SQL SERVER AS A RUNTIME HOST unload the AppDomain. On resource failures, the CLR will unwind the entire managed stack of the session that takes the resource failure. If that session has any locks, the entire AppDomain that session is in is unloaded. This is because having locks indicates there is some shared state to synchronize, and thus that shared state is not likely to be consistent if just the session was aborted. In certain cases this might mean that finally blocks in CLR code may not run. In addition, finalizers, hooks that programmers can use to do necessary but not time-critical resource cleanup, might not get run. Except in UNSAFE mode (discussed later in the chapter), finalizers are not permitted in CLR code that runs in SQL Server. Stack overflow conditions cannot be entirely prevented, and are usually handled by implementing exceptional condition handling in the program. If the program does not handle this condition, the CLR will catch these exceptions, unwind the stack, and abort the thread if needed. In exceptional circumstances, such as when memory allocation during a stack overflow causes an out-of-memory condition, recycling the App Domain may be necessary. In all the cases just mentioned, SQL Server will maintain transactional semantics. In the case of AppDomain recycling, this is needed to assert that the principal concern is reliability, if needed, at the expense of performance. In addition, all the Framework class libraries (FX libraries) that SQL Server will load have gone through a careful review and testing to ensure that they clean up all memory and other resources after a thread abort or an AppDomain unload. We ll discuss AppDomains shortly. Code Loading Since in SQL Server users are not allowed to run arbitrary programs for reliability reasons, code (an assembly) is loaded a little differently than in other runtime hosts. The user or DBA must preload the code into the database and define which portions are invocable from Transact-SQL. Preloading and defining code uses ordinary SQL Server Data Definition Language (DDL). Loading code as a stream of bytes from the database rather than from the file system makes SQL Server s class loader unique. Later in this chapter, we ll look at the exact DDL that is used to create an assembly in SQL Server (that is, load or refresh the bytes of code in the database) and manage the behavior of the assemblies. The class libraries that make up the .NET Framework are treated differently from ordinary user code in that they are loaded from the global assembly cache and are not defined to SQL Server or stored in SQL Server.
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