mirror of https://github.com/docker/docs.git
208 lines
9.7 KiB
Markdown
208 lines
9.7 KiB
Markdown
---
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description: Get an in-depth overview of the Docker platform including what it can
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be used for, the architecture it employs, and its underlying technology.
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keywords: what is a docker, docker daemon, why use docker, docker architecture, what
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to use docker for, docker client, what is docker for, why docker, uses for docker,
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what is docker container used for, what are docker containers used for
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title: Docker overview
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aliases:
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- /introduction/understanding-docker/
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- /engine/userguide/basics/
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- /engine/introduction/understanding-docker/
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- /engine/understanding-docker/
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- /engine/docker-overview/
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- /get-started/overview/
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- /get-started/docker-overview/
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---
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Docker is an open platform for developing, shipping, and running applications.
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Docker enables you to separate your applications from your infrastructure so
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you can deliver software quickly. With Docker, you can manage your infrastructure
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in the same ways you manage your applications. By taking advantage of Docker's
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methodologies for shipping, testing, and deploying code, you can
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significantly reduce the delay between writing code and running it in production.
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## The Docker platform
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Docker provides the ability to package and run an application in a loosely isolated
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environment called a container. The isolation and security lets you run many
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containers simultaneously on a given host. Containers are lightweight and contain
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everything needed to run the application, so you don't need to rely on what's
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installed on the host. You can share containers while you work,
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and be sure that everyone you share with gets the same container that works in the
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same way.
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Docker provides tooling and a platform to manage the lifecycle of your containers:
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* Develop your application and its supporting components using containers.
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* The container becomes the unit for distributing and testing your application.
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* When you're ready, deploy your application into your production environment,
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as a container or an orchestrated service. This works the same whether your
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production environment is a local data center, a cloud provider, or a hybrid
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of the two.
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## What can I use Docker for?
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### Fast, consistent delivery of your applications
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Docker streamlines the development lifecycle by allowing developers to work in
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standardized environments using local containers which provide your applications
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and services. Containers are great for continuous integration and continuous
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delivery (CI/CD) workflows.
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Consider the following example scenario:
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- Your developers write code locally and share their work with their colleagues
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using Docker containers.
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- They use Docker to push their applications into a test environment and run
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automated and manual tests.
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- When developers find bugs, they can fix them in the development environment
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and redeploy them to the test environment for testing and validation.
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- When testing is complete, getting the fix to the customer is as simple as
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pushing the updated image to the production environment.
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### Responsive deployment and scaling
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Docker's container-based platform allows for highly portable workloads. Docker
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containers can run on a developer's local laptop, on physical or virtual
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machines in a data center, on cloud providers, or in a mixture of environments.
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Docker's portability and lightweight nature also make it easy to dynamically
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manage workloads, scaling up or tearing down applications and services as
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business needs dictate, in near real time.
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### Running more workloads on the same hardware
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Docker is lightweight and fast. It provides a viable, cost-effective alternative
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to hypervisor-based virtual machines, so you can use more of your server
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capacity to achieve your business goals. Docker is perfect for high density
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environments and for small and medium deployments where you need to do more with
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fewer resources.
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## Docker architecture
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Docker uses a client-server architecture. The Docker client talks to the
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Docker daemon, which does the heavy lifting of building, running, and
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distributing your Docker containers. The Docker client and daemon can
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run on the same system, or you can connect a Docker client to a remote Docker
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daemon. The Docker client and daemon communicate using a REST API, over UNIX
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sockets or a network interface. Another Docker client is Docker Compose,
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that lets you work with applications consisting of a set of containers.
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### The Docker daemon
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The Docker daemon (`dockerd`) listens for Docker API requests and manages Docker
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objects such as images, containers, networks, and volumes. A daemon can also
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communicate with other daemons to manage Docker services.
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### The Docker client
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The Docker client (`docker`) is the primary way that many Docker users interact
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with Docker. When you use commands such as `docker run`, the client sends these
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commands to `dockerd`, which carries them out. The `docker` command uses the
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Docker API. The Docker client can communicate with more than one daemon.
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### Docker Desktop
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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](/desktop/index.md).
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### Docker registries
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A Docker registry stores Docker images. Docker Hub is a public
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registry that anyone can use, and Docker looks for images on
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Docker Hub by default. You can even run your own private registry.
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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 pushes
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your image to your configured registry.
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### Docker objects
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When you use Docker, you are creating and using images, containers, networks,
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volumes, plugins, and other objects. This section is a brief overview of some
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of those objects.
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#### Images
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An image is a read-only template with instructions for creating a Docker
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container. Often, an image is based on another image, with some additional
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customization. For example, you may build an image which is based on the `ubuntu`
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image, but installs the Apache web server and your application, as well as the
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configuration details needed to make your application run.
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You might create your own images or you might only use those created by others
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and published in a registry. To build your own image, you create a Dockerfile
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with a simple syntax for defining the steps needed to create the image and run
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it. Each instruction in a Dockerfile creates a layer in the image. When you
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change the Dockerfile and rebuild the image, only those layers which have
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changed are rebuilt. This is part of what makes images so lightweight, small,
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and fast, when compared to other virtualization technologies.
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#### Containers
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A container is a runnable instance of an image. You can create, start, stop,
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move, or delete a container using the Docker API or CLI. You can connect a
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container to one or more networks, attach storage to it, or even create a new
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image based on its current state.
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By default, a container is relatively well isolated from other containers and
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its host machine. You can control how isolated a container's network, storage,
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or other underlying subsystems are from other containers or from the host
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machine.
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A container is defined by its image as well as any configuration options you
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provide to it when you create or start it. When a container is removed, any changes to
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its state that aren't stored in persistent storage disappear.
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##### Example `docker run` command
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The following command runs an `ubuntu` container, attaches interactively to your
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local command-line session, and runs `/bin/bash`.
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```console
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$ docker run -i -t ubuntu /bin/bash
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```
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When you run this command, the following happens (assuming you are using
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the default registry configuration):
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1. If you don't have the `ubuntu` image locally, Docker pulls it from your
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configured registry, as though you had run `docker pull ubuntu` manually.
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2. Docker creates a new container, as though you had run a `docker container create`
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command manually.
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3. Docker allocates a read-write filesystem to the container, as its final
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layer. This allows a running container to create or modify files and
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directories in its local filesystem.
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4. Docker creates a network interface to connect the container to the default
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network, since you didn't specify any networking options. This includes
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assigning an IP address to the container. By default, containers can
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connect to external networks using the host machine's network connection.
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5. Docker starts the container and executes `/bin/bash`. Because the container
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is running interactively and attached to your terminal (due to the `-i` and `-t`
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flags), you can provide input using your keyboard while Docker logs the output to
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your terminal.
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6. When you run `exit` to terminate the `/bin/bash` command, the container
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stops but isn't removed. You can start it again or remove it.
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## The underlying technology
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Docker is written in the [Go programming language](https://golang.org/) and takes
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advantage of several features of the Linux kernel to deliver its functionality.
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Docker uses a technology called `namespaces` to provide the isolated workspace
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called the container. When you run a container, Docker creates a set of
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namespaces for that container.
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These namespaces provide a layer of isolation. Each aspect of a container runs
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in a separate namespace and its access is limited to that namespace.
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## Next steps
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- [Install Docker](/get-started/get-docker.md)
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- [Get started with Docker](/get-started/introduction/_index.md)
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