How does a database ensure consistency during concurrent transactions?

A database ensures consistency during concurrent transactions through mechanisms like locking, timestamping, and using transaction logs.

Databases use a variety of techniques to maintain consistency during concurrent transactions. One of the most common methods is locking. In this method, the database system locks the data items that a transaction is accessing, preventing other transactions from modifying them until the first transaction is complete. This ensures that transactions do not interfere with each other and that the database remains in a consistent state. There are two types of locks: shared locks and exclusive locks. Shared locks allow multiple transactions to read (but not write to) the same data item simultaneously, while exclusive locks allow only one transaction to read and write a data item.

Another method used to ensure consistency is timestamping. Each transaction is given a unique timestamp when it starts. The database system uses these timestamps to determine the order in which transactions should access data items. If a transaction tries to access a data item that has been accessed by a later transaction, the database system will roll back the transaction and restart it with a new timestamp. This prevents conflicts and ensures that all transactions are executed in a serialisable manner.

Transaction logs are another tool used by database systems to maintain consistency. A transaction log is a record of all changes made to the database by each transaction. If a transaction is interrupted (for example, due to a system crash or power failure), the database system can use the transaction log to roll back the transaction and restore the database to its previous state. This ensures that the database remains consistent even in the event of system failures.

In addition to these techniques, database systems also use protocols such as the two-phase commit protocol to ensure consistency across multiple databases. In this protocol, a transaction is divided into two phases: a prepare phase, in which the transaction is executed and the changes are logged, and a commit phase, in which the changes are actually applied to the database. This ensures that all databases involved in the transaction agree on the outcome before any changes are made, maintaining consistency across all databases.