Docker插件机制简介

Docker公司对Docker的扩展分成三个级别，从低到高分别是：User-facing API，该API是Docker提供用于串联出一些场景的API；第二层是插件（Plugins），也是今天的主要介绍内容，Plugins和Docker是两个相互独立的进程，它们之间通过一些预定的通讯协议进行功能扩展；第三层是叫Drivers，是Docker实现功能的一些驱动，例如一些文件存储的驱动。目前，大多数Drivers都是官方出的，当有特定需求或者是用到独特技术时，开发者可以自己写相应的驱动。

卷插件工作原理

从下图可以看出，卷插件是一个独立运行的进程

image.png

卷插件需要提供两种REST API：

Voluem-Plugin REST API
用于docker和插件通信
CLI REST API
命令行工具管理插件

已有的卷插件

Convoy：
一种基于本地存储的单机版插件，支持的驱动有Device Mapper、VFS、EBS等
Flocker：
另外一个功能很强大的卷插件，支持多种后台存储驱动，包括OpenStack Cinder，AWS EBS，EMC ScaleIO、ZFS等。

Convoy 试用

# 下载安装插件
wget https://github.com/rancher/convoy/releases/download/v0.5.2/convoy.tar.gz
tar xvzf convoy.tar.gz
sudo cp convoy/convoy convoy/convoy-pdata_tools /usr/local/bin/
sudo mkdir -p /etc/docker/plugins/
sudo bash -c 'echo "unix:///var/run/convoy/convoy.sock" > /etc/docker/plugins/convoy.spec'

# 启动插件进程
convoy daemon --drivers vfs --driver-opts path=/data

# 创建卷
convoy create volume_data

# docker 使用
docker run -itd -v volume_data:/volume_data --volume-driver=convoy ubuntu /bin/bash

convoy 源码分析

https://github.com/rancher/convoy/blob/master/daemon/daemon.go

我们看一下，convoy都提供了哪些REST API

CLI REST API

    m := map[string]map[string]requestHandler{
        "GET": {
            "/info":            s.doInfo,
            "/volumes/list":    s.doVolumeList,
            "/volumes/":        s.doVolumeInspect,
            "/snapshots/":      s.doSnapshotInspect,
            "/backups/list":    s.doBackupList,
            "/backups/inspect": s.doBackupInspect,
        },
        "POST": {
            "/volumes/create":   s.doVolumeCreate,
            "/volumes/mount":    s.doVolumeMount,
            "/volumes/umount":   s.doVolumeUmount,
            "/snapshots/create": s.doSnapshotCreate,
            "/backups/create":   s.doBackupCreate,
        },
        "DELETE": {
            "/volumes/":   s.doVolumeDelete,
            "/snapshots/": s.doSnapshotDelete,
            "/backups":    s.doBackupDelete,
        },
    }

Volume-Plugin REST API

    pluginMap := map[string]map[string]http.HandlerFunc{
        "POST": {
            "/Plugin.Activate":           s.dockerActivate,
            "/VolumeDriver.Create":       s.dockerCreateVolume,
            "/VolumeDriver.Remove":       s.dockerRemoveVolume,
            "/VolumeDriver.Mount":        s.dockerMountVolume,
            "/VolumeDriver.Unmount":      s.dockerUnmountVolume,
            "/VolumeDriver.Path":         s.dockerVolumePath,
            "/VolumeDriver.Get":          s.dockerGetVolume,
            "/VolumeDriver.List":         s.dockerListVolume,
            "/VolumeDriver.Capabilities": s.dockerCapabilities,
        },
    }

convoy daemon的启动参数

[root@localhost ~]# convoy daemon --help
NAME:
   convoy daemon - start convoy daemon

USAGE:
   convoy daemon [command options] [arguments...]

OPTIONS:
   --debug                          Debug log, enabled by default
   --log                            specific output log file, otherwise output to stdout by default
   --root "/var/lib/rancher/convoy"             specific root directory of convoy, if configure file exists, daemon specific options would be ignored
   --drivers [--drivers option --drivers option]        Drivers to be enabled, first driver in the list would be treated as default driver
   --driver-opts [--driver-opts option --driver-opts option]    options for driver
   --mnt-ns                             Specify mount namespace file descriptor if user don't want to mount in current namespace. Support by Device Mapper and EBS
   --ignore-docker-delete                   Do not delete volumes when told to by Docker
   --create-on-docker-mount                 Create a volume if docker asks to do a mount and the volume doesn't exist.
   --cmd-timeout                        Set timeout value for executing each command. One minute (1m) by default and at least one minute.
   --ignore-config-file                     Avoid loading the existing config file when starting daemon, and use the command line options instead (not including driver options)

其中两个参数非常重要

--root:
specific root directory of convoy, if configure file exists, daemon specific options would be ignored
--ignore-config-file:
Avoid loading the existing config file when starting daemon, and use the command line options instead (not including driver options)