128 CHAPTER 5 PERMUTATIONS PATTERN Implementation E (Web host music)
128 CHAPTER 5 PERMUTATIONS PATTERN Implementation E B V In the implementation of the Permutations pattern, there are two concerns: associating a representation with a resource, and authorizing a user to access a resource or representation. The implementation of the two concerns requires the creation of a URL rewriter component. The purpose of the URL rewriter component is to inspect the request and decide which content should be generated. Rewriting URLs In Figure 5-9, the URL /bankaccount/login was redirected to reference the URL /bankaccount/ login.jsp. The redirection in HTTP server terms is called rewriting the URL. By using URL rewriting, it is possible to alter the URL being requested without doing a physical HTTP redirect. Even though there are times to use a physical HTTP redirect (for example, after the bank account login), doing so for each URL is a waste of resources. The URL rewriter component changes the URL to another value; this action is very common on many web servers. Some readers may want to point out that the Apache HTTPD web server has a very capable URL-rewriting module called mod_rewrite, which can be used in lieu of writing a separate URL rewriter component. The mod_rewrite module is a very good URL rewriter, but its ability to return content based on the Accept HTTP header is limited. Hence for most cases it is necessary to write a URL rewriter component. Conceptually, though, writing a URL rewriting component is identical to the functionality being offered by the module mod_rewrite. When rewriting URLs, the logic to rewrite them needs to be put into something an HTTP server calls a filter. Filters are not handlers, in that filters are not responsible for generating the output of a specific request. Filters process all requests, and they will modify input streams, output streams, HTTP parameters, or whatever a filter needs to accomplish. Filters are often used for logging or authentication purposes. In ASP.NET, a filter can be defined in two places. The first place is in the file global.asax, and the second place is a component that is referenced by the web application configuration file. HTTP servers have multiple phases in which a filter can process the HTTP request. There are phases before the handler processes the request, phases to manage authentication, and phases after the handler has processed the request. The reason for the different phases is that the HTTP request has different states at each phase. In the case of ASP.NET, the filter phase used to implement the URL rewriting component is OnBeginRequest. The phase OnBeginRequest occurs before any of the other filters are called. The phase is ideally suited for rewriting a URL because the authentication, logging, and so on have not been executed. When performing HTTP authentication, it is important to authenticate by using the rewritten URL, and not the original. This is because depending on the content requested, a client may or may not be granted access. For simplicity purposes, the URL rewriter component is added to the file global.asax. Additionally, when adding filters to an ASP.NET application, the filter applies to that ASP.NET application only, and not other ASP.NET applications. If instead you wanted a global filter, you would need to write an Internet Server API (ISAPI) filter. The URL rewriter component will have two definitions: URL rewriter component and rewrite component. The purpose of the URL rewriter component is to provide an entry point and identify a URL that needs to be rewritten. The purpose of the rewrite component is to rewrite a given URL. In C# interface terms, both components are defined as follows:
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