DockerCLI/docs/reference/glossary.md

description	keywords
Glossary of terms used around Docker	glossary, docker, terms, definitions

Glossary

A list of terms used around the Docker project.

aufs

aufs (advanced multi layered unification filesystem) is a Linux filesystem that Docker supports as a storage backend. It implements the union mount for Linux file systems.

base image

An image that has no parent is a base image.

bridge

In terms of generic networking, a bridge is a Link Layer device which forwards traffic between network segments. A bridge can be a hardware device or a software device running within a host machine's kernel.

In terms of Docker, a bridge network uses a software bridge which allows containers connected to the same bridge network to communicate, while providing isolation from containers which are not connected to that bridge network. The Docker bridge driver automatically installs rules in the host machine so that containers on different bridge networks cannot communicate directly with each other.

The default bridge network, which is also named bridge, behaves differently from user-defined bridge networks. Containers connected to the default bridge network can communicate with each other across the bridge by IP address but cannot resolve each other's container name to an IP address unless they are explicitly linked using the --link flag to docker run.

For more information about Docker networking, see Understand container communication.

btrfs

btrfs (B-tree file system) is a Linux filesystem that Docker supports as a storage backend. It is a copy-on-write filesystem.

build

build is the process of building Docker images using a Dockerfile. The build uses a Dockerfile and a "context". The context is the set of files in the directory in which the image is built.

cgroups

cgroups is a Linux kernel feature that limits, accounts for, and isolates the resource usage (CPU, memory, disk I/O, network, etc.) of a collection of processes. Docker relies on cgroups to control and isolate resource limits.

Also known as : control groups

Compose

Compose is a tool for defining and running complex applications with Docker. With compose, you define a multi-container application in a single file, then spin your application up in a single command which does everything that needs to be done to get it running.

Also known as : docker-compose, fig

copy-on-write

Docker uses a copy-on-write technique and a union file system for both images and containers to optimize resources and speed performance. Multiple copies of an entity share the same instance and each one makes only specific changes to its unique layer.

Multiple containers can share access to the same image, and make container-specific changes on a writable layer which is deleted when the container is removed. This speeds up container start times and performance.

Images are essentially layers of filesystems typically predicated on a base image under a writable layer, and built up with layers of differences from the base image. This minimizes the footprint of the image and enables shared development.

For more about copy-on-write in the context of Docker, see Understand images, containers, and storage drivers.

container

A container is a runtime instance of a docker image.

A Docker container consists of

• A Docker image
• Execution environment
• A standard set of instructions

The concept is borrowed from Shipping Containers, which define a standard to ship goods globally. Docker defines a standard to ship software.

data volume

A data volume is a specially-designated directory within one or more containers that bypasses the Union File System. Data volumes are designed to persist data, independent of the container's life cycle. Docker therefore never automatically delete volumes when you remove a container, nor will it "garbage collect" volumes that are no longer referenced by a container.

Docker

The term Docker can refer to

• The Docker project as a whole, which is a platform for developers and sysadmins to develop, ship, and run applications
• The docker daemon process running on the host which manages images and containers

Docker for Mac

Docker for Mac is an easy-to-install, lightweight Docker development environment designed specifically for the Mac. A native Mac application, Docker for Mac uses the macOS Hypervisor framework, networking, and filesystem. It's the best solution if you want to build, debug, test, package, and ship Dockerized applications on a Mac. Docker for Mac supersedes Docker Toolbox as state-of-the-art Docker on macOS.

Docker for Windows

Docker for Windows is an easy-to-install, lightweight Docker development environment designed specifically for Windows 10 systems that support Microsoft Hyper-V (Professional, Enterprise and Education). Docker for Windows uses Hyper-V for virtualization, and runs as a native Windows app. It works with Windows Server 2016, and gives you the ability to set up and run Windows containers as well as the standard Linux containers, with an option to switch between the two. Docker for Windows is the best solution if you want to build, debug, test, package, and ship Dockerized applications from Windows machines. Docker for Windows supersedes Docker Toolbox as state-of-the-art Docker on Windows.

Docker Hub

The Docker Hub is a centralized resource for working with Docker and its components. It provides the following services:

• Docker image hosting
• User authentication
• Automated image builds and work-flow tools such as build triggers and web hooks
• Integration with GitHub and Bitbucket

Dockerfile

A Dockerfile is a text document that contains all the commands you would normally execute manually in order to build a Docker image. Docker can build images automatically by reading the instructions from a Dockerfile.

filesystem

A file system is the method an operating system uses to name files and assign them locations for efficient storage and retrieval.

Examples :

• Linux : ext4, aufs, btrfs, zfs
• Windows : NTFS
• macOS : HFS+

image

Docker images are the basis of containers. An Image is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime. An image typically contains a union of layered filesystems stacked on top of each other. An image does not have state and it never changes.

libcontainer

libcontainer provides a native Go implementation for creating containers with namespaces, cgroups, capabilities, and filesystem access controls. It allows you to manage the lifecycle of the container performing additional operations after the container is created.

libnetwork

libnetwork provides a native Go implementation for creating and managing container network namespaces and other network resources. It manage the networking lifecycle of the container performing additional operations after the container is created.

link

links provide a legacy interface to connect Docker containers running on the same host to each other without exposing the hosts' network ports. Use the Docker networks feature instead.

node

A node is a physical or virtual machine running an instance of the Docker Engine in swarm mode.

Manager nodes perform swarm management and orchestration duties. By default manager nodes are also worker nodes.

Worker nodes execute tasks.

overlay network driver

Overlay network driver provides out of the box multi-host network connectivity for docker containers in a cluster.

overlay storage driver

OverlayFS is a filesystem service for Linux which implements a union mount for other file systems. It is supported by the Docker daemon as a storage driver.

registry

A Registry is a hosted service containing repositories of images which responds to the Registry API.

The default registry can be accessed using a browser at Docker Hub or using the docker search command.

repository

A repository is a set of Docker images. A repository can be shared by pushing it to a registry server. The different images in the repository can be labeled using tags.

Here is an example of the shared nginx repository and its tags

service

A service is the definition of how you want to run your application containers in a swarm. At the most basic level a service defines which container image to run in the swarm and which commands to run in the container. For orchestration purposes, the service defines the "desired state", meaning how many containers to run as tasks and constraints for deploying the containers.

Frequently a service is a microservice within the context of some larger application. Examples of services might include an HTTP server, a database, or any other type of executable program that you wish to run in a distributed environment.

service discovery

Swarm mode service discovery is a DNS component internal to the swarm that automatically assigns each service on an overlay network in the swarm a VIP and DNS entry. Containers on the network share DNS mappings for the service via gossip so any container on the network can access the service via its service name.

You don’t need to expose service-specific ports to make the service available to other services on the same overlay network. The swarm’s internal load balancer automatically distributes requests to the service VIP among the active tasks.

swarm

A swarm is a cluster of one or more Docker Engines running in swarm mode.

Docker Swarm

Do not confuse Docker Swarm with the swarm mode features in Docker Engine.

Docker Swarm is the name of a standalone native clustering tool for Docker. Docker Swarm pools together several Docker hosts and exposes them as a single virtual Docker host. It serves the standard Docker API, so any tool that already works with Docker can now transparently scale up to multiple hosts.

Also known as : docker-swarm

swarm mode

Swarm mode refers to cluster management and orchestration features embedded in Docker Engine. When you initialize a new swarm (cluster) or join nodes to a swarm, the Docker Engine runs in swarm mode.

tag

A tag is a label applied to a Docker image in a repository. tags are how various images in a repository are distinguished from each other.

Note : This label is not related to the key=value labels set for docker daemon

task

A task is the atomic unit of scheduling within a swarm. A task carries a Docker container and the commands to run inside the container. Manager nodes assign tasks to worker nodes according to the number of replicas set in the service scale.

The diagram below illustrates the relationship of services to tasks and containers.

Toolbox

Docker Toolbox is a legacy installer for Mac and Windows users. It uses Oracle VirtualBox for virtualization.

For Macs running OS X El Capitan 10.11 and newer macOS releases, Docker for Mac is the better solution.

For Windows 10 systems that support Microsoft Hyper-V (Professional, Enterprise and Education), Docker for Windows is the better solution.

Union file system

Union file systems implement a union mount and operate by creating layers. Docker uses union file systems in conjunction with copy-on-write techniques to provide the building blocks for containers, making them very lightweight and fast.

For more on Docker and union file systems, see Docker and AUFS in practice, Docker and Btrfs in practice, and Docker and OverlayFS in practice

Example implementations of union file systems are UnionFS, AUFS, and Btrfs.

virtual machine

A virtual machine is a program that emulates a complete computer and imitates dedicated hardware. It shares physical hardware resources with other users but isolates the operating system. The end user has the same experience on a Virtual Machine as they would have on dedicated hardware.

Compared to containers, a virtual machine is heavier to run, provides more isolation, gets its own set of resources and does minimal sharing.

Also known as : VM