通常，一个完整的应用都是由多个容器组合而成，我们可以手动的去单独启动和管理每个容器，但是这样会很不方便。Docker Compose是这样的一个应用，它可以方便的帮助我们构建、运行和扩展一套由多个容器组成的应用。

1. 安装Docker Compose

参考Docker的github下载程序：

curl -L https://github.com/docker/compose/releases/download/1.23.0/docker-compose-`uname -s`-`uname -m` -o /usr/local/bin/docker-compose
chmod +x /usr/local/bin/docker-compose

参考Install command completion添加docker-compose命令自动补全功能：

# 先安装CentOS自己的bash-completion
[root@node2 ~]# yum -y install bash-completion
# 添加Docker Compose的command completion
[root@node2 ~]# curl -L https://raw.githubusercontent.com/docker/compose/1.23.0/contrib/completion/bash/docker-compose -o /etc/bash_completion.d/docker-compose
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 13238  100 13238    0     0   9156      0  0:00:01  0:00:01 --:--:--  9161
[root@node2 ~]# source /etc/bash_completion.d/docker-compose

2. 使用docker-compose

Docker Compose通过读取docker-compose.yml配置文件，来启动相关的服务。一个docker-compose.yml配置示例如下：

version: "3.5"
services:
  web:
    image: fundamentalsofdocker/ch08-web:1.0
    ports:
      - 3000:3000
  db:
    image: fundamentalsofdocker/ch08-db:1.0
    volumes:
      - pets-data:/var/lib/postgresql/data

volumes:
  pets-data:

上面的配置解释如下：

• version：不同的docker compose有不同的格式，这里我们使用3.5版本格式。
• services：服务，通常由多个容器组合而成。
  • web：容器的名称。同时还指定该容器使用哪个镜像(该镜像需要存在于仓库中，这里是docker hub)，开放的端口。
  • db：数据库容器的名称。同时也指定了相应的镜像，以及挂载的数据卷与容器对应目录的关系。
• volumes：创建的数据卷。

Tips：容器之间可以通过上面定义的容器名，来进行数据通信交互。

进入到该docker-compose.yml所在的目录，这里是compose-test目录，通过docker-compose up命令启动服务：

[root@node2 compose-test]# docker-compose up
Creating network "compose-test_default" with the default driver
Creating volume "compose-test_pets-data" with default driver
Pulling web (fundamentalsofdocker/ch08-web:1.0)...
1.0: Pulling from fundamentalsofdocker/ch08-web
605ce1bd3f31: Pull complete
188ade417c9f: Pull complete
ad8771290e5e: Pull complete
Pulling db (fundamentalsofdocker/ch08-db:1.0)...
Pulling db (fundamentalsofdocker/ch08-db:1.0)...
1.0: Pulling from fundamentalsofdocker/ch08-db
ff3a5c916c92: Pull complete
ac3385cd756f: Pull complete
Creating compose-test_web_1_1c7e1b7ce98c ... done
Creating compose-test_db_1_14e7442081f6  ... done
Attaching to compose-test_web_1_ec5cf32a5b62, compose-test_db_1_785716b0e082
...
db_1_785716b0e082 | 2018-10-31 01:56:00.122 UTC [20] LOG:  database system was shut down at 2018-10-31 01:55:42 UTC
db_1_785716b0e082 | 2018-10-31 01:56:00.144 UTC [1] LOG:  database system is ready to accept connections
web_1_ec5cf32a5b62 | Listening at 0.0.0.0:3000
web_1_ec5cf32a5b62 | Connecting to DB
web_1_ec5cf32a5b62 | Connected!
...以上输出内容省略了部分

简单说明一下：上面的过程分两步，一是拉取镜像。二是启动服务，并打印日志。
启动完成后，通过浏览器访问http://主机IP:3000/pet，可以查看服务。
按Ctrl+C可结束服务并退出。再次运行时，速度会快很多，因为不用再拉取镜像，相关的数据卷也已经创建完毕。

将服务放在后台运行：

[root@node2 compose-test]# docker-compose up -d
Starting compose-test_web_1_ec5cf32a5b62 ... done
Starting compose-test_db_1_785716b0e082  ... done

查看docker compose进程：

[root@node2 compose-test]# docker-compose ps 
             Name                            Command              State           Ports         
------------------------------------------------------------------------------------------------
compose-test_db_1_785716b0e082    docker-entrypoint.sh postgres   Up      5432/tcp              
compose-test_web_1_ec5cf32a5b62   /bin/sh -c node src/server.js   Up      0.0.0.0:3000->3000/tcp

关闭服务：

[root@node2 compose-test]# docker-compose down
Stopping compose-test_db_1_785716b0e082  ... done
Stopping compose-test_web_1_ec5cf32a5b62 ... done
Removing compose-test_db_1_785716b0e082  ... done
Removing compose-test_web_1_ec5cf32a5b62 ... done
Removing network compose-test_default

Tips：你会发现停止web服务总是比较慢，那是因为db服务能够正确的响应系统发出的SIGTERM信号，正常停止服务。而web不会，所以docker在10秒超时后，强制停用了它。

3. 可伸缩的docker compose服务

当我们上面的服务中，一个web服务形成瓶颈的时候，我们可以快速地的扩充几个web服务出来。命令如下：

[root@node2 compose-test]# docker-compose up --scale web=3
Creating network "compose-test_default" with the default driver
Creating volume "compose-test_pets-data" with default driver
WARNING: The "web" service specifies a port on the host. If multiple containers for this service are created on a single host, the port will clash.
Creating compose-test_db_1_4a3325f40a66  ... done
Creating compose-test_web_1_34a682b911cd ... done
Creating compose-test_web_2_d1270f3e5ab6 ... error
Creating compose-test_web_3_4eee803fe1dc ... error

ERROR: for compose-test_web_2_d1270f3e5ab6  Cannot start service web: driver failed programming external connectivity on endpoint compose-test_web_2_b339ca2150cf (f9b0c19850ea1218bfa73e96df65e8c048b2936504c2f07baa85a66bbaa304e0): Bind for 0.0.0.0:3000 failed: port is already allocated

ERROR: for compose-test_web_3_4eee803fe1dc  Cannot start service web: driver failed programming external connectivity on endpoint compose-test_web_3_af3222197ecd (101305704ef4a4da659f0aaa53931a8ef7aa103e7a9fc4b3806c98f7a76a2911): Bind for 0.0.0.0:3000 failed: port is already allocated

ERROR: for web  Cannot start service web: driver failed programming external connectivity on endpoint compose-test_web_2_b339ca2150cf (f9b0c19850ea1218bfa73e96df65e8c048b2936504c2f07baa85a66bbaa304e0): Bind for 0.0.0.0:3000 failed: port is already allocated
ERROR: Encountered errors while bringing up the project.

我们发现当创建第二和第三个web服务的时候，发生了错误，主要是因为端口冲突问题。我们在docker-compose.yml中指定端口映射的部分进行修改，让主机随机启动三个端口进行映射即可：

version: "3.5"
services:
  web:
    image: fundamentalsofdocker/ch08-web:1.0
    ports:
      - 3000
  db:
    image: fundamentalsofdocker/ch08-db:1.0
    volumes:
      - pets-data:/var/lib/postgresql/data

volumes:
  pets-data:

再次启动：

# 先关闭刚才启动的（虽然没有完全启动成功）
[root@node2 compose-test]# docker-compose down
[root@node2 compose-test]# docker-compose up -d --scale web=3
Creating network "compose-test_default" with the default driver
Creating compose-test_db_1_6a526a0954dc  ... done
Creating compose-test_web_1_5f57a97855bf ... done
Creating compose-test_web_2_37757fe7fff7 ... done
Creating compose-test_web_3_5db14966adca ... done

查看：

[root@node2 compose-test]# docker-compose ps
             Name                            Command              State            Ports         
-------------------------------------------------------------------------------------------------
compose-test_db_1_a2388d8552ee    docker-entrypoint.sh postgres   Up      5432/tcp               
compose-test_web_1_5445bb723cbe   /bin/sh -c node src/server.js   Up      0.0.0.0:32772->3000/tcp
compose-test_web_2_9253be834670   /bin/sh -c node src/server.js   Up      0.0.0.0:32773->3000/tcp
compose-test_web_3_c4178b44052d   /bin/sh -c node src/server.js   Up      0.0.0.0:32774->3000/tcp