p2的相关概念
1.The role of P2
p2的核心构架是agent[代理]，这个agent是抽象的概念，它不是真正的代理对象，它
真实是一个逻辑实体，它用来说明：metadata元数据管理profiles，通过管理
下载aritfacts从仓库repositories中,使用engine来操作profiles.
artifacts被安装或更新，它大多是一些bundles
metadata说明：依赖的信息，它从产品配置文件或manifests中抽取得到
profiles:represent runnable configurations of software,recorded intalled artifact.

2.p2 Architecture
the directors and engine can be remote

All metadata is captured in {installable unit}IUs

IU has an ID and a version,the combination of which must be globally unique.


agent
Director ->[]Metadata
TouchPoints ->[]Runtines ->[]Eclipse,other,os
Engine ->[]app.part,profiles
Repositories ->[]Artifact

director is the brains{大脑}of the p2 operation.
: the metadata available in the known repositories
the profile being managed
a set of install,uninstall,update operations

Engine : to execute a given set of install,unstall,and update operation.

when a bundle is installed into an Equinox system,its start level
and auto-start state need to configured,This is done by the Equinox
touchpoint.

3.Using the p2 Api
p2暴露一些集合，关于provisional Api
你可以完全自已的安装向导，以及整个工作流程。
IProfileRegistry
IInstallableUnit
IPlanner
IEngine
IMetadataRepositoryManager
IArtifactRepositoryManager

4.publishing Metadata
三种创建repositories方法：
导出向导，PDE Build,使用publisher