1. MySQL

Reference: juejin. Cn/post / 684490…

1.1 Isolation Level

In theory, when a transaction accesses data, other transactions should be queued, and when the transaction commits, other transactions can continue to access the data. However, this has a significant impact on performance, so the database designer proposes various isolation levels to maximize the system’s ability to process transactions concurrently, but this is achieved at the expense of some isolation

  • READ UNCOMMITTED: A transaction reads data modified by another UNCOMMITTED transaction
  • READ COMMITTED: A transaction can only READ data COMMITTED by another transaction, and each time another transaction changes and commits the data, the transaction can query the latest value. This isolation level is called unrepeatable reads
  • REPEATABLE READ (REPEATABLE degrees) : a transaction can only be READ another has submitted the affairs of the modified data, but the first READ after a record, even if the other transactions to modify the value of the record and submit, READ this again after the transaction record, reading is still the first READ value, rather than READ different data every time