The Art of SQL Server FILESTREAM - Red Gate Software

Chapter 3: Accessing FILESTREAM Data from Client ApplicationsUnderstanding Streaming Access toFILESTREAM DataIn order to gain streaming access to the FILESTREAM data, SQL Server exposes amanaged object for the .NET developers and a Win32 API for the Windows programmers.Having instantiated an SqlFileStream object, representing the FILESTREAM datafile (in case of .NET), or opened a FILESTREAM data file (using the SQL Server Win32API), you can read or write to the FILESTREAM data file using the FILE manipulationfunctions provided by the client library or the .NET programming language.So how does this differ from the traditional model of storing the BLOB data in the filesystem and storing the path to the data file in a relational table? How does SQL Servermaintain transactional consistency between the relational and the FILESTREAM data?Well, when using the FILESTREAM streaming APIs, you are not accessing theFILESTREAM data directly; SQL Server opens the file handle and passes it to you throughits own wrapper APIs. This allows SQL Server to keep total control over the operationsand maintain transactional consistency between the relational data in the tables and theBLOB data in the FILESTREAM store.Accessing FILESTREAM data through the streaming APIs involves the steps below,irrespective of the client library or programming language you use.1. Start a transaction – You need a transaction to gain streaming access to theFILESTREAM data.2. Retrieve the path name and transaction context – You need to obtain a logical pathname to the FILESTREAM data file and a pointer to the current transaction contextto access the FILESTREAM data.3. Open the FILESTREAM data file – Gain access to the FILESTREAM data file eitherusing the .NET wrapper class or the Win32 FILESTREAM API.90