Docker overview (2024)

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

Docker provides the ability to package and run an application in a loosely isolatedenvironment called a container. The isolation and security lets you run manycontainers simultaneously on a given host. Containers are lightweight and containeverything needed to run the application, so you don't need to rely on what'sinstalled on the host. You can share containers while you work,and be sure that everyone you share with gets the same container that works in thesame way.

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

• Develop your application and its supporting components using containers.
• The container becomes the unit for distributing and testing your application.
• When you're ready, deploy your application into your production environment,as a container or an orchestrated service. This works the same whether yourproduction environment is a local data center, a cloud provider, or a hybridof the two.

What can I use Docker for?

Fast, consistent delivery of your applications

Docker streamlines the development lifecycle by allowing developers to work instandardized environments using local containers which provide your applicationsand services. Containers are great for continuous integration and continuousdelivery (CI/CD) workflows.

Consider the following example scenario:

• Your developers write code locally and share their work with their colleaguesusing Docker containers.
• They use Docker to push their applications into a test environment and runautomated and manual tests.
• When developers find bugs, they can fix them in the development environmentand redeploy them to the test environment for testing and validation.
• When testing is complete, getting the fix to the customer is as simple aspushing the updated image to the production environment.

Responsive deployment and scaling

Docker's container-based platform allows for highly portable workloads. Dockercontainers can run on a developer's local laptop, on physical or virtualmachines in a data center, on cloud providers, or in a mixture of environments.

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

Running more workloads on the same hardware

Docker is lightweight and fast. It provides a viable, cost-effective alternativeto hypervisor-based virtual machines, so you can use more of your servercapacity to achieve your business goals. Docker is perfect for high densityenvironments and for small and medium deployments where you need to do more withfewer resources.

Docker uses a client-server architecture. The Docker client talks to theDocker daemon, which does the heavy lifting of building, running, anddistributing your Docker containers. The Docker client and daemon canrun on the same system, or you can connect a Docker client to a remote Dockerdaemon. The Docker client and daemon communicate using a REST API, over UNIXsockets or a network interface. Another Docker client is Docker Compose,that lets you work with applications consisting of a set of containers.

The Docker daemon

The Docker daemon (dockerd) listens for Docker API requests and manages Dockerobjects such as images, containers, networks, and volumes. A daemon can alsocommunicate with other daemons to manage Docker services.

The Docker client

The Docker client (docker) is the primary way that many Docker users interactwith Docker. When you use commands such as docker run, the client sends thesecommands to dockerd, which carries them out. The docker command uses theDocker API. The Docker client can communicate with more than one daemon.

Docker Desktop

Docker Desktop is an easy-to-install application for your Mac, Windows or Linux environment that enables you to build and share containerized applications and microservices. Docker Desktop includes the Docker daemon (dockerd), the Docker client (docker), Docker Compose, Docker Content Trust, Kubernetes, and Credential Helper. For more information, see Docker Desktop.

Docker registries

A Docker registry stores Docker images. Docker Hub is a publicregistry that anyone can use, and Docker looks for images onDocker Hub by default. You can even run your own private registry.

When you use the docker pull or docker run commands, Docker pulls the required images from your configured registry. When you use the docker push command, Docker pushesyour image to your configured registry.

Docker objects

When you use Docker, you are creating and using images, containers, networks,volumes, plugins, and other objects. This section is a brief overview of someof those objects.

Images

An image is a read-only template with instructions for creating a Dockercontainer. Often, an image is based on another image, with some additionalcustomization. For example, you may build an image which is based on the ubuntuimage, but installs the Apache web server and your application, as well as theconfiguration details needed to make your application run.

You might create your own images or you might only use those created by othersand published in a registry. To build your own image, you create a Dockerfilewith a simple syntax for defining the steps needed to create the image and runit. Each instruction in a Dockerfile creates a layer in the image. When youchange the Dockerfile and rebuild the image, only those layers which havechanged are rebuilt. This is part of what makes images so lightweight, small,and fast, when compared to other virtualization technologies.

Containers

A container is a runnable instance of an image. You can create, start, stop,move, or delete a container using the Docker API or CLI. You can connect acontainer to one or more networks, attach storage to it, or even create a newimage based on its current state.

By default, a container is relatively well isolated from other containers andits host machine. You can control how isolated a container's network, storage,or other underlying subsystems are from other containers or from the hostmachine.

A container is defined by its image as well as any configuration options youprovide to it when you create or start it. When a container is removed, any changes toits state that aren't stored in persistent storage disappear.

Example docker run command

The following command runs an ubuntu container, attaches interactively to yourlocal command-line session, and runs /bin/bash.

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

When you run this command, the following happens (assuming you are usingthe default registry configuration):

1. If you don't have the ubuntu image locally, Docker pulls it from yourconfigured registry, as though you had run docker pull ubuntu manually.

2. Docker creates a new container, as though you had run a docker container createcommand manually.

3. Docker allocates a read-write filesystem to the container, as its finallayer. This allows a running container to create or modify files anddirectories in its local filesystem.

4. Docker creates a network interface to connect the container to the defaultnetwork, since you didn't specify any networking options. This includesassigning an IP address to the container. By default, containers canconnect to external networks using the host machine's network connection.

5. Docker starts the container and executes /bin/bash. Because the containeris running interactively and attached to your terminal (due to the -i and -tflags), you can provide input using your keyboard while Docker logs the output toyour terminal.

6. When you run exit to terminate the /bin/bash command, the containerstops but isn't removed. You can start it again or remove it.

The underlying technology

Docker is written in theGo programming language and takesadvantage of several features of the Linux kernel to deliver its functionality.Docker uses a technology called namespaces to provide the isolated workspacecalled the container. When you run a container, Docker creates a set ofnamespaces for that container.

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

• Install Docker
• Get started with Docker