Day23 affairs*** (write a little bit first, and have time to add later)

The concept of affairs

• (1) What is a transaction?
  The role of the transaction: to ensure that a set of operations either succeed or fail at the same time during the process.
  1. Purpose: Keep the data consistent in the case of multiple concurrency.
  2. Purpose: If an incorrect operation occurs, it can be corrected
• (2) What are the characteristics of transactions?
  Transaction characteristics (ACID)
  atomicity: A transaction is the smallest unit of execution and cannot be divided.
  Consistency: The data must be consistent before and after the transaction is specified.
  Persistence: Once the transaction is committed or rolled back, the lasting change in the database.
  Isolation: Isolation is used to ensure that multiple users operate the same data in the database without interference.

Concurrency issues that occur when transactions are not isolated

• (1) Dirty read: One transaction reads another transaction and has not yet submitted data.
• (2) Non-repeatable read: The data read multiple times in a transaction is inconsistent. One transaction reads the modified data of another transaction.
• (3) Phantom read (virtual read): A transaction reads the data of insert.

Transaction term

• What is the isolation level of a transaction?

Isolation level	Dirty read	Non-repeatable	Phantom reading
Read uncommitted	may	may	may
Read committed	impossible	may	may
Repeatable read	impossible	impossible	可能
Serializable	impossible	impossible	impossible