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CHAPTER 4 CACHE CONTROLLER PATTERN
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public class ValidationLibrarianServlet extends HttpServlet { protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws javax.servlet.ServletException, java.io.IOException { String isbn = getISBNFromURL(req.getRequestURI()); try { String etagvalue = req.getHeader("If-Match"); Librarian librarian = LibrarianBuilder.create(etagvalue); Book book = librarian.checkOutBook(isbn); if(etagvalue != null && book.hashCode() == Integer.parseInt(etagvalue)) { resp.setStatus(304, "Not modified"); return; } resp.setHeader("ETag", Integer.toString( book.hashCode())); generateGetContent(resp, book); } catch (Exception ex) { throw new ServletException( "LibrarianServlet generated error", ex); EBVN } } protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws javax.servlet.ServletException, java.io.IOException { try { Librarian librarian = LibrarianBuilder.create("empty"); Book book = getDetailsFromRequest(req); librarian.checkInBook(book); generatePutContent(resp, book); } catch (Exception ex) { throw new ServletException( "LibrarianServlet generated error", ex); } } } In the example code, a number of incomplete methods are beyond the scope of this pattern because they are implementation details specific to a code base. Starting with the method goGet, which is called when the HTTP GET method is called, the ISBN is retrieved. At the beginning of this chapter, the URL /ajax/books/[ISBN].xml was used to uniquely identify a book. The method getISBNFromURL will parse the URL and retrieve the desired ISBN. Having multiple
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CHAPTER 4 CACHE CONTROLLER PATTERN
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URLs associated with a single servlet is not difficult. Specifically for Java, the administrator would change the web.xml file to associate the base URL /ajax/books with the ValidationLibrarianServlet. After having extracted the ISBN number, the ETag identifier is retrieved from the request by using the method req.getHeader( "If-Match"). The retrieved instance is passed as a parameter to the method LibrarianBuilder.create. Depending on the value of the ETag, a decorated LibrarianSaveToStorage class is created. The method checkOutBook is called, and an instance will be retrieved that indicates either that an HTTP 304 should be returned, or that a new instance has been instantiated and output should be generated. If output is generated, an ETag identifier is generated and added to the HTTP output. The method doPut is called whenever an HTTP PUT is called. The implementation is relatively simple in that the decorated Librarian classes are instantiated, and the Book class parameters are retrieved and added to the underlying storage mechanism by using the method checkInBook. Because the Librarian classes are decorated, the hash code value will be automatically identified with the ISBN of the book. The examples illustrated a relatively simple HTTP GET and PUT. Let s say that you want to search for a book based on the title. Then the URL /ajax/books/search author=[name] could be used, and ValidationLibrarianServlet would need to be extended to include the functionality.
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Pattern Highlights
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E B V N Let s wrap all of this up and consider what the Cache Controller pattern accomplishes. The
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purpose of the Cache Controller pattern is to provide a temporary cache by complementing the Internet infrastructure. The idea is not to re-create yet another caching infrastructure, because the Internet already does that very well. In the scope of writing web applications, the cache infrastructure to use is HTTP validation. Even though HTTP validation is not typically used for scripts, it can and should be. The following points are important highlights of the Cache Controller pattern: When using a cache, it is preferable to use the HTTP Validation model. The HTTP Expiration model is less useful because expiration says content is good for a certain time frame regardless of what happens to the server. When using HTTP validation for writing a cache, only the client actually caches the information. The server is responsible for generating the entity tags and for comparing old with new entity tags. This means that the server has to keep a sense of history with respect to the changing state of the objects. There are two ways to implement HTTP validation: letting the HTTP server do the heavy lifting, or creating a server-side processor that does everything. When letting the HTTP server do the heavy lifting, the server framework (for example, JSP, Servlet, ASP.NET) is responsible for updating the static content pieces managed by the HTTP server.
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