Before 2016, Docker was only available on Linuxoperating systems. After the first version for MicrosoftWindows was released, Docker rapidly became one of the most popular software platforms for building, testing, and deploying applications.
This open-source platform develops, ships, and runs applications in docker containers, making it a fast and functional option for software development teams.
Got a new application slated for development? We have all the important information on how you can manage your infrastructure, scale your applications, and get the most out of this container runtime.
What is Docker?
Docker is an open-source platform as a service (PaaS) that uses containers rather than virtual machines to streamline application development. While Linux containers (LXC) have been around for decades, Docker has made more developers aware of this methodology.
Managing the lifecycle of containers with the Docker tools and platform allows developers to collaborate faster and more reliably than ever before.
A docker build is developed in a container that provides a standard way to run code. That same container can be used for testing and distribution.
Instead of using resources at the hardware level, Docker uses containers to run at the operating system level. This eliminates the need for a virtual machine, requires fewer system resources, and reduces the delay between writing code and production.
What is Docker used for?
The simplified development process makes Docker the best choice for many app development projects. Docker is an excellent option for builds that need to be distributed and compatible with multiple environments because code can be written once and reliably run anywhere the container is sent.
Docker is compatible with:
Linux
MicrosoftWindows
Apple macOS
Amazon Web Services (AWS)
MicrosoftAzure
IBM Cloud
Ubuntu
The simple commands in Docker allow one to automate containerization and save time with application programming interfaces (APIs). Applications can be deployed and scaled with all the libraries, system tools, code, and runtime stored in a container.
Docker can be used to share work, push applications to test environments, debug in the development environment, and to push the updated version to production.
How do containers work?
Containers are quick and easy virtualized runtime environments. They make it easy to manage your underlying infrastructure by holding a package of code, dependencies, configuration files, and anything else your software may need to function. They can be isolated from other running containers, providing better security and preventing conflict between applications.
Multiple containers can run on the same host and take up less space than the same process on a VM. Teams working with Docker containers can simultaneously make changes in different environments and not affect the application's performance.
What is a container image?
A container shouldn’t be confused with a container image.
Docker container images provide the filesystem on the Docker Engine that a container uses to run. These intermediate images are created when a new command is run in the Dockerfile.
A Docker image has everything needed to run the application and configuration for the container, such as metadata and environment variables.
Docker Architecture
Docker’s architecture is a client-server model. There are 4 main components involved in this model.
Docker Client
The client is where a user interacts with Docker using the command line interface (CLI) to pull images from the registry. The docker client can reside on a remote host or the same host as the daemon. A docker client can even communicate with more than one daemon.
The common commands used in the Docker client are:
docker build
docker pull
docker run
Docker Host
The Docker host is where you’ll find the Docker daemon and Docker objects.
The docker daemon is the key player in this piece of the architecture, as it is responsible for pulling and building container images and all container-related actions.
The daemon will receive commands through the CLI or a RESTAPI if 2 systems are interfacing over the internet.
Docker Objects
Objects are the components used in developing a containerized application. There are a handful of object types, including the docker image, docker container, dockerfile, volumes, and networks. We’ve covered containers and images; now, let’s look at networks and storage in Docker.
Network and Storage Components
Networks
Docker uses networking to communicate with other containers, the host machine, and the outside world.
Two basic types of networks are available: the default Docker networks and user-defined networks.
Default Docker networks are host, bridge, and none. Host allows for communication to, you guessed it, the host. Bridge connects to the outside world or between containers. None means the containers are running on their container-specific stack.
User-Defined networks are configurable by the administrator to customize exactly what you’d like your application to do. Some common user-defined networks include a user-defined bridge network, Overlay networks to connect containers on separate hosts to communicate with each other, and Macvlan networks which use MAC addresses instead of IP addresses.
Storage
Docker offers storage in 4 options:
Data volumes: Persistent storage on the host file system
Data Volume Container: A dedicated container hosts a volume for other containers and runs independently of the application container
Directory Mounts: Located in the directory of the host machine
Storage Plug-ins: A way to map storage to an external source
Docker Hub and Registries
Registries contain Docker repositories. These are hosted locations to download and store images. Docker Hub is a cloud-based library and community for container images. You’ll find all sorts of open-source projects in this comprehensive repository.
Teams and software companies use Docker Hub for access between developers working on the project and the client or user during the build process. Docker Hub also provides mechanisms to formulate container images from Bitbucket and GitHub.
What kinds of applications can be used with Docker?
Any type of application can be used with Docker. Docker works seamlessly with container orchestration. This makes it a preferred platform for microservice architecture applications that automate deployment, scaling, and management of containerized applications.
Kubernetes and Docker Swarm are two examples of microservice applications.
Docker containers are portable, making this platform an excellent choice for cloud-based applications. Containers are also perfect for applications using continuous integration and continuous delivery (CI/CD) workflows (think: applications that push phone updates).
Deploying a Dockerized Application
When building Dockerized applications, DevOps usually go through a roadmap of building images, running your image as a container, then using the containers for development. Once everything is coded, it is essential to run tests and look for any bugs in the script.
Once the code has been debugged, the application can be configured for CI/CD. You can use DockerGitHub Actions to build and push those images. Make sure to define and run the workflow before deploying your application.
How is Docker used to simplify the process of deploying applications? If you have Docker Desktop installed, you can easily deploy your workloads in parallel, on Kubernetes, Swarm, and as standalone containers.
Docker also supports AWS Elastic Container Services and Azure Container Instances using native Docker commands. If you are using AWS ECS, you’ll need Docker ComposeCLI to quickly and easily switch from a local context to a cloud context.
What are the advantages of using Docker over competitors?
Docker can be used in a wide variety of environments. Users have the flexibility to take their own configuration, put that into the code, and deploy it.
Some of the major advantages of using Docker include:
Portability: Containers make for highly portable workloads. Docker can run on a laptop, in cloud providers, on physical or virtual machines in a data center, or a combination of all/any of the above.
Scalability: Multiple containers can be placed in a single host, and new containers can be generated quickly.
Saves money: Run more code on the same servers because of the lightweight framework Docker provides.
Build software faster: With the virtualization of the OS, there’s no startup lag, and simple deployment means applications are ready sooner.
Security: When working with a container ecosystem, there are fewer software dependencies and less access needing to be granted.
What is the difference between Docker and a virtual machine?
A virtual machine is a digital version of a computer. It is a file or image that looks and acts like a computer, and has a CPU, memory, disks, and connects to the internet if needed.
VMs are completely independent of one another and the physical host machine. The hypervisor, or virtual machine manager, lets you run different operating systems on different virtual machines at the same time. This takes up valuable computing resources in the process.
Docker doesn't need a hypervisor or guest operating systems. Instead, Docker containers share the host operating system and take advantage of several features of the Linux kernel to deliver its functionality. By using namespaces to provide the isolated containers, Docker provides everything needed to run an application.
Docker Glossary
If you’re just starting with Docker, or need a refresher before an interview, here’s a Docker glossary to help you retain all the information.
DockerFile is a text file with instructions on how to build Docker container image.
Docker daemon is a service that creates and manages Docker images.
Docker Host is the server that the Docker daemon runs on.
Docker image is the read-only source code for executable applications, often pulled from repositories.
Docker Container means the live running instances of Docker images that can be adjusted with Docker commands and interacted with by users.
Docker commands are the arguments sent to docker run are used to perform a specific operation for a container. Find a list of commands in the Docker documentation.
Docker registry is the storage and distribution system for Docker images and provides a place to track versions.
Docker Hub is a public repository of Docker images.
Docker Desktop is the application to build and share containerized applications. Docker Desktop includes Docker Hub, Docker Engine, Docker CLI, Docker Compose, Kubernetes, and more.
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Docker’s popularity exploded in 2019 after Microsoft announced that Docker would run on Windows without a Virtual Machine or Hyper-V. In 2020, it became a worldwide platform due to its easy-to-use implementations.
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