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This article is about how different isolation levels in the database have different effects on transaction processing. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

In the database operation, in order to effectively ensure the correctness of concurrent reading data, the transaction isolation level is proposed. In the standard SQL specification, four transaction isolation levels are defined, and different isolation levels handle transactions differently.

General databases, including four isolation levels, different isolation levels deal with transactions differently.

Unauthorized read

Also known as read uncommitted (Read Uncommitted): dirty reads are allowed, but updates are not allowed to be lost. If one transaction has already started writing data, the other transaction does not allow simultaneous write operations, but allows other transactions to read this row of data. This isolation level can be achieved through an exclusive write lock.

Authorized read

Also known as read commit (Read Committed): allows non-repeatable reads, but does not allow dirty reads. Transactions that read data allow other transactions to continue to access the row's data, but uncommitted write transactions will prevent other transactions from accessing the row. It can be achieved through "instant shared read lock" and "exclusive write lock".

Repeatable read (Repeatable Read)

Non-repeatable and dirty reads are prohibited, but phantom reading data may sometimes occur. Transactions that read data will prohibit writing transactions (but read transactions are allowed), and write transactions will prohibit any other transactions. Can be achieved through "shared read lock" and "exclusive write lock".

Serialization (Serializable)

Provide strict transaction isolation. It requires transaction serialization execution, and transactions can only be executed one after another, not concurrently. Transaction serialization cannot be achieved through row-level locks alone, and other mechanisms must be used to ensure that newly inserted data is not accessed by the transaction that has just performed the query operation.

The higher the isolation level, the better the integrity and consistency of the data, but the greater the impact on concurrency performance. For most applications, priority can be given to setting the isolation level of the database system to Read Committed. It can avoid dirty reading and has good concurrency performance. Although it can lead to concurrency problems such as unrepeatable reads, phantom reads, and second-class missing updates, in individual cases where such problems may occur, the application can use pessimistic or optimistic locks.

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