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Kenta Takeuchi
Kenta Takeuchi

Posted on • Originally published at bmf-tech.com

What is Docker? A Beginner's Complete Guide to Containers

#docker #virtualenvironment

This article was originally published on bmf-tech.com.

What is Docker

  • A platform developed by Docker Inc. for creating, distributing, and running container-based virtual environments

  • Uses Linux container technology

    • Containers utilize the host machine's kernel to isolate processes and users
    • Lightweight and fast

  • Middleware and various environment settings can be managed as code (=Infrastructure as Code)

    • Regardless of local or production environments
    • Anyone can create the same environment
    • Easy redistribution and reuse of environments

  • The essence of Docker

    • Linux Container (LXC)
    • A container management tool to simplify customization of containers in LXC (such as creating configuration files and installing files via shell scripts)

  • Docker for Mac

    • Uses the virtualization tool HyperKit, which is included by default on Mac, to launch a virtual machine and run Linux to enable Docker

What is a Container (Linux Container)

  • A set of processes isolated from the rest of the system

    • Runs from a separate image that provides all the files necessary to support the processes

  • Shares the OS and kernel, isolating application processes from the rest of the system (runs on a single OS)

Differences Between Other Virtual Environments and Containers

General PC

  • Configuration 
  [Host OS]
  [Hard Disk]
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Host OS Type (sometimes defined as hypervisor type using host OS in a broad sense)

  • Configuration 
  [Guest OS]
  [Virtualization Software]
  [Host OS]
  [Hard Disk]
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  • Advantages

    • Easy to achieve virtualization
    • High freedom in OS selection

  • Disadvantages

    • High consumption of disk and memory

Hypervisor Type (using hard disk pattern)

  • Configuration 
  [Guest OS]
  [Hypervisor]
  [Hard Disk]
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  • Advantages

    • Can control hardware directly without needing host OS processing, resulting in faster processing speed

  • Disadvantages

    • Not easy to achieve virtual environments (may not be able to use the host OS as is, or may require a dedicated physical server)

Container Type 

[Container Management Software]
[Host OS]
[Hard Disk]
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  • Advantages

    • Easy to achieve virtual environments
    • Low consumption of disk and memory

  • Disadvantages

    • Can only use OS that utilizes the Linux kernel

Overview of Docker Images and Containers 

                Docker repository(Ex. Docker Hub)

                         ↓ (pull)

Dockerfile   →     Docker Image    →    Docker Container
                 (build)                           (run)   
                                      ↓ (commit)

                             Docker Image    →    Docker Container
                                                       (run)
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※ Differences between Host OS Type and Hypervisor Type
Think IT - What are the differences between Host Type and Hypervisor Type? Overview of VMware vSphere Hypervisor is a useful reference.

  • Host OS Type
    • Install virtualization software on the OS
    • Access to hardware is via the host OS, causing overhead and not fully utilizing performance.
  • Hypervisor Type
    • Install directly on the server without needing a host OS.
    • Can directly control hardware, making it easier to achieve performance.

Basic Commands

  • docker build

    • Create an image from a Dockerfile

  • docker run

    • Launch a container from an image

  • docker commit

    • Create an image from a container

Additional Information

  • What is a Kernel

    • Software responsible for the basic functions of the OS
    • It's generally difficult to use the OS with just the kernel, so it's commonly used in combination with other software
    • Kernel + Software = Distribution
    • CentOS or Ubuntu → Linux kernel + software distribution

References

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