Everything must happen as a single unit of work to be acceptable to both parties. Obviously,
it??™s unacceptable to the cashier if you just take the gum, and you wouldn??™t accept
not receiving change for your purchase.
When you make a change to the database, data isn??™t written directly to the data files.
Instead, the database server first writes your change to the transaction log on your
behalf. When the transaction is complete, it is marked as committed in the transaction
log (a step you often initiate by explicitly executing COMMIT TRAN). This does not mean that
the data is moved from the transaction log to the data file. A committed transaction only
means that all elements of the transaction have completed successfully. The buffer cache
may be updated, but not necessarily the data file.
Periodically, a checkpoint occurs. This tells SQL Server to ensure that all committed
transactions, whether held in the buffer cache or in the transaction log file itself, are written
to the appropriate data file(s). Checkpoints can occur in several ways:
??? Someone literally issues a CHECKPOINT command.
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