You can also integrate Docker into your continuous integration and continuous deployment (CI/CD) workflow.

Consider the following example scenario. Your developers write code locally and share their work with their colleagues using Docker containers. They can use Docker to push their applications into a test environment and execute automated and manual tests. When developers find problems, they can fix them in the development environment and redeploy them to the test environment for testing. When testing is complete, getting the fix to the customer is as simple as pushing the updated image to the production environment.

Responsive deployment and scaling

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, in the Cloud, or in a mixture of environments.

Docker's portability and lightweight nature also make it easy to dynamically manage workloads, scaling up or tearing down applications and services as business needs dictate, in near real time.

Running more workloads on the same hardware

Docker is lightweight and fast. It provides a viable, cost-effective alternative to hypervisor-based virtual machines, allowing you to use more of your compute capacity to achieve your business goals. This is useful in high density environments and for small and medium deployments where you need to do more with fewer resources.

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. The Docker client and 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 REST API.

The Docker daemon

The Docker daemon runs on a host machine. The user uses the Docker client to interact with the daemon.

The Docker client

The Docker client, in the form of the docker binary, is the primary user interface to Docker. It accepts commands and configuration flags from the user and communicates with a Docker daemon. One client can even communicate with multiple unrelated daemons.

Inside Docker

To understand Docker's internals, you need to know about images, registries, and containers.

Docker images

A Docker image is a read-only template with instructions for creating a Docker container. For example, an image might contain an Ubuntu operating system with Apache web server and your web application installed. You can build or update images from scratch or download and use images created by others. An image may be based on, or may extend, one or more other images. A docker image is described in text file called a Dockerfile, which has a simple, well-defined syntax. For more details about images, see How does a Docker image work?.

Docker images are the build component of Docker.

Docker containers

A Docker container is a runnable instance of a Docker image. You can run, start, stop, move, or delete a container using Docker API or CLI commands. When you run a container, you can provide configuration metadata such as networking information or environment variables. Each container is an isolated and secure application platform, but can be given access to resources running in a different host or container, as well as persistent storage or databases. For more details about containers, see How does a container work?.

Docker containers are the run component of Docker.

Docker registries

A docker registry is a library of images. A registry can be public or private, and can be on the same server as the Docker daemon or Docker client, or on a totally separate server. For more details about registries, see How does a Docker registry work?

Docker registries are the distribution component of Docker.

Docker services

A Docker service allows a swarm of Docker nodes to work together, running a defined number of instances of a replica task, which is itself a Docker image. You can specify the number of concurrent replica tasks to run, and the swarm manager ensures that the load is spread evenly across the worker nodes. To the consumer, the Docker service appears to be a single application. Docker Engine supports swarm mode in Docker 1.12 and higher.

Docker services are the scalability component of Docker.

How does a Docker image work?

Docker images are read-only templates from which Docker containers are instantiated. Each image consists of a series of layers. Docker uses union file systems 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.

These layers are one of the reasons Docker is so lightweight. When you change a Docker image, such as when you update an application to a new version, a new layer is built and replaces only the layer it updates. The other layers remain intact. To distribute the update, you only need to transfer the updated layer. Layering speeds up distribution of Docker images. Docker determines which layers need to be updated at runtime.

An image is defined in a Dockerfile. Every image starts from a base image, such as 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. The base image is defined using the FROM keyword in the dockerfile.

Note: Docker Hub is a public registry and stores images.

The docker image is built from the base image using a simple, descriptive set of steps we call instructions, which are stored in a Dockerfile. Each instruction creates a new layer in the image. Some examples of Dockerfile instructions are:

• Specify the base image (FROM)
• Specify the maintainer (MAINTAINER)
• Run a command (RUN)
• Add a file or directory (ADD)
• Create an environment variable (ENV)
• What process to run when launching a container from this image (CMD)

Docker reads this Dockerfile when you request a build of an image, executes the instructions, and returns the image.

How does a Docker registry work?

A Docker registry stores Docker images. After you build a Docker image, you can push it to a public registry such as Docker Hub or to a private registry running behind your firewall. You can also search for existing images and pull them from the registry to a host.

Docker Hub is a public Docker registry which serves a huge collection of existing images and allows you to contribute your own. For more information, go to Docker Registry and Docker Trusted Registry.

Docker store allows you to buy and sell Docker images. For image, you can buy a Docker image containing an application or service from the software vendor, and use the image to deploy the application into your testing, staging, and production environments, and upgrade the application by pulling the new version of the image and redeploying the containers. Docker Store is currently in private beta.

How does a container work?

A container uses the host machine's Linux kernel, and consists of any extra files you add when the image is created, along with metadata associated with the container at creation or when the container is started. Each container is built from an image. The image defines the container's contents, which process to run when the container is launched, and a variety of other configuration details. 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 UnionFS as we saw earlier) in which your application runs.

What happens when you run a container?

When you use the docker run CLI command or the equivalent API, the Docker Engine client instructs the Docker daemon to run a container. This example tells the Docker daemon to run a container using the ubuntu Docker image, to remain in the foreground in interactive mode (-i), and to run the /bin/bash command.

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

When you run this command, Docker Engine does the following:

1. Pulls the ubuntu image: Docker Engine checks for the presence of the ubuntu image. If the image already exists locally, Docker Engine uses it for the new container. Otherwise, then Docker Engine pulls it from Docker Hub.

2. Creates a new container: Docker uses the image to create a container.

3. 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.

4. Allocates a network / bridge interface: Creates a network interface that allows the Docker container to talk to the local host.

5. Sets up an IP address: Finds and attaches an available IP address from a pool.

6. Executes a process that you specify: Executes the /bin/bash executable.

7. Captures and provides application output: Connects and logs standard input, outputs and errors for you to see how your application is running, because you requested interactive mode.

Your container is now running. You can manage and interact with it, use the services and applications it provides, and eventually stop and remove it.

The underlying technology

Docker is written in Go and takes advantage of several features of the Linux kernel to deliver its functionality.

Namespaces

Docker uses a technology called namespaces to provide the isolated workspace called the container. When you run a container, Docker creates a set of namespaces for that container.

These namespaces provide a layer of isolation. Each aspect of a container runs in a separate namespace and its access is limited to that namespace.

Docker Engine uses namespaces such as the following on Linux:

• 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 filesystem mount points (MNT: Mount).
• The uts namespace: Isolating kernel and version identifiers. (UTS: Unix Timesharing System).

Control groups

Docker Engine on Linux also relies on another technology called control groups (cgroups). A cgroup limits an application to a specific set of resources. Control groups allow Docker Engine to share available hardware resources to containers and optionally enforce limits and constraints. For example, you can limit 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 UnionFS to provide the building blocks for containers. Docker Engine can use multiple UnionFS variants, including AUFS, btrfs, vfs, and DeviceMapper.

Container format

Docker Engine combines the namespaces, control groups, and UnionFS into a wrapper called a container format. The default container format is libcontainer. In the future, Docker may support other container formats by integrating with technologies such as BSD Jails or Solaris Zones.

Next steps

• Read about Installing Docker Engine.
• Get hands-on experience with the Get Started With Docker tutorial.
• Check out examples and deep dive topics in the Docker Engine User Guide.