Let’s talk about the composition of Docker containers and the components in them

With the rapid development of cloud computing and containerization technology, more and more people are beginning to use Docker containers for application deployment. So, what exactly is included in a Docker container? This article will introduce you to the composition of Docker containers and the components in them.

First of all, we need to understand what a Docker container is. Simply put, a Docker container refers to an application running in the Docker engine. It can be created using Docker images and can be instantiated, run and terminated via the Docker engine. The beauty of Docker containers is that they are very lightweight and can run on different environments without changes.

The structure of a Docker container can be divided into four parts, namely namespace, control group, Union file system and container format.

Namespace

Namespace is a mechanism provided by the Linux kernel to isolate resources between processes. Docker containers can run multiple containers on the same host with the help of namespaces. Each container has its own independent namespace, allowing complete isolation of resources between them. Docker utilizes the following six namespaces:

1. PID namespace: assigns a unique process ID to each container
2. Network namespace: assigns a unique network address to each container
3. Mount namespace: Assign a unique file system mount point to each container
4. IPC namespace: Allocate a unique inter-process communication resource to each container
5. UTS Namespace: Assign a unique host name and domain name to each container
6. User namespace: Assign a unique user ID and group ID to each container

Control group

Control group (cgroup) is another mechanism provided by the Linux kernel to limit the resource usage of the process. Docker relies on control groups to limit the CPU, memory, network bandwidth and other resources used by containers. This ensures that containers do not interfere with each other, and also provides better control over the resource allocation of the entire system.

Union File System

Union file system is another key technology used by Docker to share files between different Docker containers. The Union file system adopts a layered mechanism. Each layer stores corresponding image files, and all layers combined form a complete file system. When a Docker container is started, the first layer in the image is used as the root file system, and the other layers are mounted read-only. When container applications need to modify files, they can only modify files above this layer, but cannot modify the base layer of the image, allowing files to be shared between different containers and reducing the storage size of each container. space.

Container format

Container format is another core component of the Docker container, which specifies the structure and metadata information of the Docker container. Docker container formats can be divided into two types, one is the LXC format adopted by Docker in the early days, and the other is the libcontainer format independently developed by Docker. LXC format allows you to create a Docker container using the Linux container (LXC) template provided by the Linux kernel. The libcontainer format is Docker's modified format based on LXC. It no longer relies on LXC templates. It is completely independently developed and supported by Docker and is considered a more flexible and efficient format.

Summary

Within the Docker container, the namespace, control group and Union file system are very important components. They all provide the core isolation function and file system mechanism of the Docker container. The container format specifies the basic structure and metadata information of the Docker container. Understanding these components of the Docker container is very helpful for understanding the implementation principles and usage of the Docker container.

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