Saturday, April 23, 2016

How SQL Server handles our requests for modifications

This is not something new with latest Microsoft SQL Server but this is still being discussed and it is unknown or unclear to many of database developers. While I was discussing transaction log of SQL Server database with few, as a part of it, how SQL Server accepts our requests and modifies records was discussed. Thought it is useful to everyone, hence making a post on it;

What really happens when we send a request to SQL Server? It can be an update or a delete. Request might be related to one record or many records. Have a look on below image;

This starts with the request. Either using an application or connecting directly to SQL Server using something like SSMS, we send the request. Once SQL Server received the request, it checks data pages related to the records. If data pages required are not in the memory (or buffer cache), it loads relevant data pages from the disk to memory. Then, remember, it modifies records in pages that are in the memory, not pages in the disk. That is what 1st and 2nd steps in the image explains.

Third step is, updating the transaction log in the disk. Once the page (or pages) in the memory are modified, they become dirty pages. Then SQL Server writes redo and undo information to the log file. During this update, pages related are locked until the transaction log is completely updated.

Once the log is updated, the acknowledgement is sent to the application. Note that, even though the data files are not updated, we receive a message saying records are successfully updated. But we do not want worry, even something happen after we received the message, SQL Server can recover committed records, making sure durability which is one of the properties of the transaction, is satisfied with SQL Server.

Later, after one ore more transactions, a process called Checkpoint writes all dirty pages back to the disk, updating data files. This is how SQL Server handles our update requests.

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