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CHAPTER 9 STATE NAVIGATION PATTERN
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Figure 9-8. Processing multiple pages in a web application by using cookies The confusion with an HTTP cookie occurs when the first web browser loads the representation associated with the resource /resource3, while the second web browser loads the resource /resource2. If the second browser attempts to navigate to the resource /resource3, the server will become confused as to what stage the web browser is really at. The server cannot distinguish between browser instances, and therefore overwrites new data over old, or old data over new, causing consistency problems. The behavior of the cookie is correct, as the cookie specification explicitly says that a cookie is associated with a domain and not a browser instance. Using a state identifier to manage the state and resources creates a solution in which the state is accumulated. Accumulation of state makes it possible to fork the state if multiple browsers are accessing multiple versions of the state. To understand the logic, consider Figure 9-9, which indicates how unique state identifiers are created.
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CHAPTER 9 STATE NAVIGATION PATTERN
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E B V N Figure 9-9. An example of the state identifier being updated after the URL is copied to another web
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browser instance In the upper-left corner of Figure 9-9 is a web browser that has downloaded some content with the URL /resource#11. The loaded content is the resource /resource with the state identifier 11. Imagine the user opening a second browser and copying the link /resource#11. The State Navigation pattern will load the resource /resource and the state associated with the identifier 11. In the second browser, the state identifier is updated to reference 12. If the second browser has the same state identifier as the first, that binds both browsers to the same state and creates concurrency problems. Imagine that the client modifies the state in the first browser; then the second browser would see the same state. This is not desirable, and therefore the state identifier 11 is copied to a new state identifier 12. Then the first and second browser instances for the time being have the same state values, but different references. The solution in Figure 9-9 needs one additional twist to make it work properly. If the browser were to request the URLs /resource#11 and /resource#12, the resource /resource would be issued twice. This relates back to the purpose of the hash character, which is a reference to a link on an HTML page. This is a good thing, because the State Navigation pattern has separated the resource from the state of the resource. So when the resource /resource#11 is called, the URLs /resource (for example, HTML page) and /resource/state (for example, HTML page state) are called. By using the XMLHttpRequest object, it is easy to separate the two URL requests, and there are multiple ways to implement the two URLs. But using two URLs is not enough. You also need to use HTTP headers to uniquely identify the request. Figure 9-10 illustrates how Figure 9-8 is fixed by using HTTP headers.
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CHAPTER 9 STATE NAVIGATION PATTERN
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Figure 9-10. Rearchitecting multiple resources to use a state identifier In Figure 9-10, each of the web browser instances is a URL, with a hash code identified state identifier that is converted into an X-Page-State HTTP header. Each instance of the web browser has an HTTP header that is unique. This is a good thing because now, even though there are two browsers, the resource /resource2 has two separate instances of the associated state. Having the unique state identifiers works, except it exposes another problem: there is no stringing together of the individual HTML pages to build a web application. What we don t know is how the states relate to each other. Visually, we know that the states 11, 12, and 14 are a single chain. And visually we know that 11 and 13 are another chain. But the server does not know that because the server does not know that state 13 is the result of opening a second browser. To finish the solution, the history of the URLs is needed. The web browser has that information because it is required for navigating the Forward and Back buttons. The simple solution would be to access the browser-exposed history object and pass those URLs to the HTTP POST. The problem with the simple solution is that it is not generally viable. Accessing the history object by using a script is a security issue, and unless the client has allowed access, will generate an exception. A more feasible solution is to add an additional HTTP header that uniquely identifies the window used to chain together the HTML pages. Specifically, the property window.name can be assigned and is ideally suited to uniquely identify the individual HTML windows. Figure 9-11 illustrates the final solution.
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