how to generate barcode in asp.net using c# 6-1. IMPLEMENTING WEB SERVICES FOR LARGE OR SLOW DATA SETS in Font

Painting Code 3/9 in Font 6-1. IMPLEMENTING WEB SERVICES FOR LARGE OR SLOW DATA SETS

CHAPTER 6 6-1. IMPLEMENTING WEB SERVICES FOR LARGE OR SLOW DATA SETS
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exist, then false is returned, indicating that the URL should be executed. If the URL does exist, then a cached object exists. The existence of a cached object means that it is not necessary to call the server, and the code s onComplete method can be called directly. The only problem with the cached object is that there is no available XMLHttpRequest instance. The solution to this problem is to create a fake instance of XMLHttpRequest. Using JavaScript, it s easy to create a fake object. Where things get tricky is that the methods have not been defined, thus they are not available. The headers are not available because they have not been stored in the object when the cached object was created. Again, the fact that the headers have not been stored in the cache highlights that this is a simple implementation of XMLHttpRequest. The fakeXMLHttp variable represents the fake XMLHttpRequest object and is used to call the code s settings.onComplete method. Because settings.onComplete is called, the cache returns a false value, indicating that Asynchronous should not make an HTTP request. The other part to the cache code is the CacheProcessAndBreakAfterRequest function, which is used to add a cached object entry. The addition of the object to cache is constrained to an HTTP status code of 200 and an HTTP GET. If the object is added to the cache, then the XMLHttpRequest property, status, statusText, responseText, and responseXML are saved. If you wanted to create a complete implementation of the fake XMLHttpRequest object and save the HTTP headers, you would put that code in the CacheProcessAndBreakAfterRequest function. As an alternative, you might be tempted to save a reference to the XMLHttpRequest instance. The problem is that the cache is not in control of the XMLHttpRequest instance, so by saving a reference, you are not saving the data, as the Asynchronous instance could reuse the XMLHttpRequest instance for another request. Therefore, whenever implementing your cache, you need to copy the information that should be saved in the cache. Implementing an HTTP Validation Client Cache The cache implementation is simple in that once a request has been executed and saved, it will never be executed again. For URLs where the data never changes, this is acceptable. However, there are URLs where the data changes, such as is the case with a real-time stock ticker, and the single request client cache is completely unacceptable. In those situations, the single request client cache needs to be extended to use HTTP validation. HTTP Expiration Caching Is a Bad Idea (Generally) When you are using HTTP validation, you are letting the Internet infrastructure manage the caching. There are two models: HTTP expiration and HTTP validation. When using HTTP expiration, you are saying that content is valid for a certain period of time. The period of time for which the data is valid depends on what the HTTP headers say. It is generally not a good idea to use the HTTP expiration model; rather, it s better to use HTTP validation and write code to help the Internet infrastructure do its work. To understand why the HTTP expiration model is problematic, consider the following scenario. Say you are running a Web site that hosts news feeds. To reduce repetitive traffic on the Web site, you enable HTTP caching and assign an expiration time of 30 minutes. (The expiration time is an arbitrary value used for illustrative purposes.) This means that when a browser downloads some content, the next version of the content will be available in 30 minutes. Indicating a wait period of 30 minutes is a bad idea in that 30 minutes, news can dramatically change. A client who has downloaded some content is then restricted to retrieving news in 30-minute cycles. Of course, the client could ignore or empty the cache, resulting in downloads
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