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Docker

Complete Guide To Docker For Beginners And Intermediates

Craig Berg

Introduction

In the earlier days of technological evolution, developers deployed applications directly on physical machines, with each equipped with an Operating System. Because of the single user-space, applications shared runtime.

Although deployment on physical machines was stable, the maintenance was long and arduous, more so when each host used a different operating system. There was no flexibility for developers and the hosted applications.

As you can imagine, this caused many issues when there was more than one application built that required regular maintenance and a standalone machine for it.

Because of the limitations of deploying applications on physical hardware and utilizing the resources of the entire host system, Virtualization technology came into being, which is when the dynamics of app development started changing.

Using tools known as hypervisors such as Hyper-V, ESX, KVM, VMware, and others, developers started being able to create virtual machines that they could use to deploy host operating systems (guest OS) on one physical machine.

Virtual machines have independent virtual machines, and applications deployed on virtual machines are what we call isolated and standalone. That means, thanks to the complete isolation of the entire infrastructure running the application, its possible to perform updates and patches on one instance of the application without affecting other applications.

The diagram below illustrates virtualization.

As technology grows, virtual machines, development tools that were once instrumental, are becoming redundant, in part because of the complexity of using Virtual Machines, but mainly because of the technological changes precipitated by the process of reducing hardware emulation. Containers are taking over the territory once owned by Virtual Machines.

By using Containers, developers can package applications with their standalone environments. Although compared to VM, Containers are less flexible because they rely on the host operating system; as we will discuss in upcoming chapters, they are a better alternative.

In this Docker guide:

• You will get a concise introduction to the Docker platform and architecture, including how to install it on the Mac, Linux, and Windows OS.
• Learn how to work with Docker Containers, including how to search, list, and pull Docker images.
• We shall look at how to work with Container Network and data management.

And so much, much more.

This guide purposes to equip you with invaluable knowledge that will help you go from new to Docker, to having the ability to work well with Containers and well on your way to Docker mastery.

Lets get started:

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Table of Content

Section 1: Docker Platform And Architecture

Docker is a containerization software developed by Solomon Hykes, the founder of Dot Cloud. It was initially meant to be an internal project. In March 2013, Docker became an open-source software using the Apache License.

Docker utilizes a hosts operating system kernel feature that allows containerization. We can illustrate Dockers platform and kernel features using the image below.

To help you develop a deeper understanding of Docker better, lets look at the facets that make up its core architecture:

Namespaces

Namespaces are the main building blocks of Docker Containers. There are various types of namespaces, with one of them acting as an isolating block from one application to another. The creation of Namespaces happens through the clone system call. Developers sometimes also attach existing namespaces. Some of the namespaces used by Docker include:

• The PID namespace
• The net namespace
• IPC namespace
• UTS namespace
• MNT namespace
• User namespace

Lets discuss each of the above namespaces in detail:

The PID Namespace

The PID namespaces in Linux system isolates the process Identification Number space. That means processes using different PID namespaces can have the same PID.

The PID namespace allows containers to deliver functionalities like suspending and resuming the set of processes within the container. It also gives developers the ability to migrate the container from one host to another while the processes running within the container maintain their original PIDs.

PIDs in a new PID namespace start at 1, somewhat like a standalone system, and calls to for k , vfor k , or clon e will produce processes with PIDs that are unique within the namespace.

As mentioned, the PID namespaces allow each container to have an isolated set of PIDs. Each PID creates a unique process hierarchy.

A parent (main) namespace can manage the children namespaces and perform actions to alter their functionality. A child namespace, however, cannot see or perform any particular action on the parent namespace node.

Other than the initial (root) PID namespace, each of the nested namespaces has a parent. The parent in this case refers to the PID namespace of the process that actually created the namespace using unshare or clon e . Thus, the PID namespaces make some sort of tree that contains all all namespaces that ultimately trace their ancestry to the root namespace.

If there are two levels of hierarchy, then at the top level, we would see the process running inside the child namespace with a different PID. Therefore, a process running within a child namespace usually has two PIDs: one for the parent namespace and one for the child namespace.

Let me give an illustration: If we start a program such as nan o , the action creates a parent and child process:

You can find more information on PID namespace from the resource page below:

http://man7.org/linux/man-pages/man7/pid_namespaces.7.html

The NET Namespace

The network namespace is a logical copy of the network stack. It has individual building blocks like firewall rules, network interfaces, and routing tables.

The default location for the network namespaces is:

/var/run/netns/NAME

By convention, a process usually inherits all its network namespace from the parent namespace, where, initially, the same namespace from the ini t process is shared by all processes.

By using the PID namespace only, we can run one program more than once in isolated environments. For example, we can run a service such as NGINX on different containers since the PIDs do not conflict, but we cannot communicate with the service on port 80 without using the net namespace.

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