Glossary

Term

Definition

AMD64 is AMD’s 64-bit extension of Intel’s x86 architecture, and is also referred to as x86_64 (or x86-64).

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.

A base image has no parent image specified in its Dockerfile. It is created using a Dockerfile with the FROM scratch directive.

boot2docker is a lightweight Linux distribution made specifically to run Docker containers. The boot2docker management tool for Mac and Windows was deprecated and replaced by docker-machine which you can install with the Docker Toolbox.

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

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 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

A cluster is a group of machines that work together to run workloads and provide high availability.

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

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.

A container is a runtime instance of a docker image.

A Docker container consists of

• A Docker image
• An 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.

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 (also called Docker Engine)

Docker Enterprise is a platform to build, ship, and run containerized applications, that you can deploy in the cloud or on-premise. It includes a tested and certified version of Docker, web UIs for managing your app resources, and support.

Docker Desktop for Mac is an easy-to-install, lightweight Docker development environment designed specifically for the Mac. A native Mac application, Docker Desktop 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 Desktop for Mac supersedes Docker Toolbox as state-of-the-art Docker on macOS.

Docker Desktop 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 Desktop 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 Desktop for Windows supersedes Docker Toolbox as state-of-the-art Docker on Windows.

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

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.

In a Dockerfile, an ENTRYPOINT is an optional definition for the first part of the command to be run. If you want your Dockerfile to be runnable without specifying additional arguments to the docker run command, you must specify either ENTRYPOINT, CMD, or both.

• If ENTRYPOINT is specified, it is set to a single command. Most official Docker images have an ENTRYPOINT of /bin/sh or /bin/bash. Even if you do not specify ENTRYPOINT, you may inherit it from the base image that you specify using the FROM keyword in your Dockerfile. To override the ENTRYPOINT at runtime, you can use --entrypoint. The following example overrides the entrypoint to be /bin/ls and sets the CMD to -l /tmp.

  $ docker run --entrypoint=/bin/ls ubuntu -l /tmp
  

• CMD is appended to the ENTRYPOINT. The CMD can be any arbitrary string that is valid in terms of the ENTRYPOINT, which allows you to pass multiple commands or flags at once. To override the CMD at runtime, just add it after the container name or ID. In the following example, the CMD is overridden to be /bin/ls -l /tmp.

  $ docker run ubuntu /bin/ls -l /tmp
  

In practice, ENTRYPOINT is not often overridden. However, specifying the ENTRYPOINT can make your images more flexible and easier to reuse.

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+

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.

A legacy GUI, bundled with Docker Toolbox, for managing Docker containers. We recommend upgrading to Docker Desktop for Mac or Docker Desktop for Windows, which have superseded Kitematic.

In an image, a layer is modification to the image, represented by an instruction in the Dockerfile. Layers are applied in sequence to the base image to create the final image. When an image is updated or rebuilt, only layers that change need to be updated, and unchanged layers are cached locally. This is part of why Docker images are so fast and lightweight. The sizes of each layer add up to equal the size of the final image.

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

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.

Machine is a Docker tool which makes it really easy to create Docker hosts on your computer, on cloud providers and inside your own data center. It creates servers, installs Docker on them, then configures the Docker client to talk to them.

Also known as : docker-machine

A Linux namespace is a Linux kernel feature that isolates and virtualizes system resources. Processes which are restricted to a namespace can only interact with resources or processes that are part of the same namespace. Namespaces are an important part of Docker’s isolation model. Namespaces exist for each type of resource, including net (networking), mnt (storage), pid (processes), uts (hostname control), and user (UID mapping). For more information about namespaces, see Docker run reference and Introduction to user namespaces.

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

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.

An image’s parent image is the image designated in the FROM directive in the image’s Dockerfile. All subsequent commands are based on this parent image. A Dockerfile with the FROM scratch directive uses no parent image, and creates a base image.

Persistent storage or volume storage provides a way for a user to add a persistent layer to the running container’s file system. This persistent layer could live on the container host or an external device. The lifecycle of this persistent layer is not connected to the lifecycle of the container, allowing a user to retain state.

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.

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.

SSH (secure shell) is a secure protocol for accessing remote machines and applications. It provides authentication and encrypts data communication over insecure networks such as the Internet. SSH uses public/private key pairs to authenticate logins.

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.

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.

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

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 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.

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.

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.

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 Desktop for Mac is the better solution.

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

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.

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

A volume is a specially-designated directory within one or more containers that bypasses the Union File System. Volumes are designed to persist data, independent of the container’s life cycle. Docker therefore never automatically deletes volumes when you remove a container, nor will it “garbage collect” volumes that are no longer referenced by a container. Also known as: data volume

There are three types of volumes: host, anonymous, and named:

• A host volume lives on the Docker host’s filesystem and can be accessed from within the container.

• A named volume is a volume which Docker manages where on disk the volume is created, but it is given a name.

• An anonymous volume is similar to a named volume, however, it can be difficult, to refer to the same volume over time when it is an anonymous volumes. Docker handle where the files are stored.

x86_64 (or x86-64) refers to a 64-bit instruction set invented by AMD as an extension of Intel’s x86 architecture. AMD calls its x86_64 architecture, AMD64, and Intel calls its implementation, Intel 64.