背景

随着微服务概念的深入人心，随着docker开发的持续进行，我们在生产的过程中将会产生大量的docker镜像，这些镜像会随着版本迭代的过程中，这些镜像将会占用大量的存储空间，本文将分析影响镜像大小的因素，随后提供镜像瘦身的思路。

Dockerfile、Docker 镜像和 Docker 容器的关系

不可避免地，我们在docker学习的过程中一定绕不开理解这三者的关系，从研发流程的角度来看来看，Dockerfile 是软件的原材料，Docker 镜像是软件的交付品，而 Docker 容器则可以认为是软件的运行的状态。从应用软件的角度来看，Dockerfile、Docker 镜像与 Docker 容器分别代表软件的三个不同阶段，Dockerfile 面向开发，Docker 镜像成为交付标准，Docker 容器则涉及部署与运维，三者缺一不可，合力充当 Docker 体系的基石。
简单来讲，Dockerfile构建出Docker镜像，通过Docker镜像运行Docker容器。后续有机会再详细分析这三者的关系

精简镜像

以下是我们精简镜像的目的：

• 更快的构建速度
• 更小的Docker镜像大小
• 更少的Docker镜像层
• 充分利用镜像缓存
• 增加Dockerfile可读性
• 让Docker容器使用起来更简单

原理

简单来讲就是：最基础的镜像+合并操作指令（减少操作指令）

瘦身行动

我们即将通过一系列的dockerfile开发，对镜像打包进行一次次的试验，以尽最大努力的减小镜像的大小
原始文件
这是一份非常糟糕的Dockerfile文件打包之后，仅作为分析用例

FROM python:3.5.6-slim-stretch
RUN apt-get update
RUN echo python -V 
RUN mkdir /code 
RUN mkdir /code/db
ADD . /code/
WORKDIR /code
RUN pip install -r requirement

运行命令：docker build -t spidermax:test_01 .
将会对镜像进行打包，docker相关程序会读取该文件（Dockerfile），以下是镜像层的部署过程

Sending build context to Docker daemon 289.8kB
Step 1/9 : FROM python:3.5.6-slim-stretch
---> 86669b8e5771
Step 2/9 : RUN apt-get update
---> Using cache
---> 1b77542b82f4
Step 3/9 : RUN echo python -V
---> Using cache
---> de6cc0a916d7
Step 4/9 : RUN mkdir /code
---> Using cache
---> 996828abe04e
Step 5/9 : RUN mkdir /code/db
---> Using cache
---> 2db7c4dfb918
Step 6/9 : ADD . /code/
---> d9d92db45b29
Step 7/9 : WORKDIR /code
---> Running in 5ae405de28fc
Removing intermediate container 5ae405de28fc
---> a7e3696c6a5b
Step 8/9 : RUN pip install -U pip
---> Running in dcf5b22f6f33
Collecting pip
Downloading https://files.pythonhosted.org/packages/d8/f3/413bab4ff08e1fc4828dfc59996d721917df8e8583ea85385d51125dceff/pip-19.0.3-py2.py3-none-any.whl (1.4MB)
Installing collected packages: pip
Found existing installation: pip 18.1
Uninstalling pip-18.1:
Successfully uninstalled pip-18.1
Successfully installed pip-19.0.3
Removing intermediate container dcf5b22f6f33
---> 949d89aabe2b
Step 9/9 : RUN pip install -r requirement
---> Running in 697c65e4161a
Collecting Django==2.1.7 (from -r requirement (line 1))
Downloading https://files.pythonhosted.org/packages/c7/87/fbd666c4f87591ae25b7bb374298e8629816e87193c4099d3608ef11fab9/Django-2.1.7-py3-none-any.whl (7.3MB)
Collecting EasyProcess==0.2.5 (from -r requirement (line 2))
Downloading https://files.pythonhosted.org/packages/45/3a/4eecc0c7995a13a64739bbedc0d3691fc574245b7e79cff81905aa0c2b38/EasyProcess-0.2.5.tar.gz
Collecting PyMySQL==0.9.3 (from -r requirement (line 3))
Downloading https://files.pythonhosted.org/packages/ed/39/15045ae46f2a123019aa968dfcba0396c161c20f855f11dea6796bcaae95/PyMySQL-0.9.3-py2.py3-none-any.whl (47kB)
Collecting pytz==2018.9 (from -r requirement (line 4))
Downloading https://files.pythonhosted.org/packages/61/28/1d3920e4d1d50b19bc5d24398a7cd85cc7b9a75a490570d5a30c57622d34/pytz-2018.9-py2.py3-none-any.whl (510kB)
Collecting selenium==3.141.0 (from -r requirement (line 5))
Downloading https://files.pythonhosted.org/packages/80/d6/4294f0b4bce4de0abf13e17190289f9d0613b0a44e5dd6a7f5ca98459853/selenium-3.141.0-py2.py3-none-any.whl (904kB)
Collecting urllib3==1.24.1 (from -r requirement (line 6))
Downloading https://files.pythonhosted.org/packages/62/00/ee1d7de624db8ba7090d1226aebefab96a2c71cd5cfa7629d6ad3f61b79e/urllib3-1.24.1-py2.py3-none-any.whl (118kB)
Building wheels for collected packages: EasyProcess
Building wheel for EasyProcess (setup.py): started
Building wheel for EasyProcess (setup.py): finished with status 'done'
Stored in directory: /root/.cache/pip/wheels/41/22/19/af15ef6264c58b625a82641ed7483ad05e258fbd8925505227
Successfully built EasyProcess
Installing collected packages: pytz, Django, EasyProcess, PyMySQL, urllib3, selenium
Successfully installed Django-2.1.7 EasyProcess-0.2.5 PyMySQL-0.9.3 pytz-2018.9 selenium-3.141.0 urllib3-1.24.1
Removing intermediate container 697c65e4161a
---> f59692faac28
Successfully built f59692faac28
Successfully tagged spidermax:test_01

通过docker images查看镜像大小

通过docker history我们来观察镜像层打包的过程

这里有若干个missing层，其实是docker打包程序程序自动优化的过程。Docker为了加快镜像构建速度，也会将每一个镜像层缓存下来。利用这个特性，可以提升工作效率，不过能用到这个特性却是有点尴尬：

• 上一个指令能够在上一层缓存中找到
• 缓存中存在一个镜像层，而且指令和即将打包的指令一模一样，即使有空格也不行！
• 如果有类似COPY或者ADD指令，如果他们引用的文件的元数据发生变化，缓存也无效

优化1

接下来我们对相同属性指令进行合并，比如RUN命令等

FROM python:3.5.6-slim-stretch
RUN apt-get update &&\
    echo python -V &&\
    mkdir /code &&\
    mkdir /code/db
ADD . /code/
WORKDIR /code
RUN pip install -U pip &&\
    pip install -r requirement

以下是编译日志

docker build -t spidermax:test_02 .
Sending build context to Docker daemon 289.8kB
Step 1/5 : FROM python:3.5.6-slim-stretch
---> 86669b8e5771
Step 2/5 : RUN apt-get update && echo python -V && mkdir /code && mkdir /code/db
---> Running in 2a879b85221f
Get:1 http://security.debian.org/debian-security stretch/updates InRelease [94.3 kB]
Get:2 http://security.debian.org/debian-security stretch/updates/main amd64 Packages [481 kB]
Ign:3 http://deb.debian.org/debian stretch InRelease
Get:4 http://deb.debian.org/debian stretch-updates InRelease [91.0 kB]
Get:5 http://deb.debian.org/debian stretch Release [118 kB]
Get:6 http://deb.debian.org/debian stretch-updates/main amd64 Packages [11.1 kB]
Get:7 http://deb.debian.org/debian stretch Release.gpg [2434 B]
Get:8 http://deb.debian.org/debian stretch/main amd64 Packages [7084 kB]
Fetched 7881 kB in 8s (966 kB/s)
Reading package lists...
python -V
Removing intermediate container 2a879b85221f
---> 5ce22d4611e9
Step 3/5 : ADD . /code/
---> af87134918ca
Step 4/5 : WORKDIR /code
---> Running in 2a5c72670be3
Removing intermediate container 2a5c72670be3
---> 0324df92356d
Step 5/5 : RUN pip install -U pip && pip install -r requirement
---> Running in 439cc1586076
Collecting pip
Downloading https://files.pythonhosted.org/packages/d8/f3/413bab4ff08e1fc4828dfc59996d721917df8e8583ea85385d51125dceff/pip-19.0.3-py2.py3-none-any.whl (1.4MB)
Installing collected packages: pip
Found existing installation: pip 18.1
Uninstalling pip-18.1:
Successfully uninstalled pip-18.1
Successfully installed pip-19.0.3
Collecting Django==2.1.7 (from -r requirement (line 1))
Downloading https://files.pythonhosted.org/packages/c7/87/fbd666c4f87591ae25b7bb374298e8629816e87193c4099d3608ef11fab9/Django-2.1.7-py3-none-any.whl (7.3MB)
Collecting EasyProcess==0.2.5 (from -r requirement (line 2))
Downloading https://files.pythonhosted.org/packages/45/3a/4eecc0c7995a13a64739bbedc0d3691fc574245b7e79cff81905aa0c2b38/EasyProcess-0.2.5.tar.gz
Collecting PyMySQL==0.9.3 (from -r requirement (line 3))
Downloading https://files.pythonhosted.org/packages/ed/39/15045ae46f2a123019aa968dfcba0396c161c20f855f11dea6796bcaae95/PyMySQL-0.9.3-py2.py3-none-any.whl (47kB)
Collecting pytz==2018.9 (from -r requirement (line 4))
Downloading https://files.pythonhosted.org/packages/61/28/1d3920e4d1d50b19bc5d24398a7cd85cc7b9a75a490570d5a30c57622d34/pytz-2018.9-py2.py3-none-any.whl (510kB)
Collecting selenium==3.141.0 (from -r requirement (line 5))
Downloading https://files.pythonhosted.org/packages/80/d6/4294f0b4bce4de0abf13e17190289f9d0613b0a44e5dd6a7f5ca98459853/selenium-3.141.0-py2.py3-none-any.whl (904kB)
Collecting urllib3==1.24.1 (from -r requirement (line 6))
Downloading https://files.pythonhosted.org/packages/62/00/ee1d7de624db8ba7090d1226aebefab96a2c71cd5cfa7629d6ad3f61b79e/urllib3-1.24.1-py2.py3-none-any.whl (118kB)
Building wheels for collected packages: EasyProcess
Building wheel for EasyProcess (setup.py): started
Building wheel for EasyProcess (setup.py): finished with status 'done'
Stored in directory: /root/.cache/pip/wheels/41/22/19/af15ef6264c58b625a82641ed7483ad05e258fbd8925505227
Successfully built EasyProcess
Installing collected packages: pytz, Django, EasyProcess, PyMySQL, urllib3, selenium
Successfully installed Django-2.1.7 EasyProcess-0.2.5 PyMySQL-0.9.3 pytz-2018.9 selenium-3.141.0 urllib3-1.24.1
Removing intermediate container 439cc1586076
---> d767a8a2d9f2
Successfully built d767a8a2d9f2
Successfully tagged spidermax:test_02

虽然在大小和原来没有啥区别

但是在镜像打包上，步骤上少了很多，这样在一定程度上就缩短镜像打包的时间，这一点从刚才的打包日志也有所体现，因为test01版本是需要9步，优化1版本是仅需要5步，打包效率提升将近1倍

优化2

我们所使用的基础镜像是python:3.5.6-slim-stretch，该基础镜像导致我们每次打包都需要在其之上进行操作，这样也导致我们的奖项将会变得非常的大，毕竟基础就已经达到135M之大，虽然说我们不应该重复造轮子，但是这样的轮子我们要不起啊

我们不妨重新反思一下我们所使用的镜像功能，无非就是python的编译器、pip以及apt功能，至于说另外的django等文件，是我们通过requirement来安装的，编译器自带的若干依赖我们没怎么用到（这么描述，不是很严谨），那我们为什么不干脆自己部署一个python环境，只要有apt，还有什么事情没有不能做的？
这里推荐使用apline这款镜像，简直是恶魔！竟然只有4M左右

Alpine 的意思是“高山的”，比如 Alpine plants高山植物，Alpine skiing高山滑雪、the alpine resort阿尔卑斯山胜地。
Alpine Linux 网站首页注明“Small！Simple！Secure！Alpine Linux is a security-oriented, lightweight Linux distribution based on musl libc and busybox.”概括了以下特点：

1. 小巧：基于Musl libc和busybox，和busybox一样小巧，最小的Docker镜像只有4MB；
2. 安全：面向安全的轻量发行版；
3. 简单：提供APK包管理工具，软件的搜索、安装、删除、升级都非常方便。
4. 适合容器使用：由于小巧、功能完备，非常适合作为容器的基础镜像。
   
   test03版本的dockerfile如下

   FROM alpine:3.7
   RUN apk update && apk upgrade &&\
   apk add python3 &&\
   apk add python3-dev &&\
   python3 -m ensurepip &&\
   if [ ! -e /usr/bin/pip ]; then ln -s pip3 /usr/bin/pip ; fi &&\
   if [[ ! -e /usr/bin/python ]]; then ln -sf /usr/bin/python3 /usr/bin/python; fi &&\
   mkdir /code &&  mkdir /code/db
   ADD . /code/
   WORKDIR /code
   RUN pip install -U pip && pip install -r requirement

   因为安装python3的时候默认会是pip3，为了安装方便我们需要将pip3修改为pip，此外还有将python3修改为python，方便我们对镜像的启动，以下是运行的日志
   docker build -t spidermax:test_03 .
   Sending build context to Docker daemon 290.3kB
   Step 1/5 : FROM alpine:3.7
   ---> 6d1ef012b567
   Step 2/5 : RUN apk update && apk upgrade && apk add python3 && apk add python3-dev && python3 -m ensurepip && if [ ! -e /usr/bin/pip ]; then ln -s pip3 /usr/bin/pip ; fi && if [[ ! -e /usr/bin/python ]]; then ln -sf /usr/bin/python3 /usr/bin/python; fi && mkdir /code && mkdir /code/db
   ---> Running in ef033831c1a9
   fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/main/x86_64/APKINDEX.tar.gz
   fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/community/x86_64/APKINDEX.tar.gz
   v3.7.3-6-g3eacd5f9a6 [http://dl-cdn.alpinelinux.org/alpine/v3.7/main]
   v3.7.3-4-g7296a289a6 [http://dl-cdn.alpinelinux.org/alpine/v3.7/community]
   OK: 9049 distinct packages available
   OK: 4 MiB in 13 packages
   (1/11) Installing libbz2 (1.0.6-r6)
   (2/11) Installing expat (2.2.5-r0)
   (3/11) Installing libffi (3.2.1-r4)
   (4/11) Installing gdbm (1.13-r1)
   因为安装python3的时候默认会是pip3，为了安装方便我们需要将pip3修改为pip，此外还有将python3修改为python，方便我们对镜像的启动，以下是运行的日志

   docker build -t spidermax:test_03 .
   Sending build context to Docker daemon 290.3kB
   Step 1/5 : FROM alpine:3.7
   ---> 6d1ef012b567
   Step 2/5 : RUN apk update && apk upgrade && apk add python3 && apk add python3-dev && python3 -m ensurepip && if [ ! -e /usr/bin/pip ]; then ln -s pip3 /usr/bin/pip ; fi && if [[ ! -e /usr/bin/python ]]; then ln -sf /usr/bin/python3 /usr/bin/python; fi && mkdir /code && mkdir /code/db
   ---> Running in ef033831c1a9
   fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/main/x86_64/APKINDEX.tar.gz
   fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/community/x86_64/APKINDEX.tar.gz
   v3.7.3-6-g3eacd5f9a6 [http://dl-cdn.alpinelinux.org/alpine/v3.7/main]
   v3.7.3-4-g7296a289a6 http://dl-cdn.alpinelinux.org/alpine/v3.7/community Installing xz-libs (5.2.3-r1)
   (6/11) Installing ncurses-terminfo-base (6.0_p20171125-r1)
   (7/11) Installing ncurses-terminfo (6.0_p20171125-r1)
   (8/11) Installing ncurses-libs (6.0_p20171125-r1)
   (9/11) Installing readline (7.0.003-r0)
   (10/11) Installing sqlite-libs (3.25.3-r0)
   (11/11) Installing python3 (3.6.5-r0)
   Executing busybox-1.27.2-r11.trigger
   OK: 66 MiB in 24 packages
   (1/2) Installing pkgconf (1.3.10-r0)
   (2/2) Installing python3-dev (3.6.5-r0)
   Executing busybox-1.27.2-r11.trigger
   OK: 79 MiB in 26 packages
   Requirement already satisfied: setuptools in /usr/lib/python3.6/site-packages
   Requirement already satisfied: pip in /usr/lib/python3.6/site-packages
   Removing intermediate container ef033831c1a9
   ---> 4250724684eb
   Step 3/5 : ADD . /code/
   ---> 8f1a939f8ac1
   Step 4/5 : WORKDIR /code
   ---> Running in 137e7b42decb
   Removing intermediate container 137e7b42decb
   ---> 29586daada18
   Step 5/5 : RUN pip install -U pip && pip install -r requirement
   ---> Running in 36baec208714
   Collecting pip
   Downloading https://files.pythonhosted.org/packages/d8/f3/413bab4ff08e1fc4828dfc59996d721917df8e8583ea85385d51125dceff/pip-19.0.3-py2.py3-none-any.whl (1.4MB)
   Installing collected packages: pip
   Found existing installation: pip 9.0.3
   Uninstalling pip-9.0.3:
   Successfully uninstalled pip-9.0.3
   Successfully installed pip-19.0.3
   Collecting Django==2.1.7 (from -r requirement (line 1))
   Downloading https://files.pythonhosted.org/packages/c7/87/fbd666c4f87591ae25b7bb374298e8629816e87193c4099d3608ef11fab9/Django-2.1.7-py3-none-any.whl (7.3MB)
   Collecting EasyProcess==0.2.5 (from -r requirement (line 2))
   Downloading https://files.pythonhosted.org/packages/45/3a/4eecc0c7995a13a64739bbedc0d3691fc574245b7e79cff81905aa0c2b38/EasyProcess-0.2.5.tar.gz
   Collecting PyMySQL==0.9.3 (from -r requirement (line 3))
   Downloading https://files.pythonhosted.org/packages/ed/39/15045ae46f2a123019aa968dfcba0396c161c20f855f11dea6796bcaae95/PyMySQL-0.9.3-py2.py3-none-any.whl (47kB)
   Collecting pytz==2018.9 (from -r requirement (line 4))
   Downloading https://files.pythonhosted.org/packages/61/28/1d3920e4d1d50b19bc5d24398a7cd85cc7b9a75a490570d5a30c57622d34/pytz-2018.9-py2.py3-none-any.whl (510kB)
   Collecting selenium==3.141.0 (from -r requirement (line 5))
   Downloading https://files.pythonhosted.org/packages/80/d6/4294f0b4bce4de0abf13e17190289f9d0613b0a44e5dd6a7f5ca98459853/selenium-3.141.0-py2.py3-none-any.whl (904kB)
   Collecting urllib3==1.24.1 (from -r requirement (line 6))
   Downloading https://files.pythonhosted.org/packages/62/00/ee1d7de624db8ba7090d1226aebefab96a2c71cd5cfa7629d6ad3f61b79e/urllib3-1.24.1-py2.py3-none-any.whl (118kB)
   Installing collected packages: pytz, Django, EasyProcess, PyMySQL, urllib3, selenium
   Running setup.py install for EasyProcess: started
   Running setup.py install for EasyProcess: finished with status 'done'
   Successfully installed Django-2.1.7 EasyProcess-0.2.5 PyMySQL-0.9.3 pytz-2018.9 selenium-3.141.0 urllib3-1.24.1
   Removing intermediate container 36baec208714
   ---> e59b7105697a
   Successfully built e59b7105697a
   Successfully tagged spidermax:test_03

检查镜像大小，会发现这个镜像和原来相比我们将其缩小了近一倍

那这样已经达到目的了吗？不还不够！作为工程师，优化永不止步！

优化3

每一次的镜像打包，docker都会去云端请求，下载相关的软件，并安装，那安装之后的安装包会怎么处理呢？——还算存留在镜像中，也就是所谓的缓存，这些是相对应用层来说的，他们只会占用我们存储空间，并没有过多的用处，因此需要的就是将这个缓存的内容删除，实现起来也很简单的带上--no-cache即可，如图见apk操作详情

修改之后的脚本为

FROM alpine:3.7
RUN apk update --no-cache && apk upgrade --update-cache --available &&\
    apk add --no-cache python3 &&\
    apk add --no-cache python3-dev &&\
    python3 -m ensurepip &&\
    if [ ! -e /usr/bin/pip ]; then ln -s pip3 /usr/bin/pip ; fi &&\
    if [[ ! -e /usr/bin/python ]]; then ln -sf /usr/bin/python3 /usr/bin/python; fi &&\
    rm -rf /var/lib/apk/* &&\
    mkdir /code &&  mkdir /code/db
ADD . /code/
WORKDIR /code
RUN pip install -U pip && pip install -r requirement

这样的话，会导致一个问题，就是每次打包都需要一次又一次的重新下载资源，这个其实也是一个效率上的矛盾，一个互相博弈的过程，镜像大小变化

当然了，我们可删除apk的缓存当然可以删除pip的缓存啦，我们可以通过直接删除pip的安装包的形式来减小文件的大小，代码如下：

FROM alpine:3.7
RUN apk update && apk upgrade --update-cache --available &&\
    apk add python3 &&\
    apk add python3-dev &&\
    python3 -m ensurepip &&\
    rm -r /usr/lib/python*/ensurepip &&\
    if [ ! -e /usr/bin/pip ]; then ln -s pip3 /usr/bin/pip ; fi &&\
    if [[ ! -e /usr/bin/python ]]; then ln -sf /usr/bin/python3 /usr/bin/python; fi &&\
    rm -rf /var/lib/apk/* &&\
    mkdir /code && mkdir /code/db
ADD . /code/
WORKDIR /code
RUN pip install -U pip && pip install -r requirement && rm -rf ~/.cache/pip

镜像的变化如下：

通过我们的一次次优化之后，我们将一个高达203M的镜像进行了优化，缩小了将近一倍，在这一次次的编译过程中，也体会到了打包速度一次比一次的提高，这样才是优化思想的应用，一次又一次的超越自己

修改之后

最后给这个dockerfile标记维护者的信息，表示说这个镜像是我提供的，如果有问题了需要帮忙解决可以联系我，也表示我对这个镜像拥有著作权，虽然我已经将这个镜像开源了

FROM alpine:3.7
MAINTAINER yerikyu "yerik_shu@139.com"
RUN apk update && apk upgrade --update-cache --available &&\
    apk add python3 &&\
    apk add python3-dev &&\
    python3 -m ensurepip &&\
    rm -r /usr/lib/python*/ensurepip &&\
    if [ ! -e /usr/bin/pip ]; then ln -s pip3 /usr/bin/pip ; fi &&\
    if [[ ! -e /usr/bin/python ]]; then ln -sf /usr/bin/python3 /usr/bin/python; fi &&\
    rm -rf /var/lib/apk/* &&\
    mkdir /code && mkdir /code/db
ADD . /code/
WORKDIR /code
RUN pip install -U pip && pip install -r requirement && rm -rf ~/.cache/pip

打包结果如下：

总结

• 编写.dockerignore文件（这个用来忽略某些不需要的缓存文件类似gitignore)
• 容器只运行单个应用(微服务的一个体现）
• 将多个RUN指令合并为一个
• 基础镜像的标签不要用latest
• 每个RUN指令后删除多余文件
• 选择合适的基础镜像(alpine版本最好)
• 设置WORKDIR和CMD
• 使用ENTRYPOINT (可选)
• 在entrypoint脚本中使用exec（本次实验未体现）
• COPY与ADD优先使用前者
• 合理调整COPY与RUN的顺序
• 设置默认的环境变量，映射端口和数据卷（这个我是在docker-compose进行设置的，后面有机会再讨论）

  参考资料

  docker镜像层：http://blog.daocloud.io/principle-of-docker-image/
  alpine官方资料：https://alpinelinux.org/