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Although establishing a connection is fairly cheap, it s not free, so when designing highly scalable applications it makes sense to minimize the total number of times an application opens and closes connections, thereby getting the most out of each open/ close cycle. This is especially true for ASP or Web Service applications that have to service dozens to thousands of connections a minute. Because an ASP/ASP.NET/Web Service application cannot (or should not) attempt to maintain a globally referenced Connection object, applications are designed around a just-in-time connection strategy. This usually means acquiring a connection just before it s needed and releasing the connection back to the connection pool right after the results are captured by the client. I discuss the connection pool a little later (see Connection management later in this chapter). I think this makes sense to an extent. If you know that your application plans to make several queries in quick succession, some might suggest a broader-scope connection paradigm that holds the connection until all of these operations are complete. This approach comes with an element of risk. One of the more common failure modes for ASP-type applications is leaking connections that is, when connections are acquired (by the Connection.Open method) but not released (with the Connection.Close method). Because the .NET Framework s garbage collector (GC) does not run that often,2 orphaned connection objects (that have not been released) might languish in the object garbage heap for minutes to hours. Because of this, any connection strategy must ensure that any connection that s acquired (opened) is release (closed). To facilitate ASP-type application connection churn, the data access providers have implemented a mechanism to cache connections it s called the connection pool. I ll devote some time to this mechanism a bit later when I discuss properties, methods, and events that the data access interfaces expose to help manage connections.
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ADO.NET 2.0 and later supports the option to use Multiple Active Resultsets (MARS) connections. Although these permit developers to reuse con-
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nections that have pending results, there are so many restrictions and evil side effects, that there are few real scenarios in which this approach provides any benefit over opening another connection. Another myth that I hope to bust is that all applications should always use a connectquery-disconnect connection strategy. I can see how this myth got started, as this is a best practice for ASP, ASP.NET, and Web Service applications. If you re building a Microsoft Windows Presentation Foundation (WPF), Windows Forms, or other client-side executable, it does not always (or even usually) make sense. Instead, I often recommend connecting to the server when connectivity is first required and maintaining the connection for the life of the application. The benefits to this approach are compelling. First, your application need not spend resources opening (and closing) connections.
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The garbage collector runs when memory resources demand it so perhaps not for some time.
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But the most significant benefit is the ability to manage connection-scoped server state. For example, you can create Temporary tables (in tempdb) that can be indexed to hold intermediate query results based on the current client. These can be used to dramatically improve query performance. Connection-specific connection settings that need be applied only once and stick with the connection scope. Server-side cursors against live data, although this is not often done. Consider that when you open a connection, SQL Server has to launch an agent to manage your requests, allocate memory, and prepare for your work. When you close a connection, the server has to clean up the mess you left behind like the maid who comes into your hotel room after that all-nighter. Any connection-owned resources, such as temporary tables or server-side cursors, have to be freed, and the agent has to be shut down. These operations can be postponed by using connection pooling, as I ll discuss later (see Managing the connection pool later in this chapter).
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