Part 2: Containers

We start at the bottom of the hierarchy of such an app, which is a container, which we cover on this page.

新的开发环境 `Container`

In the past, if you were to start writing a Python app, your first order of business was to install a Python runtime onto your machine. But, that creates a situation where the environment on your machine needs to be perfect for your app to run as expected, and also needs to match your production environment.

With Docker, you can just grab a portable Python runtime as an image, no installation necessary. Then, your build can include the base Python image right alongside your app code, ensuring that your app, its dependencies, and the runtime, all travel together.

These portable images are defined by something called a Dockerfile.

用 `Dockerfile` 定义 `Container`

文件 Dockerfile 定义了文件进入 container 的规则。令 container 像向网络接口请求数据一样向 host 机器请求数据，它与 host 其他环境完全分离，所以你需要定义一个端口映射到 host 的外界并且定义可以进入环境的文件。这些内容都可以写在 Dockerfile 这个文件中。

`Dockerfile`

一个基本的 Dockerfile 示例如下：

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# Use an official Python runtime as a parent image
FROM python:2.7-slim

# Set the working directory to /app
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Install any needed packages specified in requirements.txt
RUN pip install --trusted-host pypi.python.org -r requirements.txt

# Make port 80 available to the world outside this container
EXPOSE 80

# Define environment variable
ENV NAME World

# Run app.py when the container launches
CMD ["python", "app.py"]

该 Dockerfile 中指明了 app.py 和 requirement.txt 两个文件未创建。

这两个文件只需要创建在与 Dockerfile 同级目录下，上面的 COPY 命令会将它们复制到镜像中，而其中的 EXPOSE 则使我们的 app.py 可以通过 HTTP 协议访问到。

接下来我们创建它们。

`app.py`

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from flask import Flask
from redis import Redis, RedisError
import os
import socket

# Connect to Redis
redis = Redis(host="redis", db=0, socket_connect_timeout=2, socket_timeout=2)

app = Flask(__name__)

@app.route("/")
def hello():
    try:
        visits = redis.incr("counter")
    except RedisError:
        visits = "<i>cannot connect to Redis, counter disabled</i>"

    html = "<h3>Hello {name}!</h3>" \
           "<b>Hostname:</b> {hostname}<br/>" \
           "<b>Visits:</b> {visits}"
    return html.format(name=os.getenv("NAME", "world"), hostname=socket.gethostname(), visits=visits)

if __name__ == "__main__":
    app.run(host='0.0.0.0', port=80)

`requirement.txt`

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Flask
Redis

运行以下命令测试环境是否正确：

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$ pip install -r requirement.txt
...

$ python app.py
 * Serving Flask app "app" (lazy loading)
 * Environment: production
   WARNING: Do not use the development server in a production environment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://0.0.0.0:80/ (Press CTRL+C to quit)

浏览器中访问 localhost，便可以看到 soket.gethostname() 的结果。

搭建 `app`

在项目根目录下运行以下命令 build 这个项目（其中 -t 选项是为了给镜像起名字，注意最后的 . 指明是当前文件夹）：

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$ docker build -t test_image .

可以通过以下的命令查看注册在 Docker 后台的镜像：

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$ docker image ls

Troubleshooting for Linux users

代理服务器可能会禁止你的 web 应用连接。如果你使用了代理服务器，添加以下内容到 Dockerfile 中，其中 ENV 后指明代理服务器的主机名和端口号：
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# Set proxy server, replace host:port with values for your servers ENV http_proxy host:port ENV https_proxy host:port
DNS 配置错误会导致 pip 安装命令失败。你可能需要更改 Docker 守护进程的 DNS 设置。使用 dns key 编辑（或创建）文件 /etc/docker/daemon.json ：
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{ "dns": ["your_dns_address", "8.8.8.8"] }
保存之后重启 docker 服务即可：
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$ sudo service docker restart

启动 `app`

通过以下命令启动服务（其中，4000:80 意为将机器的 4000 端口映射到容器内部的 80 端口）：

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$ docker run -p 4000:80 test_image

在浏览器中访问 localhost:4000 可以看到 docker 的运行结果。curl 命令也是同样的效果。

CTRL+C 停止运行程序，在 windows 系统上或者 Linux 后台运行时可以使用以下命令先获取 CONTAINER ID 后停止运行程序：

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$ docker container ls
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS                  NAMES
6ff52e6cfdbc        test_image          "python app.py"     10 seconds ago      Up 8 seconds        0.0.0.0:4000->80/tcp   confident_hopper

$ docker container stop 6ff52e6cfdbc

通过 detached mode，可以令 app 在后台运行，比如使用以下的命令：

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$ docker run -d -p 4000:80 test_image

镜像的可移植性

首先，当你想要将容器部署到生产环境时，你需要知道如何将容器推送到注册表 (registry)。

A registry is a collection of repositories, and a repository is a collection of images—sort of like a GitHub repository, except the code is already built. An account on a registry can create many repositories. The docker CLI uses Docker’s public registry by default.

Log in with your Docker ID

在 Docker 官网 https://hub.docker.com 上注册账户，并用以下的命令在本地机器上登陆账户：

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$ docker login

Tag the image

将本地的一个镜像与注册表上的仓库关联起来的方式是 username/repository:tag。tag 是可选的，但是推荐使用，因为它是注册表给每个镜像一个版本名称的机制。

Now, put it all together to tag the image. Run docker tag image with your username, repository, and tag names so that the image uploads to your desired destination. The syntax of the command is:

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$ docker tag <image> <username>/<repository>:<tag>

Run docker image ls to see your newly tagged image.

Publish the image

以下的命令可以用于上传仓库：

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$ docker push <username>/<repository>:<tag>

Once complete, the results of this upload are publicly available. If you log in to Docker Hub, you see the new image there, with its pull command.

Pull and run the image from the remote repository

现在，你可以通过以下的命令在任何一个机器上运行你的 app：

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$ docker run -p 4000:80  <username>/<repository>:<tag>

如果 image 在本机并不可用，Docker 会自动从远端拉取镜像。

命令总结

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docker build -t friendlyhello .  # Create image using this directory's Dockerfile
docker run -p 4000:80 friendlyhello  # Run "friendlyhello" mapping port 4000 to 80
docker run -d -p 4000:80 friendlyhello         # Same thing, but in detached mode
docker container ls                                # List all running containers
docker container ls -a             # List all containers, even those not running
docker container stop <hash>           # Gracefully stop the specified container
docker container kill <hash>         # Force shutdown of the specified container
docker container rm <hash>        # Remove specified container from this machine
docker container rm $(docker container ls -a -q)         # Remove all containers
docker image ls -a                             # List all images on this machine
docker image rm <image id>            # Remove specified image from this machine
docker image rm $(docker image ls -a -q)   # Remove all images from this machine
docker login             # Log in this CLI session using your Docker credentials
docker tag <image> username/repository:tag  # Tag <image> for upload to registry
docker push username/repository:tag            # Upload tagged image to registry
docker run username/repository:tag                   # Run image from a registry

Part 2: Containers

新的开发环境 Container

用 Dockerfile 定义 Container

Dockerfile

app.py

requirement.txt

搭建 app