Docker Tutorial - Learn Docker essentials by containerizing Wordpress смотреть онлайн

Let's learn about docker!

So, what is Docker? Docker allows us to share and distribute software more easily.

It gives us a platform to “package, ship, and run any app, anywhere.” To be able to do this, Docker uses containers. Think of a container as a modified runtime environment for a program, where it prevents the program from accessing any protected resources.

So, what does this mean? We run software in containers to achieve isolation or basically create sandboxed runtime environments for applications to run in. It’s sandboxed in the sense that containers run only in the user space and programs that run inside of a container can only access the sub space allocated for the container. The container is essentially a private and isolated running process the operating system.

It’s important to understand that before Docker, we achieved isolation for software to run in through virtual machines or VMs. So, what’s the difference between achieving isolation through containers vs. VMs?

Docker containers achieve isolation at the OS level by leveraging container technology already provided by the operating system. This is not the same as virtualization technology used by virtual machines. With virtual machines you create isolation by distributing the physical machines hardware with hardware virtualization on which then you can install programs and operating systems.

To create and run VMs you need a program known as the hypervisor, which sits between the hardware and the VMs that need to be virtualized. Each VM comes with a whole copy of an operating system, which is known as the guest OS. Then finally in the guest OS of the virtual machine is where you get to run your programs and application. The problem is that VMs are slow to create and a guest OS adds significant resource overhead just for you to be able to run a program.

When it comes to a docker container, it gets to interface directly with the kernel from the host OS without any additional layer such as a guest operating system. So, it’s much faster to build and run containers and be able to run programs or applications inside of containers without additional resource overhead.

Let’s drill down some more to really understand the problems Docker is solving by allowing us ship software in containers, where programs run in isolation.

Imagine that you created this application and now you want to share it with others. It turns out that this is not an easy task. In order to install and use your software You’d have to tell me what operating system I should have, you’d have to give me a copy of your code, tell me to install various tools and package management systems, download a bunch of libraries and frameworks the application may depend on, give me instructions on to how to configure everything properly and hope that I’ve done everything correctly just to be able to run your app.

You can eliminate your worries by using Docker. Let’s recall that Docker allows us to run applications inside of containers, so where does Docker get all the information needed to build and run an app? The whole process of containerizing an application with Docker starts with a Dockerfile. With a Dockerfile we provide a set of instructions, which are essentially commands that need to be executed to assemble a docker image. Once we have a docker image we can publish the image to a public or private docker registry. A docker registry is basically a centralized system that’s responsible for storage and delivery of docker images. The largest docker registry where you can browse hundreds of thousands of container images is called docker hub. To share your applications with anyone publicly you can publish container images to docker hub or let’s say when you’re working in a company and wish to distribute privately, then you can publish to a private docker registry that’s created and maintained by the company.