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]
```

Host OS Type (sometimes defined as hypervisor type using host OS in a broad sense)

Configuration

[Guest OS]
[Virtualization Software]
[Host OS]
[Hard Disk]

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]
```
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]

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)

※ 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