Docker介绍

Docker concepts

Docker is a platform for developers and sysadmins to develop, deploy, and run applications with containers.

The use of Linux containers to deploy applications is called containerization.

Containers are not new, but their use for easily deploying applications is.

Containerization is increasingly popular because containers are:

• Flexible: Even the most complex applications can be containerized.
• Lightweight: Containers leverage and share the host kernel.
• Interchangeable: You can deploy updates and upgrades on-the-fly.
• Portable: You can build locally, deploy to the cloud, and run anywhere.
• Scalable: You can increase and automatically distribute container replicas.
• Stackable: You can stack services vertically and on-the-fly.

Images and containers

A container is launched by running an image.

An image is an executable package that includes everything needed to run an application--the code, a runtime, libraries, environment variables, and configuration files.

A container is a runtime instance of an image--what the image becomes in memory when executed (that is, an image with state, or a user process).

You can see a list of your running containers with the command, docker ps, just as you would in Linux.

Containers and virtual machines

A container runs natively on Linux and shares the kernel of the host machine with other containers.

It runs a discrete process, taking no more memory than any other executable, making it lightweight.

By contrast, a virtual machine (VM) runs a full-blown “guest” operating system with virtual access to host resources through a hypervisor.

In general, VMs provide an environment with more resources than most applications need.

Prepare your Docker environment

Install

a maintained version of Docker Community Edition (CE) 

or

Enterprise Edition (EE) on a supported platform.

For full Kubernetes Integration

• Kubernetes on Docker for Mac is available in 17.12 Edge (mac45) or 17.12 Stable (mac46) and higher.
• Kubernetes on Docker for Windows is available in 18.02 Edge (win50) and higher edge channels only.

Test Docker version

1. Run docker --version and ensure that you have a supported version of Docker:

   docker --version
   
   Docker version 17.12.0-ce, build c97c6d6
   

2. Run docker info or (docker version without --) to view even more details about your docker installation:

   docker info
   
   Containers: 0
    Running: 0
    Paused: 0
    Stopped: 0
   Images: 0
   Server Version: 17.12.0-ce
   Storage Driver: overlay2
   ...
   

To avoid permission errors (and the use of sudo), add your user to the docker group. Read more.

Test Docker installation

1. Test that your installation works by running the simple Docker image, hello-world:

   docker run hello-world
   
   Unable to find image 'hello-world:latest' locally
   latest: Pulling from library/hello-world
   ca4f61b1923c: Pull complete
   Digest: sha256:ca0eeb6fb05351dfc8759c20733c91def84cb8007aa89a5bf606bc8b315b9fc7
   Status: Downloaded newer image for hello-world:latest
   
   Hello from Docker!
   This message shows that your installation appears to be working correctly.
   ...
   

2. List the hello-world image that was downloaded to your machine:

   docker image ls
   

3. List the hello-world container (spawned by the image) which exits after displaying its message. If it were still running, you would not need the --all option:

   docker container ls --all
   
   CONTAINER ID     IMAGE           COMMAND      CREATED            STATUS
   54f4984ed6a8     hello-world     "/hello"     20 seconds ago     Exited (0) 19 seconds ago
   
   
   

   Recap and cheat sheet

   ## List Docker CLI commands
   docker
   docker container --help
   
   ## Display Docker version and info
   docker --version
   docker version
   docker info
   
   ## Execute Docker image
   docker run hello-world
   
   ## List Docker images
   docker image ls
   
   ## List Docker containers (running, all, all in quiet mode)
   docker container ls
   docker container ls --all
   docker container ls -aq
   
   
   

   Conclusion of part one

   Containerization makes CI/CD seamless. For example:

   • applications have no system dependencies
   • updates can be pushed to any part of a distributed application
   • resource density can be optimized.

   With Docker, scaling your application is a matter of spinning up new executables, not running heavy VM hosts.