Overview of Docker for Beginners

What Is Docker?

There are two ways of looking at Docker. The first approach involves seeing Docker containers as really lightweight Virtual Machines. The second approach is to see Docker as a software packaging and delivery platform. This latter approach proved a lot more helpful to human developers and resulted in widespread adoption of the technology.

An Overview of Docker Containers

Traditionally, cloud service providers used Virtual Machines to isolate running applications from one another. A hypervisor, or host operating system, provides virtual CPU, memory and other resources to many guest operating systems. Each guest OS works as if it is running on actual physical hardware, and it is, ideally, unaware of other guests running on the same physical server.

VMware was one of the first to popularize this concept. However, there are several problems with this virtualization. First of all, the provisioning of resources takes time. Each virtual disk image is large and bulky and getting a VM ready for use can take up to a minute!

Second, and a more important issue, was the inefficient utilization of system resources. OS kernels are control freaks that want to manage everything that’s supposedly available to them. So when a guest OS thinks 2GB of memory is available to it, it takes control of that memory even if the applications running on that OS uses only half of it.

On the other hand, when we run containerized applications, we virtualize the operating system (your standard libraries, packages, etc) itself, not the hardware. Now, instead of providing virtual hardware to a VM, you provide a virtual OS to your application. You can run multiple applications and impose limitations on their resource utilization if you want, and each application will run oblivious to the hundreds of other containers it is running alongside.

Docker — As a Developer’s Tool

One of the problems that developers have is the difference between the production server, where the applications run, and their own dev machines (usual laptops and workstations) where applications are developed. Let’s imagine that you have Windows 10 running on your desktop but you want to write applications for Ubuntu 18.04. Maybe you are using Python v3.6 to write your application, while the Ubuntu server is still running at 3.4.

There are just too many variables to take into account and so we use Docker to abstract that complexity away. Docker can be installed on any OS, even Windows and Mac OS X are well-supported. So you can package your code into a Docker image, run and test it locally using Docker to guaranteed that the containers that were created from that Docker image will behave the same way in production.

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