Relational databases manage concurrent access through a combination of locking mechanisms, isolation levels, and transaction management. When multiple users or applications attempt to access database resources simultaneously, the database has to ensure data integrity and consistency. To achieve this, it employs locking, where a user’s access to certain resources (like rows, tables, or databases) can be restricted. For instance, if one transaction is updating a record, other transactions trying to access that record might be put on hold until the first transaction completes, preventing any conflicting changes.
Different isolation levels provide varying degrees of protection against concurrency issues, such as dirty reads, non-repeatable reads, and phantom reads. For example, if a transaction operates at the "Read Committed" isolation level, it will only read data that has been committed, ensuring that it does not see changes that are not finalized. On the other hand, a higher isolation level like "Serializable" provides a stricter guarantee that transactions will not interfere with each other, but it can lead to increased contention and reduced performance because more locks are needed and transactions may need to be retried if they conflict.
Transaction management is crucial for maintaining data consistency in concurrent environments. Databases typically implement a mechanism called the two-phase commit (2PC) to ensure that all parts of a distributed transaction either succeed or fail as a unit. This means that if any part of the transaction fails, the entire operation rolls back, ensuring that the database remains in a consistent state. Overall, the combination of locking, isolation levels, and transaction management allows relational databases to effectively handle concurrent access, ensuring that multiple users can work without disrupting each other's transactions.