Location of State Data in Visual Basic .NET

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Location of State Data
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Session in process Session out of process
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Performance, Scalability, and Fault Tolerance
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High performance; low scalability; low fault tolerance; web farms must use sticky sessions; fully load-balanced web farms not supported Decreased performance; low scalability; improved fault tolerance (ASP.NET process can reset without losing state data); web farms must use sticky sessions; fully load-balanced web farms not supported Decreased performance; high scalability; high fault tolerance
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Session on state server
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In conclusion, while storing state data on the web server (or in a state server) provides the simplest programming model, you must make some obvious sacrifices with regard to complexity and performance in order to achieve scalability and fault tolerance.
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Transferring State to or from the Client
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The second option to consider is transferring all state from the server to the client, and back to the server again, on each page request. The idea here is that the web server never maintains any state data it gets all state data along with the page request, works with the data, and then sends it back to the client as part of the resulting page. This approach provides high scalability and fault tolerance with very little complexity in your infrastructure: since the web servers never maintain state data, you can implement a fully loadbalanced web farm without worrying about server-side state issues. On the other hand, there are some drawbacks. First of all, all the state data is transferred over what is typically the slowest link in the system: the connection between the user s browser and the web server. Moreover, that state is transferred twice for each page: from the server to the browser, and then from the browser back to the server. Obviously, this can have serious performance implications over a slow network link (like a modem), and can even affect an organization s overall network performance due to the volume of data being transferred on each page request. The other major drawback is the complexity of the application s code. There s no automatic mechanism that puts all state data into each page; you must do that by hand. Often this means creating hidden fields on each page in which you can store state data that s required, but that the user shouldn t see. The pages can quickly become very complex as you add these extra fields. This can also be a security problem. When state data is sent to the client, that data becomes potentially available to the end user. In many cases, an application s state data will include internal
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CHAPTER 10 s WEB FORMS UI
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information that s not intended for direct consumption by the user. Sometimes, this information may be sensitive, so sending it to the client could create a security loophole in the system. Although you could encrypt this data, that would incur extra processing overhead and could increase the size of the data sent to/from the client, so performance would be decreased. To avoid such difficulties, applications often minimize the amount of data stored in the page by re-retrieving it from the original database on each page request. All you need to keep in the page, then, is the key information to retrieve the data and any data values that have changed. Any other data values can always be reloaded from the database. This solution can dramatically increase the load on your database server, but continues to avoid keeping any state on the web server. In conclusion, while this solution offers good scalability and fault tolerance, it can be quite complex to program, and can often result in a lot of extra code to write and maintain. Additionally, it can have a negative performance impact, especially if your users connect over low-speed lines.
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State in a File or Database
The final solution to consider is the use of temporary files (or database tables of temporary data) in which you can store state data. Such a solution opens the door to other alternatives, including the creation of data schemas that can store state data so that it can be retrieved in parts, reported against, and so forth. Typically, these activities aren t important for state data, but they can be important if you want to keep the state data for a long period of time. Most state data just exists between page calls, or at most, for the period of time during which the user is actively interacting with the site. Some applications, however, keep state data for longer periods of time, thereby allowing the user s session to last for days, weeks, or months. Persistent shopping carts and wish lists are examples of long-term state data that s typically stored in a meaningful format in a database. Whether you store state as a single BLOB of data or in a schema, storing it in a file or a database provides good scalability and fault tolerance. It can also provide better performance than sending the state to and from the client workstation, since communicating with a database is typically faster than communicating with the client. In situations like these, the state data isn t kept on the client or the web server, so you can create fully load-balanced web farms, as shown in Figure 10-5.
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