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CHAPTER 8 PERSISTENT COMMUNICATIONS PATTERN
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protected void doPost(HttpServletRequest request, HttpServletResponse response) throws javax.servlet.ServletException, java.io.IOException Iterator iter = _users.iterator(); boolean didFind = false; while(iter.hasNext()) { String user = (String)iter.next(); if(user.compareTo(_user) == 0) { didFind = true; break; } } if(!didFind) { _users.add(_user); _version ++; } response.setStatus(200, "All ok"); }
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In the implementation of doPost, the sent data of the HTTP POST is not processed because our presence-detection example needs only to detect the user identity. In the implementation of doPost, what is processed is the user identity. If the user identification (_user) already exists in the list of present users (_users), nothing happens. If the user does not exist, the user is EBVN added to the users list, and the version (_version) number is incremented to indicate a change of state. Regardless of whether a user is added, the HTTP 200 code is returned, indicating a successful operation. It could be argued that if the user already exists in the users list, an error should be returned. However, that is not entirely appropriate; if a user is already added to the list, that does not indicate an error condition, but a condition of doing something repeatedly. And doing something repeatedly may be inefficient, but it is not wrong. Following is the implementation of the doGet and getSentVersion methods: private int getSentVersion(HttpServletRequest request) { Cookie[] cookies = request.getCookies(); if(cookies != null) { for(int c1 = 0; c1 < cookies.length; c1 ++) { if(cookies[ c1].getName().compareTo("VersionId") == 0) { return new Integer( cookies[ c1].getValue()).intValue(); } } } return 0; }
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CHAPTER 8 PERSISTENT COMMUNICATIONS PATTERN
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protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { int sentVersion = getSentVersion(request); int waitCount = 0; while(waitCount < 100) { if(sentVersion < _version) { PrintWriter out = response.getWriter(); Iterator iter = _users.iterator(); while(iter.hasNext()) { String user = (String)iter.next(); out.println("User (" + user + ")"); } response.addCookie(new Cookie("VersionId", new Integer(_version).toString())); return; } try { Thread.currentThread().sleep(1000); } catch (InterruptedException e) { } waitCount ++; } EBVN response.setStatus(408, "No change"); } The doGet method implementation is similar to the doGet method implementation illustrated earlier, in the Calling the ServerCommunicator Intelligently section. The difference is that HTTP cookies are used to track the version number that the client has. The method getSentVersion extracts the version number from the client-sent cookies. If the version number does not exist, a value of 0 is returned. Then the server goes through the looping process of checking for a version number difference. If a version number difference is present, the output is generated, and the cookie VersionId is sent with the version number of generated content.
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Example: Server Push
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For each example, the level of complexity has increased, and the only remaining scenario to explain is the server push. What makes a server push unique is that each client accessing the global resource (for example, http://mydomain.com/global/resource) has a unique child URL (for example, http://mydomain.com/global/resource/unique-child). In previous examples, the URL used was a global resource that was shared among individual users. This time the URL must be unique because when the server pushes content, it is individualized. As a side note, when referencing the unique URLs for the scope of this example, the unique identifier will always be a username or user identifier. That does not need to be the case; it could be a feed identifier, message queue, and so on. The unique identifier represents some type of unique resource that distinguishes itself from the other resources. It also does not mean that a single user is allocated a single resource. It could be that multiple users share the same unique resource.
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CHAPTER 8 PERSISTENT COMMUNICATIONS PATTERN
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Some Thoughts on Specifying URLs
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Before continuing with the server push implementation, a bit more thought to specifying the URL has to be given. For the Permutations pattern, I illustrated how to generate and retrieve URLs. You saw techniques for associating a representation with a resource based on the needs of the client. When using the server push, the URL must be unique, and this can be a problem because we don t know what the URL is in the first place. The question is how to figure out that the URL http://mydomain.com/global/resource/unique-child is unique based on the global resource http://mydomain.com/global/resource. Using a Hard-Coded URL A hard-coded URL is a URL that is written directly into the HTML, as illustrated by the following example: <html> <head> <title>Hard Code Reference</title> <script language="JavaScript" src="../lib/factory.js"></script> <script language="JavaScript" src="../lib/asynchronous.js"></script> <script language="JavaScript" type="text/javascript"> var asynchronous = new Asynchronous();
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