Now with AJAX (when implemented correctly),
despite making a round-trip to the server, the overall size of the packets of data
getting passed will be a lot smaller because you are just going to update part of the page;
the entire page will not ???flash??? as it refreshes the user interface with the update.
Beneath the resource tier comes the data retrieval tier. In a clean architecture, this is
kept separate from the logic so that if data sources change, you don??™t need to get into the
business logic plumbing and rip it out. It should provide a clean interface to the data
layer. Visual Studio 2005 offers you the facility to create a proxy to an existing web service,
which you can use to implement a data retrieval tier. In this application, the Price History
web service from Yahoo! provides CSV over HTTP, so you will implement a web service
that wraps this functionality and can easily be proxied.
The business logic tier is where you add value to your resources through aggregation,
integration, and calculation. For example, when calculating the P/E, discussed earlier,
with price information coming from one resource and earnings from another, instead of
integrating and calculating these on the page level, you aggregate the information in the
business logic tier where the function performing the calculation calls the data retrieval
tier to get the information from both resources and then performs the calculation.
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