Locking Non-Existent InnoDB Rows for Concurrent Insert Prevention

In database management systems, ensuring data integrity and preventing concurrent access conflicts is crucial. In InnoDB, the challenge arises when attempting to search and insert a non-existent row without interruptions.

The Traditional Approach

Initially, some may suggest utilizing the LOCK IN SHARE MODE and FOR UPDATE keywords on existing rows. However, this method falls short when attempting to lock non-existent rows.

Limitations of SELECT ... FOR UPDATE

While SELECT ... FOR UPDATE prevents concurrent inserts on existing records, it fails when attempting to lock non-existent rows. Concurrent sessions can still lock the same non-existent row using SELECT ... FOR UPDATE, leading to race conditions and potential deadlocks or key errors during insert operations.

A Better Solution

Due to MySQL's lack of a native locking mechanism for non-existent rows, alternative approaches are necessary. Two viable options include:

1. Semaphore Tables: Semaphore tables provide a way to track and control concurrent access to database resources, including row insertions. By creating a semaphore row for each potential non-existent row, concurrent transactions can acquire a lock on the semaphore row before attempting to insert a new row.
2. Table-Level Locking: In scenarios where row-level locking is not feasible, locking the entire table may be necessary. This ensures that no concurrent transactions can modify the table, preventing race conditions and ensuring reliable data insertions.

Conclusion

To prevent race conditions and ensure the integrity of database insertions on non-existent rows, utilizing semaphore tables or locking the entire table offers effective workarounds albeit with potential performance implications.