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CHAPTER 8 PERSISTENT COMMUNICATIONS PATTERN
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Figure 8-1. Yahoo! message board that displays messages and the availability of the message creator The different services generate very different types of data. When a message is posted, it is read-only. Messages are typically not changed, but they may be deleted. In contrast, the status of the user will change, and more important, a reader does not know when the status will change. Generating content for these two dissimilar streams is complicated because when a message is generated and displayed in an HTML page, it will not change. Yet the status of the user icon can change while the reader is inspecting the postings. A possible strategy when generating the content is to consider the status of the user as single-shot display-only status. The single-shot display strategy will generate a status as the content is generated, and after that the status is not updated. Most likely, a reader will glance over the messages and decide within a minute or so whether to contact the message poster. If the message poster was available as the content was generated, chances are pretty high that the message poster will be around for a minute or so. The problem with the strategy is that if the
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CHAPTER 8 PERSISTENT COMMUNICATIONS PATTERN
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HTML page is loaded and the reader does not read the message until 10 minutes later due to a coffee break, the status will be out-of-date. Another strategy is to poll the server on the status of the message author. When the page is generated, an initial status is defined. Then, after a minute or whatever period decided by the generated script, the status is updated. The problem with polling is that the status is current only when the poll request has been made. Between polls, the state is considered stale and not representative. So to keep a poll representative, you poll more often, but what is a good polling frequency Let s say that you poll every second; well, then the waiting time is not significant from a reader perspective. However, for a real-time system that is an eternity. The point is that the poll time needs to be adjusted to the nature of the data and that there is no single best universal poll frequency. Yet another strategy is for the server to contact the client and inform the client of a change in status. That strategy seems to be the best but is not possible because of the Internet infrastructure. (I will explain the details later, in the Architecture section.) Regardless of the strategy implemented, there is a need for the server to update the client with new information for one reason or another.
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The main argument for using the Persistent Communications pattern is for the web browser (client) to be able to receive information updates from the server. The server has the ability to communicate to the client, without the server having to wait for the client to ask for an inforE B V N mation update. The pattern has three main implementation types that are direct solutions to the problem that they solve: Status updates: A status update is a piece of global information that a client is interested in and that is stored on a server. Multiple clients see the same representation of the data. An example is a ticker tape of current stock prices, like the ticker tape on a financial television program. The information is not intended for a specific user, as all users see the same information. To view the information, users do not need to identify themselves. It does not mean that the data is free for all. In other words, a group of authorized people can access the resource, but the resource is not dependent on those accessing the resource. Presence detection: Presence detection occurs when multiple clients are interested in the same global resource. The global resource has the same representation for all clients, except that the state of the resource is dependent on the clients viewing the resource. Figure 8-1, which shows the status of the message creator, is an example of presence detection. The resource might be updated by an external process but can also be updated by the individual users. Server push: Server push occurs when multiple clients register with a global resource but are allocated a unique resource. It is the server s discretion on how the identified user is cross-referenced to the unique resource. An example is a user-defined view of a ticker tape. The stock prices are identical for all users, but which stock prices are shown is unique to each user. The server has to manage which information is pushed to which user. The clients indicate only what they are interested in.
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