What lock was used in practice for transaction processing?

In practice, database-level locks are often used for transaction processing, and in Java, corresponding synchronization mechanisms are employed.

In the context of Java and transaction management, the following are commonly used:

• Pessimistic Locking — locking records or tables for the duration of a transaction to prevent conflicts. In JPA/Hibernate, this is implemented through LockMode.PESSIMISTIC_WRITE and similar.

• Optimistic Locking — assumes conflicts are rare and checks the record version upon saving. In Java, this is implemented via version fields (@Version in JPA).

For synchronization in a multithreaded environment, the following are often used:

• ReentrantLock — a more flexible lock than synchronized, with support for timeouts and interruption.
• synchronized — the basic locking mechanism in Java.

The choice depends on performance requirements and load characteristics. In database transactions, it is usually preferable to use the built-in locking mechanisms of the DBMS, while in Java, optimistic locking is used to reduce contention.