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Introduction to docker

Docker is an open source application container engine that allows developers to package their applications and dependency packages into a portable container, publish them to any popular Linux machine, and virtualize them. Containers are fully sandboxed and will not have any interface with each other.

We choose the relatively basic docker at the beginning, rather than K8S, mainly in order to learn the basic knowledge as much as possible, and then learn K8S will be more handy.

Docker is divided into two versions, CE and EE:

Community Edition (CE) Community Edition for short

Enterprise Edition (EE) check Enterprise Edition

Needless to say, we definitely use the community version, and we will use the CE version for the following articles.

Docker architecture

Docker engine is the CS architecture of a scene, with CLI in the outer layer, API layer in the middle, and core components in the inside, as shown in the following figure:

For the architecture and actual use of docker, please refer to the following figure:

Several important concepts of docker:

1.Registry

The Docker repository is used to hold the image, which can be understood as a code repository in code control.

Docker Hub (https://hub.docker.com) provides a large collection of images for use.

2.Image

The Docker image is a template for creating a Docker container.

3.Container

A container is an application or group of applications that run independently.

4.Host

A physical or virtual machine is used to execute Docker daemons and containers.

5.Daemon

Dockerd, the background service process of docker.

6.Client

The Docker client uses Docker API to communicate with the Docker daemon through the command line or other tools.

Docker installation

Let's take Ubuntu 18.04 as an example. Installing docker is a relatively simple command as follows:

Simple use of docker

For the time being, we are not going to explain how to make a mirror image. We will first focus on use, and then learn how to make our own image when we are familiar with it. The idea of the container is very simple: Build   once,Run   anywhere. As you can see from the architecture diagram above, the simplest core process is build > pull > run,build to create an image, pull is to obtain a specified image from the image hub, and the last step is to run to run the image.

Since build is a little complicated to make an image, let's skip it first, start with pull and run, and directly use what someone else on https://hub.docker.com/ has done.

Understand the run mechanism

Literally, run just runs a mirror instance. In fact, it can be divided into two steps internally. The first step is to load the image into the container and create the instance (we can call it create), and the second step is to run the container instance loaded with the image (we can call it start).

Let's do an experiment first, and we'll run docker run hello-world twice.

With this output, we can see that every time we run, a new container instance will be recreated. If we want to stop or open the container instance, we should use stop and start. If we delete the instance, we can use rm. See the following demonstration:

You can see that there is only one container instance left, and we can test the remaining containers with stop and start. It is very important to understand this run mechanism, which is also the pit that the author stepped on at the beginning of contact and learning. I always think that run means to run, and many times run runs the same instance, so it is very basic and critical to understand the mechanism of run first. That's all for today, and in the next issue we will continue to learn more about the use of docker.

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