Logging and Transaction of SQL Server

SQL Server ensures that any change to the data is ACID-compliant, i.e., it uses transactions to ensure that any operation either totally completes or is undone if fails, but never leave the database in an intermediate state. Using transactions, a sequence of actions can be grouped together, with the guarantee that either all actions will succeed or none will. SQL Server implements transactions using a write-ahead log. Any changes made to any page will update the in-memory cache of the page, simultaneously all the operations performed will be written to a log, along with the transaction ID which the operation was a part of. Each log entry is identified by an increasing Log Sequence Number (LSN) which ensure that no event overwrites another. SQL Server ensures that the log will be written onto the disc before the actual page is written back. This enables SQL Server to ensure integrity of the data, even if the system fails. If both the log and the page were written before the failure, the entire data is on persistent storage and integrity is ensured. If only the log was written (the page was either not written or not written completely), then the actions can be read from the log and repeated to restore integrity. If the log wasn't written, then also the integrity is maintained, even though the database is in a state when the transaction as if never occurred. If it was only partially written, then the actions associated with the unfinished transaction are discarded. Since the log was only partially written, the page is guaranteed to have not been written, again ensuring data integrity. Removing the unfinished log entries effectively undoes the transaction. SQL Server ensures consistency between the log and the data every time an instance is restarted.

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