What is the difference between PHP pessimistic locking and optimistic locking?

Pessimistic Lock

As the name suggests, it is very pessimistic. Every time you go to get data, I think others will modify it, so I lock it every time I get the data. If others want to get the data, they will be blocked until they get the lock.

Traditional relational databases use many such lock mechanisms, such as row locks, table locks, read locks, write locks, etc., which are all locked before operations. It refers to a conservative attitude towards data being modified by the outside world (including other current transactions of the system and transaction processing from external systems). Therefore, the data is kept in a locked state during the entire data processing process.

The implementation of pessimistic locking often relies on the locking mechanism provided by the database (only the locking mechanism provided by the database layer can truly guarantee the exclusivity of data access. Otherwise, even if the locking mechanism is implemented in this system, it cannot Guarantee that data will not be modified by external systems).

Optimistic Lock

As the name suggests, it is very optimistic. Every time you go to get the data, you think that others will not modify it. Therefore, it will not be locked, but when updating, it will be judged whether others have updated the data during this period. You can use mechanisms such as version numbers.

Optimistic locking is suitable for multi-read application types, which can improve throughput. If the database provides a mechanism similar to write_condition, it actually provides optimistic locking.

Summary

Both locks have their own advantages and disadvantages. One cannot be considered better than the other. Optimistic locking is suitable for write comparisons. In rare cases, that is, when conflicts rarely occur, this can save the cost of locks and increase the overall throughput of the system. But if conflicts often occur, the upper-layer application will continue to retry, which actually reduces performance, so in this case, pessimistic locking is more appropriate.

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