A quick guide to Docker

Docker provides a way to automatically deploy software in a safe and repeatable environment, which marks the beginning of a revolution in the way computing platforms are developed. Today, Docker is widely used in Internet companies. This article will help you quickly get started with Docker in ten minutes.

What is Docker

What is Docker?

Introduction on the official website:

Enterprise Container Platform for High-Velocity Innovation. Securely build, share and run any application, anywhere

Baidu Encyclopedia tells us:

Docker is an open source application container engine that allows developers to package their applications and dependent packages into a portable container and then publish them to any popular Linux machine. It can also achieve virtualization. The container uses a complete sandbox mechanism and there is no interface between them.

Introduction to the First DOCKER Book:

Docker is an open source engine that can automatically deploy developed applications to containers. Written by the team of Docker (www.docker.com, formerly dotCloud, an established supplier in the PaaS market) and released under the Apache 2.0 open source protocol.

What makes Docker special:

Docker is a lightweight virtualization technology with a fast startup speed (most Docker containers can be started in less than 1 second). Hundreds or thousands of containers can run simultaneously on a single piece of hardware, making rapid expansion and elastic scaling easy. It is said that in 2016, JD.com used 150,000 Docker clusters to ensure system stability in the high-concurrency scenario of 618.

Docker is cross-platform and supports Windows, Macos, and Linux. It can "build once, run everywhere", solving a series of problems caused by the inconsistency between the development environment and the production environment, allowing developers and operation and maintenance personnel to live in harmony.

Docker is open source and hosted on GitHub.

Docker Idea

Associating Docker's core concept from Docker's logo

Docker's logo is a big whale carrying containers, which is definitely the most vivid description and explanation of Docker.

In contrast to the transportation industry, before the advent of containers, goods could not be handled in a unified standard manner. For example, some goods were fragile and needed to be handled with care, while others did not. Therefore, a large amount of manpower is needed for cargo transfer between various modes of transportation such as railways, roads, and oceans, which is extremely inefficient and costly. The emergence of containers solved this problem. Any goods can be placed in this magical box. Then, in all transportation scenarios such as roads, railways, and oceans, the box is sealed during transportation. Moreover, the transfer work in the middle can be done by large machinery, greatly improving efficiency.

Docker formally borrowed the idea of ​​standard containers and applied the container idea to the software field. Docker provides a standardized container-based transportation system for code, which can package any application and its dependent environment (such as code, configuration files, JDK, Tomcat, etc.) into a container that can run on almost all operating systems.

Docker Core Concepts

Mirror

Mirroring is the cornerstone of Docker, and users can run their own containers based on mirroring.

The basis of the image is Docker's union file system, which is layered and each image is a layer. Since each layer has other layers on top, that is, images can be created on top of other images (base images). Let me use a picture to help you understand. The picture is from the Internet, please delete if infringed.

storehouse

The warehouse is where user images are stored. The official Docker warehouse address is https://hub.docker.com. There are many images on Docker Hub, including the simplest hello-world, MySQL, etc. Of course we can also have our own private warehouse.

container

Containers provide isolated runtimes for applications. Each container contains a complete and exclusive user environment, and changes in the operating environment in one container will not affect the operating environment of other containers, allowing applications to run in the same way almost anywhere.

The container is started based on the image, and one or more processes can run in the container. When creating a container process, the Namespace parameters required by the process are specified so that the container can only "see" the resources, files, devices, status, or configuration limited by the current Namespace. Therefore, a container is just a special process, and the essence of a container is a process.

Docker Installation

Take CentOS 7 as an example to install Docker.

Check the system kernel version

Docker runs on CentOS 7, which requires a 64-bit operating system and a kernel version of 3.10 or above.

Confirm that the Linux kernel that meets the requirements has been installed on this machine. Use the command uname -r to check the kernel version information.

[root@localhost ~]# uname -r
3.10.0-957.el7.x86_64

Install Docker in CentOS 7

Use the yum install -y docker command to install Docker. "-y" means do not ask and use the default configuration for installation.

Start the Docker service and set it to start automatically at boot

Use the following command:

systemctl start docker.service
systemctl enable docker.service

Enter docker version and the returned version information indicates that Docker is installed successfully.

[root@localhost ~]# docker version
Client:
 Version: 1.13.1
 API version: 1.26
 Package version: docker-1.13.1-96.gitb2f74b2.el7.centos.x86_64
 Go version: go1.10.3
 Git commit: b2f74b2/1.13.1
 Built: Wed May 1 14:55:20 2019
 OS/Arch: linux/amd64

Server:
 Version: 1.13.1
 API version: 1.26 (minimum version 1.12)
 Package version: docker-1.13.1-96.gitb2f74b2.el7.centos.x86_64
 Go version: go1.10.3
 Git commit: b2f74b2/1.13.1
 Built: Wed May 1 14:55:20 2019
 OS/Arch: linux/amd64
 Experimental: false

Docker in Action - Hello World

How can we learn from the classic “Hello World”?

Pull the image

In fact, this image already exists on DockerHub, named "hello-world". Pull the image directly from DockerHub. The command is similar to Git: docker pull hello-world

[root@localhost docker]# docker pull hello-world
Using default tag: latest
Trying to pull repository docker.io/library/hello-world ... 
latest: Pulling from docker.io/library/hello-world
1b930d010525: Pull complete 
Digest: sha256:41a65640635299bab090f783209c1e3a3f11934cf7756b09cb2f1e02147c6ed8
Status: Downloaded newer image for docker.io/hello-world:latest

View Mirror

View the pulled Docker image: docker images

[root@localhost ~]# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
docker.io/hello-world latest fce289e99eb9 5 months ago 1.84 kB

Run the image

Run the image: docker run hello-world . If you see the following content printed out, it means the operation is successful.

[root@localhost docker]# docker run hello-world

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
  (amd64)
 3. The Docker daemon creates a new container from that image which runs the
  executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
  to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

Docker Common Commands

docker images : List local images

docker pull image_name: pull the image. If no specific image tag is specified, the image with the latest tag will be automatically pulled.

docker search image_name: Search for images on Docker Hub

docker start container: start and run the container

docker run [OPTIONS] image_name: Create and start a container based on the image

Common options:
-d: Run the container in the background and return the container ID;
-i: Run the container in interactive mode, usually used with -t;
-t: reallocate a pseudo input terminal for the container, usually used together with -i;
-P: Random port mapping, the internal port of the container is randomly mapped to the high port of the host
-p: specifies port mapping in the format of: host port: container port

docker logs container: get container log information

docker attach container: enter the container

exit: exit the container

docker exec container command: execute a command in a running container

docker stop container: stop the container

docker rm container: delete a container

docker save -o image_name.tar image_name: export the image

docker ps: view running containers

docker ps -a: view the list of containers in the system

docker top container: view the processes in the container

docker stop daemon_dave: stop the daemon container

Docker build image

How to build an image?

Building your own image requires two steps:

Write a Dockerfile. Dockerfile tells Docker how to make an image and what each step looks like.

• The process by which Docker executes the instructions in the Dockerfile is as follows:
  • Docker runs a container from a base image
  • Execute an instruction to modify the container
  • Commit to a new image layer
  • Docker then runs a new container based on the image just submitted
  • Execute the next instruction in the Dockerfile until all instructions are executed.
• Build using the docker build command.

Writing a Dockerfile

The first command in every Dockerfile must be FROM. The FROM instruction specifies an existing image, telling Docker that subsequent instructions are based on this image. For example: FROM java:8

The MAINTAINER directive is used to identify the owner of the image and contact information. For example: MAINTAINER James "×××@example.com"

The VOLUME instruction is used to add volumes to a container created from an image. A volume can be a specific directory that exists in one or more containers. This directory can bypass the union file system and provide shared data and data persistence capabilities.

The CMD instruction is used to specify a command to be run when a container is started.

The ENTRYPOINT instruction is very similar to the CMD instruction.

The WORKDIR instruction is used to set a working command inside the container when creating a new container from an image. The program specified by the ENTRYPOINT or CMD instruction will be executed in this directory.

The ENV instruction is used to set environment variables during the image building process. For example: ENV TEST_PATH /home/test

The RUN instruction is used to run the specified command in the current image. For example: RUN apt-get install -y nginx

The EXPOSE instruction is used to tell Docker that the application inside the container will use the specified port of the container. For example: EXPOSE 80

The ADD instruction is used to copy files and directories from the build environment to the image. For example: ADD docker-0.0.1-SNAPSHOT.jar app.jar

The COPY instruction is similar to ADD, except that COPY is only concerned with copying local files within the build context, without extracting or decompressing them.

The LABEL instruction is used to add metadata to a Docker image. For example: LABEL name=test description="a container is used to test"

Dockerfile example:

FROM java:8
MAINTAINER James "×××@example.com"
VOLUME /tmp
ADD docker-0.0.1-SNAPSHOT.jar app.jar
ENTRYPOINT ["java","-Djava.security.egd=file:/dev/./urandom","-jar","/app.jar"]

Execute docker build

After executing the docker build command, you will see "BUILD SUCCESS" indicating that the build was successful. You can now use the docker run command to run the project.

Summarize

This article introduces what Docker is, Docker ideas, Docker core concepts, Docker installation, etc. After reading this article, you can get started with Docker, but you have only taken the first step in a long journey. There is no end to learning, so let’s encourage each other.

The above is the full content of this article. I hope it will be helpful for everyone’s study. I also hope that everyone will support 123WORDPRESS.COM.