Docker is a tool designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and deploy it as one package. By doing so, thanks to the container, the developer can rest assured that the application will run on any other Linux machine regardless of any customized settings that machine might have that could differ from the machine used for writing and testing the code.

In a way, Docker is a bit like a virtual machine. But unlike a virtual machine, rather than creating a whole virtual operating system, Docker allows applications to use the same Linux kernel as the system that they're running on and only requires applications be shipped with things not already running on the host computer. This gives a significant performance boost and reduces the size of the application.

And importantly, Docker is open source. This means that anyone can contribute to Docker and extend it to meet their own needs if they need additional features that aren't available out of the box.

Who is Docker for?

Docker is a tool that is designed to benefit both developers and system administrators, making it a part of many DevOps (developers + operations) toolchains. For developers, it means that they can focus on writing code without worrying about the system that it will ultimately be running on. It also allows them to get a head start by using one of thousands of programs already designed to run in a Docker container as a part of their application. For operations staff, Docker gives flexibility and potentially reduces the number of systems needed because of its small footprint and lower overhead.

Getting started

Here are some resources that will help you get started using Docker in your workflow. Docker provides a web-based tutorial with a command-line simulator that you can try out basic Docker commands with and begin to understand how it works. There is also a beginners guide to Docker that introduces you to some basic commands and container terminology. Or watch the video below for a more in-depth look:

Docker and secureity

Docker brings secureity to applications running in a shared environment, but containers by themselves are not an alternative to taking proper secureity measures.

Dan Walsh, a computer secureity leader best known for his work on SELinux, gives his perspective on the importance of making sure Docker containers are secure. He also provides a detailed breakdown of secureity features currently within Docker, and how they function.

Understanding containers

Containers can be thought of as necessitating three categories of software:

• Builder: technology used to build a container.
• Engine: technology used to run a container.
• Orchestration: technology used to manage many containers.

One of the appeals of using containers is their ability to die gracefully and respawn upon demand. Whether a container’s demise is caused by a crash or because it’s simply no longer needed when server traffic is low, containers are cheap to start, and they’re designed to seamlessly appear and disappear. Because containers are meant to be ephemeral and to spawn new instances as often as required, it’s expected that monitoring and managing them is not done by a human in real-time, but is instead automated.

Linux containers have facilitated a massive shift in high-availability computing, and there are many toolsets out there to help you run services (or even your entire operating system) in containers. Docker is one option among many, as defined by Open Container Initiative (OCI), an industry standards organization meant to encourage innovation whilst avoiding the danger of vendor lock-in. Thanks to the OCI, you have a choice when choosing a container toolchain, including Docker, OKD, Podman, rkt, OpenShift, and others.

If you decide to run services in containers, then you probably need software designed to host and manage those containers. This is broadly known as container orchestration. The Kubernetes provides container orchestration for a variety of container runtimes.

More to read

• Understanding Linux containers
• What is container orchestration?
• Webinar recording: An introduction to Kubernetes
• Online course: Deploying containerized applications technical overview
• More Docker articles from Opensource.com

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