POEAA笔记 DTO

Granularity is one of the most tricky issues with Remote Facade. Some people like to make fairly small Remote Facades, such as one per use case. I prefer a coarser grained structure with much fewer Remote Facades. For even a moderate-sized application I might have just one and even for a large application I may have only half a dozen. This means that each Remote Facade has a lot of methods, but since these methods are small I don't see this as a problem.

 

A single Data Transfer Object usually contains more than just a single server object. It aggregates data from all the server objects that the remote object is likely to want data from. Thus, if a remote object requests data about an order object, the returned Data Transfer Object will contain data from the order, the customer, the line items, the products on the line items, the delivery information all sorts of stuff.

 

An important factor for serialization is the synchronization of the Data Transfer Object on each side of the wire. In theory, whenever the server changes the definition of the Data Transfer Object, the client updates as well but in practice this may not happen. Accessing a server with an out-of-date client always leads to problems, but the serialization mechanism can make the problems more or less painful. With a pure binary serialization of a Data Transfer Object the result will be that its communication is entirely lost, since any change to its structure usually causes an error on deserialization. Even an innocuous change, such as adding an optional field, will have this effect. As a result direct binary serialization can introduce a lot of fragility into the communication lines.

Other serialization schemes can avoid this. One is XML serialization, which can usually be written in a way that makes the classes more tolerant of changes. Another is a more tolerant binary approach, such as serializing the data using a dictionary. Although I don't like using a dictionary as the Data Transfer Object, it can be a useful way of doing a binary serialization of the data, since that introduces some tolerance into the synchronization.