The Art of SQL Server FILESTREAM - Red Gate Software

Chapter 12: Planning, Configuration and Best Practicesincoming request, the web server tells the client whether the server has a newer versionof the file or not. The file needs to be downloaded only if the server has a newer versionor if the cache is expired.In the case of a FILESTREAM application, the data is stored and managed within thedatabase. The web server does not know about the modified status of the file. As a result,in most cases, the file is downloaded every time a request comes in. This adds considerableoverhead to the application, the database, and the web server.Having a time stamp or date column on the table that keeps track of modificationsdone on the FILESTREAM value will help you to implement cache control in yourFILESTREAM web handler. When a request comes in from a client, you can examine thecache control headers to see whether the file on the server is newer than the file on theclient. If the file has not been modified since the client downloaded it, your handler cansend back an HTTP 304 response which tells the client to use its local copy of the file.Use a computed column to retrieve the file sizeWhile working with FILESTREAM databases, you might need to know the size of theBLOB value stored in the FILESTREAM column. To avoid calculating the length of eachBLOB value every time you need to query, you might decide to create a column andupdate it at the time of writing the FILESTREAM value.An easier option is to create a PERSISTED computed column that stores the length of thefile. Listing 12-6 shows how to add a computed column that stores the size of the BLOBvalue stored in a FILESTREAM column.ALTER TABLE ItemsADD FileSize AS DATALENGTH(ItemImage) PERSISTED;Listing 12-6:Adding a computed column to store the size of the BLOB value in a FILESTREAM column.455