Database Internals

Chapter 5. Transaction Processing and Recovery

In this book, we’ve taken a bottom-up approach to database system concepts: we first learned about storage structures. Now, we’re ready to move to the higher-level components responsible for buffer management, lock management, and recovery, which are the prerequisites for understanding database transactions.

A transaction is an indivisible logical unit of work in a database management system, allowing you to represent multiple operations as a single step. Operations executed by transactions include reading and writing database records. A database transaction has to preserve atomicity, consistency, isolation, and durability. These properties are commonly referred as ACID [HAERDER83]:

Atomicity

Transaction steps are indivisible, which means that either all the steps associated with the transaction execute successfully or none of them do. In other words, transactions should not be applied partially. Each transaction can either commit (make all changes from write operations executed during the transaction visible), or abort (roll back all transaction side effects that haven’t yet been made visible). Commit is a final operation. After an abort, the transaction can be retried.

Consistency

Consistency is an application-specific guarantee; a transaction should only bring the database from one valid state to another valid state, maintaining all database invariants (such as constraints, referential integrity, and others). Consistency is ...