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Markdown
334 lines
13 KiB
Markdown
---
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description: "Glossary of terms used around Docker"
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keywords: "glossary, docker, terms, definitions"
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---
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<!-- This file is maintained within the docker/cli GitHub
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# Glossary
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A list of terms used around the Docker project.
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## base image
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An image that has no parent is a **base image**.
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## bridge
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In terms of generic networking, a bridge is a Link Layer device which forwards
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traffic between network segments. A bridge can be a hardware device or a
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software device running within a host machine's kernel.
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In terms of Docker, a bridge network uses a software bridge which allows
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containers connected to the same bridge network to communicate, while providing
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isolation from containers which are not connected to that bridge network.
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The Docker bridge driver automatically installs rules in the host machine so
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that containers on different bridge networks cannot communicate directly with
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each other.
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The default bridge network, which is also named `bridge`, behaves differently
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from user-defined bridge networks. Containers connected to the default `bridge`
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network can communicate with each other across the bridge by IP address but
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cannot resolve each other's container name to an IP address unless they are
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explicitly linked using the `--link` flag to `docker run`.
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For more information about Docker networking, see
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[Understand container communication](https://docs.docker.com/engine/userguide/networking/default_network/container-communication/).
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## btrfs
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btrfs (B-tree file system) is a Linux [filesystem](#filesystem) that Docker
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supports as a storage backend. It is a [copy-on-write](https://en.wikipedia.org/wiki/Copy-on-write)
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filesystem.
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## build
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build is the process of building Docker images using a [Dockerfile](#dockerfile).
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The build uses a Dockerfile and a "context". The context is the set of files in the
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directory in which the image is built.
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## cgroups
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cgroups is a Linux kernel feature that limits, accounts for, and isolates
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the resource usage (CPU, memory, disk I/O, network, etc.) of a collection
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of processes. Docker relies on cgroups to control and isolate resource limits.
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*Also known as : control groups*
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## Compose
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[Compose](https://github.com/docker/compose) is a tool for defining and
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running complex applications with Docker. With compose, you define a
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multi-container application in a single file, then spin your
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application up in a single command which does everything that needs to
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be done to get it running.
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*Also known as : docker-compose, fig*
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## copy-on-write
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Docker uses a
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[copy-on-write](https://docs.docker.com/engine/userguide/storagedriver/imagesandcontainers/#/the-copy-on-write-strategy)
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technique and a [union file system](#union-file-system) for both images and
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containers to optimize resources and speed performance. Multiple copies of an
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entity share the same instance and each one makes only specific changes to its
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unique layer.
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Multiple containers can share access to the same image, and make
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container-specific changes on a writable layer which is deleted when
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the container is removed. This speeds up container start times and performance.
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Images are essentially layers of filesystems typically predicated on a base
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image under a writable layer, and built up with layers of differences from the
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base image. This minimizes the footprint of the image and enables shared
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development.
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For more about copy-on-write in the context of Docker, see [Understand images,
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containers, and storage
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drivers](https://docs.docker.com/engine/userguide/storagedriver/imagesandcontainers/).
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## container
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A container is a runtime instance of a [docker image](#image).
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A Docker container consists of
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- A Docker image
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- Execution environment
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- A standard set of instructions
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The concept is borrowed from Shipping Containers, which define a standard to ship
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goods globally. Docker defines a standard to ship software.
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## data volume
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A data volume is a specially-designated directory within one or more containers
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that bypasses the Union File System. Data volumes are designed to persist data,
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independent of the container's life cycle. Docker therefore never automatically
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delete volumes when you remove a container, nor will it "garbage collect"
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volumes that are no longer referenced by a container.
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## Docker
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The term Docker can refer to
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- The Docker project as a whole, which is a platform for developers and sysadmins to
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develop, ship, and run applications
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- The docker daemon process running on the host which manages images and containers
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## Docker for Mac
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[Docker for Mac](https://docs.docker.com/docker-for-mac/) is an easy-to-install,
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lightweight Docker development environment designed specifically for the Mac. A
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native Mac application, Docker for Mac uses the macOS Hypervisor framework,
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networking, and filesystem. It's the best solution if you want to build, debug,
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test, package, and ship Dockerized applications on a Mac. Docker for Mac
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supersedes [Docker Toolbox](#toolbox) as state-of-the-art Docker on macOS.
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## Docker for Windows
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[Docker for Windows](https://docs.docker.com/docker-for-windows/) is an
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easy-to-install, lightweight Docker development environment designed
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specifically for Windows 10 systems that support Microsoft Hyper-V
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(Professional, Enterprise and Education). Docker for Windows uses Hyper-V for
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virtualization, and runs as a native Windows app. It works with Windows Server
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2016, and gives you the ability to set up and run Windows containers as well as
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the standard Linux containers, with an option to switch between the two. Docker
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for Windows is the best solution if you want to build, debug, test, package, and
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ship Dockerized applications from Windows machines. Docker for Windows
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supersedes [Docker Toolbox](#toolbox) as state-of-the-art Docker on Windows.
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## Docker Hub
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The [Docker Hub](https://hub.docker.com/) is a centralized resource for working with
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Docker and its components. It provides the following services:
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- Docker image hosting
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- User authentication
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- Automated image builds and work-flow tools such as build triggers and web hooks
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- Integration with GitHub and Bitbucket
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## Dockerfile
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A Dockerfile is a text document that contains all the commands you would
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normally execute manually in order to build a Docker image. Docker can
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build images automatically by reading the instructions from a Dockerfile.
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## filesystem
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A file system is the method an operating system uses to name files
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and assign them locations for efficient storage and retrieval.
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Examples :
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- Linux : ext4, btrfs, zfs
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- Windows : NTFS
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- macOS : HFS+
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## image
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Docker images are the basis of [containers](#container). An Image is an
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ordered collection of root filesystem changes and the corresponding
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execution parameters for use within a container runtime. An image typically
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contains a union of layered filesystems stacked on top of each other. An image
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does not have state and it never changes.
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## libcontainer
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libcontainer provides a native Go implementation for creating containers with
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namespaces, cgroups, capabilities, and filesystem access controls. It allows
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you to manage the lifecycle of the container performing additional operations
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after the container is created.
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## libnetwork
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libnetwork provides a native Go implementation for creating and managing container
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network namespaces and other network resources. It manage the networking lifecycle
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of the container performing additional operations after the container is created.
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## link
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links provide a legacy interface to connect Docker containers running on the
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same host to each other without exposing the hosts' network ports. Use the
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Docker networks feature instead.
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## node
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A [node](https://docs.docker.com/engine/swarm/how-swarm-mode-works/nodes/) is a physical or virtual
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machine running an instance of the Docker Engine in swarm mode.
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**Manager nodes** perform swarm management and orchestration duties. By default
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manager nodes are also worker nodes.
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**Worker nodes** execute tasks.
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## overlay network driver
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Overlay network driver provides out of the box multi-host network connectivity
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for docker containers in a cluster.
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## overlay storage driver
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OverlayFS is a [filesystem](#filesystem) service for Linux which implements a
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[union mount](https://en.wikipedia.org/wiki/Union_mount) for other file systems.
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It is supported by the Docker daemon as a storage driver.
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## registry
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A Registry is a hosted service containing [repositories](#repository) of [images](#image)
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which responds to the Registry API.
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The default registry can be accessed using a browser at [Docker Hub](#docker-hub)
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or using the `docker search` command.
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## repository
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A repository is a set of Docker images. A repository can be shared by pushing it
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to a [registry](#registry) server. The different images in the repository can be
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labeled using [tags](#tag).
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Here is an example of the shared [nginx repository](https://hub.docker.com/_/nginx/)
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and its [tags](https://hub.docker.com/r/library/nginx/tags/)
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## service
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A [service](https://docs.docker.com/engine/swarm/how-swarm-mode-works/services/) is the definition of how
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you want to run your application containers in a swarm. At the most basic level
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a service defines which container image to run in the swarm and which commands
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to run in the container. For orchestration purposes, the service defines the
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"desired state", meaning how many containers to run as tasks and constraints for
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deploying the containers.
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Frequently a service is a microservice within the context of some larger
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application. Examples of services might include an HTTP server, a database, or
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any other type of executable program that you wish to run in a distributed
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environment.
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## service discovery
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Swarm mode [service discovery](https://docs.docker.com/engine/swarm/networking/#use-swarm-mode-service-discovery) is a DNS component
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internal to the swarm that automatically assigns each service on an overlay
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network in the swarm a VIP and DNS entry. Containers on the network share DNS
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mappings for the service via gossip so any container on the network can access
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the service via its service name.
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You don’t need to expose service-specific ports to make the service available to
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other services on the same overlay network. The swarm’s internal load balancer
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automatically distributes requests to the service VIP among the active tasks.
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## swarm
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A [swarm](https://docs.docker.com/engine/swarm/) is a cluster of one or more Docker Engines running in [swarm mode](#swarm-mode).
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## Docker Swarm
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Do not confuse [Docker Swarm](https://github.com/docker/swarm) with the [swarm mode](#swarm-mode) features in Docker Engine.
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Docker Swarm is the name of a standalone native clustering tool for Docker.
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Docker Swarm pools together several Docker hosts and exposes them as a single
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virtual Docker host. It serves the standard Docker API, so any tool that already
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works with Docker can now transparently scale up to multiple hosts.
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*Also known as : docker-swarm*
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## swarm mode
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[Swarm mode](https://docs.docker.com/engine/swarm/) refers to cluster management and orchestration
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features embedded in Docker Engine. When you initialize a new swarm (cluster) or
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join nodes to a swarm, the Docker Engine runs in swarm mode.
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## tag
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A tag is a label applied to a Docker image in a [repository](#repository).
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tags are how various images in a repository are distinguished from each other.
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*Note : This label is not related to the key=value labels set for docker daemon*
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## task
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A [task](https://docs.docker.com/engine/swarm/how-swarm-mode-works/services/#/tasks-and-scheduling) is the
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atomic unit of scheduling within a swarm. A task carries a Docker container and
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the commands to run inside the container. Manager nodes assign tasks to worker
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nodes according to the number of replicas set in the service scale.
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The diagram below illustrates the relationship of services to tasks and
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containers.
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## Union file system
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Union file systems implement a [union mount](https://en.wikipedia.org/wiki/Union_mount) and operate by creating
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layers. Docker uses union file systems in conjunction with
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[copy-on-write](#copy-on-write) techniques to provide the building blocks for
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containers, making them very lightweight and fast.
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For more on Docker and union file systems, see [OverlayFS storage driver](https://docs.docker.com/storage/storagedriver/overlayfs-driver/),
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and [Btrfs storage driver](https://docs.docker.com/storage/storagedriver/btrfs-driver/).
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Example implementations of union file systems are
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[UnionFS](https://en.wikipedia.org/wiki/UnionFS), and [Btrfs](https://btrfs.wiki.kernel.org/index.php/Main_Page).
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## virtual machine
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A virtual machine is a program that emulates a complete computer and imitates dedicated hardware.
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It shares physical hardware resources with other users but isolates the operating system. The
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end user has the same experience on a Virtual Machine as they would have on dedicated hardware.
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Compared to containers, a virtual machine is heavier to run, provides more isolation,
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gets its own set of resources and does minimal sharing.
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*Also known as : VM*
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