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CHAPTER 11 DATA ACCESS LAYER SERVICES
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Hopefully, you can see some of the benefits of the ObjectDataSource model over the model exposed by the SqlDataSource control. While you must take some consideration at the business object layer of the application, a set of types dedicated to feeding the data access controls still creates a better separation of these layers in your application architecture than using the code-behind for individual Web Forms will. You get a cleaner separation of layers with this model, where the specialized methods feeding the ObjectDataSource controls are concerned only with creating and returning appropriate instances of types for the methods they expose, and the data-binding logic is completely handled by the data source control. This looser coupling between layers leads to easier maintenance and better reuse across different presentation tier elements, and even across different applications.
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It is very easy to establish a dependency between a cache entry and an XML file. This is a great feature, as it enables you to load, parse, and cache an XML document for exactly as long as you need to: until you make a change to the document. Make a change to the doc, and the cache entry is automatically purged. This type of dependency has very limited applications, though. It s very common to get XML data via a URL, or Web Service, or maybe even the database. In fact, it is still much more common to get data from the database than it is from XML in the first place. When it comes to loading data out of a database table and putting it into the cache, you have no similar capability to automatically purge the entry when a change is made to the data. The best you can do in 1.x of the Framework is to establish a timeout on the cache entry, and accept some level of latency. For example, if you put the data in the cache and have an absolute expiration of 60 seconds, any change you make to the data will take, at most, 59 seconds to show up. This solution is not attractive for a couple of reasons. Sometimes this level of latency is not acceptable. A change to the data may need to show up on the site immediately. Conversely, sometimes your queries are so expensive that you don t want to go back to the database at 60-second intervals to refresh it, especially since there s no guarantee the data has even changed during that time. What you d really like to do is create a cache entry, and tell the cache that if, for example, the data in the authors table changes, then purge the cache entry so you ll know to go back to the database to refresh it. This feature has been added in version 2.0 of the Framework, but only for SQL Server 7 and later. The implementation for SQL Server 2005 is very different than it is for versions preceding it. Here, we ll examine the feature in SQL Server 7 and 2000. Currently, XML file cache invalidation is done with the FileDependency class. SQL Server cache invalidation is done with a new class, named SQLCacheDependency. This type uses a combination of triggers on the database and polling from the client to determine when a cache entry has expired (see Figure 11-6).
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CHAPTER 11 DATA ACCESS LAYER SERVICES
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Figure 11-6. SQL cache invalidation infrastructure
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The first thing you must do to use SQL cache invalidations is set up a specific database to support the infrastructure. A command line tool, aspnet_regsql.exe, ships with version 2.0 of the Framework that automates a lot of the different database server setup that s required for different features of ASP .NET 2.0 (membership, personalization, session state, etc.). The tool supports a console window command line mode and a Win32 Wizard mode. To set up a specific database, you could use the following command line: aspnet_regsql -S (local) -U sa -P 123123 -d Pubs ed Most of the switches are connection information. You could also use a connection string: aspnet_regsql -C server=.;uid=sa;pwd=123123;database=pubs ed So you can see that, besides connection information, the only other switch you re passing is -ed. This tells the tool to enable the database you ve specified for SQL cache invalidation, which creates a table named AspNet_SqlCacheTablesForChangeNotification in the database you ve specified. This is the table that gets polled by the dependency, watching for changes. You ll see how changes get recorded in this table soon. There are also half a dozen stored procedures that get created. These are used for registering and unregistering tables, and polling and listing registered tables. After a database is prepared for cache invalidations, you ll need to configure the tables you re interested in watching. This registration adds triggers to the tables that will record when something changes in the AspNet_SqlCacheTablesForChangeNotification table. This is the table that gets polled by ASP .NET, and is how the word gets back to the cache dependency that an entry needs to be purged. The command line this time needs to specify the command, -et, for enable table, and then the -t switch to specify the table name: aspnet_regsql -C server=.;uid=sa;pwd=123123;database=pubs -et -t authors
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