Chen Jian

   这一节介绍了三种模式：Domain Object, Transaction Script 和 Table Module。 对第一种模式还不是非常明白，以后慢慢体会

Two issuses about concurrency that’re difficult to handle:     1. offline concurrency(transcations among several databases or serveral requests)    2. multi-thread concurrency in application server side Two basic ways of controlling concurrency    1. isolation    2. to make some data immutable Optimistic Lock： detect conflictions and then solve them。 The lock is only held …

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An object that acts as a Gateway to a database table 与Row Data Gateway不同，它对应的不是表中的一行，而是整个表，所以它的方法可以都做成静态的

     业务层对表现层提供的服务应该是Application Service，而不是Business Service      以博客为例， 有一个Business Service可能是 getPostById()，即ID找到某篇文章；而它对应的App Service则应该是 visitPostById()。两者看上去没什么区别，但实际上 visit post比起简单的 get post 可能要复杂一些，比如验证、授权、增加博客访问次数等，如果直接将getPostById()方法提供给表现层，那么刚才提到的附加工作就要交给表现层来做，这无疑将加重表现层的代码重复，减弱业务层的代码重用。 因此，App Serivce 和 Biz Service应该区分为不同的模块。        这也是 Application Service 模式(Core J2EE Patterns)的要求。Application Service 是直接跟 use-case一致的服务接口，而Business Service则要稍微通用一些       

1.为什么要分层？   a.分层可以使你将精力集中在某一层。比如FTP使用者和开发者不须要考虑物理层和数据链路层的细节。如果不分层，要学的就多了   b.分了层，可以在保留服务接口的前提下更换实现的细节。   c.分层可以将耦合限制在两层之间，不存在3-Players情形   d.分层利于标准化，这样才能将不同厂商的零件组装在一起   e.分层后，一个下层可以为多种上层使用，比如TCP可以为FTP用，也可以为HTTP用 2.注意Layer和Tier的区别。一般来说Layer基于逻辑的区分，而Tier基于物理上的区分。比如，我们有Presentation Layer, Domain Login Layer和Data Source Layer 三个Layer；在实际上只有两个Tier (可以译为“端”)： 客户端和服务器 3.一般情况上可以将所有程序都放在服务端运行，客户端只用一个浏览器，这样最方便部署和升级；但是有时候也可以将部分程序放在客户端，这主要是出于响应速度和断线操作（responsiveness or disconnected operation）的考虑 附：Fowler的经典语录 a. One pretty absolute rule is that nothing should depend on the presentation   

"Organize Business Logic by procedures that carry out what needs to be done in a transaction " 1.A transaction script organizes all its business logic as a single procedure, making calls directly to the DB all through a thin dao layer 2.When applying this pattern, try to separate business logic from the layered architechure 3. …

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An object that wraps a record data structure in an external resource, such as a row in a database table, and adds some domain logic to that object. Active Record uses the most obvious approach: put data access logic into the domain object. This way all people know how to read and write their data …

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A Table Module organizes domain logic with one class per table in the database, and a single instance of a class contains the various procedures that will act on the data. The primary distinction with Domain Model is that if you have many orders, a Domain Model will have one order object per order, while …

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