Docker Registry服务启动过程浅析

当我们pull一个registry镜像或者自己制作一个镜像之后，使用命令docker run -d -p 5000:5000 registry，就可以启动一个私有容器服务，那么究竟是怎么做到的呢？

首先docker ps显示cmd是 "docker-registry"，但是启动的时候并没有输入。
　　root@host-10-9-27-62:/home/opuser# docker ps
　　CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
　　ffca4ca34c3f registry "docker-registry" 24 hours ago Up 22 hours 0.0.0.0:5000->5000/tcp desperate_lumiere

记得dockerfile中可以指定启动docker后执行的命令，下面看看创建registry镜像的Dockerfile
　　root@ffca4ca34c3f:/# cat docker-registry/Dockerfile
# VERSION 0.1
# DOCKER-VERSION 0.7.3
# AUTHOR: Sam Alba <sam@docker.com>
# DESCRIPTION: Image with docker-registry project and dependecies
# TO_BUILD: docker build -rm -t registry .
# TO_RUN: docker run -p 5000:5000 registry

# Latest Ubuntu LTS
FROM ubuntu:14.04

# Update
RUN apt-get update \
# Install pip
&& apt-get install -y \
swig \
python-pip \
# Install deps for backports.lzma (python2 requires it)
python-dev \
python-mysqldb \
python-rsa \
libssl-dev \
liblzma-dev \
libevent1-dev \
# Install deps for building gevent
curl \
cython \
&& rm -rf /var/lib/apt/lists/*

COPY . /docker-registry
COPY ./config/boto.cfg /etc/boto.cfg

# Install core
RUN pip install /docker-registry/depends/docker-registry-core

# Install gevent 1.0.1 using updated config.guess and config.sub
RUN curl https://pypi.python.org/packages/source/g/gevent/gevent-1.0.1.tar.gz | tar -xzf - -C / \
&& curl -o /gevent-1.0.1/libev/config.guess \
'http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD' \
&& curl -o /gevent-1.0.1/libev/config.sub \
'http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD' \
&& cp -pf /gevent-1.0.1/libev/config.guess /gevent-1.0.1/c-ares/config.guess \
&& cp -pf /gevent-1.0.1/libev/config.sub /gevent-1.0.1/c-ares/config.sub \
&& pip install /gevent-1.0.1

# Install registry
RUN pip install file:///docker-registry#egg=docker-registry[bugsnag,newrelic,cors]

RUN patch \
$(python -c 'import boto; import os; print os.path.dirname(boto.__file__)')/connection.py \
< /docker-registry/contrib/boto_header_patch.diff

ENV DOCKER_REGISTRY_CONFIG /docker-registry/config/config_sample.yml
ENV SETTINGS_FLAVOR dev

EXPOSE 5000

CMD ["docker-registry"]

可以看到最后一句CMD ["docker-registry"] ，就是这里了。

那么这个命令是怎么启动registry服务的呢？进入registry容器里
　　root@ffca4ca34c3f:/# which docker-registry
　　/usr/local/bin/docker-registry

　　root@ffca4ca34c3f:/# cat /usr/local/bin/docker-registry
#!/usr/bin/python
# EASY-INSTALL-ENTRY-SCRIPT: 'docker-registry==1.0.0-dev','console_scripts','docker-registry'
__requires__ = 'docker-registry==1.0.0-dev'
import sys
from pkg_resources import load_entry_point

if __name__ == '__main__':
sys.exit(
load_entry_point('docker-registry==1.0.0-dev', 'console_scripts', 'docker-registry')()
)
这里用到了Python的一个方法，不太清楚，可以参考文章
　　http://blog.sina.com.cn/s/blog_85998e380101bojs.html
　　http://www.cnblogs.com/babykick/archive/2012/03/09/2387808.html
继续往下看

　　root@ffca4ca34c3f:/# cat /usr/local/lib/python2.7/dist-packages/docker_registry-1.0.0_dev-py2.7.egg-info/entry_points.txt
[console_scripts]
docker-registry = docker_registry.run:run_gunicorn
到这里就大概知道什么回事了，其实是调用了run.py的run_gunicorn方法

　　root@ffca4ca34c3f:/# cat /usr/local/lib/python2.7/dist-packages/docker_registry/run.py
def run_gunicorn():
"""Exec gunicorn with our wsgi app.
......

args = [
gunicorn_path, 'gunicorn',
'--access-logfile', env.source('GUNICORN_ACCESS_LOG_FILE'),
'--error-logfile', env.source('GUNICORN_ERROR_LOG_FILE'),
'--max-requests', '100',
'-k', 'gevent',
'--graceful-timeout', env.source('GUNICORN_GRACEFUL_TIMEOUT'),
'-t', env.source('GUNICORN_SILENT_TIMEOUT'),
'-w', env.source('GUNICORN_WORKERS'),
'-b', address,
]

......

args += env.source('GUNICORN_OPTS')
args.append('docker_registry.wsgi:application')
# Stringify all args and call
os.execl(*[str(v) for v in args])

run_gunicorn()调用了gunicorn程序，在host上ps的时候可以看到，registry容器启动了5个gunicorn进程。
　　root 17466 0.0 0.9 21504 19264 ? Ss Dec09 0:00 /usr/bin/python /usr/local/bin/gunicorn --access-logfile - --error-logfil
　　root 17480 1.2 2.2 48896 45888 ? S Dec09 15:37 /usr/bin/python /usr/local/bin/gunicorn --access-logfile - --error-logfil
　　root 17481 1.2 2.2 48960 46080 ? S Dec09 15:34 /usr/bin/python /usr/local/bin/gunicorn --access-logfile - --error-logfil
　　root 17482 1.2 2.2 49088 46080 ? S Dec09 15:35 /usr/bin/python /usr/local/bin/gunicorn --access-logfile - --error-logfil
　　root 17483 1.2 2.2 49664 46720 ? S Dec09 15:34 /usr/bin/python /usr/local/bin/gunicorn --access-logfile - --error-logfil

参考资料
　　Gunicorn - Python WSGI HTTP Server for UNIX
　　www.gunicorn.org