What are the four characteristics of database transactions?

The four characteristics of database transactions are: 1. Atomicity, a transaction is the logical working unit of the database, and all operations in the transaction are either done or not; 2. Consistency, before and after a transaction is executed Everything must be in a consistent state thereafter; 3. Isolation; 4. Durability. Once a transaction is submitted, its changes to the data in the database should be permanent.

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If a database claims to support transaction operations, then the database It must have the following four characteristics:

1. Atomicity

Atomicity means that all operations included in the transaction either all succeed or all fail. Roll, this is the same concept as the function of transactions introduced in the previous two blogs. Therefore, if the transaction operation is successful, it must be fully applied to the database. If the operation fails, it cannot have any impact on the database.

2. Consistency

Consistency means that a transaction must transform the database from one consistency state to another consistency state, that is to say, a A transaction must be in a consistent state both before and after it is executed.

Take transfers as an example, assuming that the total money of user A and user B is 5,000, then no matter how the transfer is between A and B, or how many times the transfer is completed, the money of the two users will be the same after the transaction is completed. The total should add up to 5000, which is the consistency of the transaction.

3. Isolation

Isolation is when multiple users access the database concurrently, such as when operating the same table, the database is opened for each user The transaction cannot be interfered by the operations of other transactions, and multiple concurrent transactions must be isolated from each other.

To achieve such an effect: for any two concurrent transactions T1 and T2, from the perspective of transaction T1, T2 either ends before T1 starts, or starts after T1 ends, so Each transaction is unaware that other transactions are executing concurrently.

Regarding transaction isolation, the database provides multiple isolation levels, which will be introduced later.

4. Durability

Durability means that once a transaction is submitted, the changes to the data in the database are permanent, even if The operation of committing transactions will not be lost even if the database system encounters a failure.

For example, when we use JDBC to operate the database, after submitting the transaction method, the user is prompted that the transaction operation is completed. When our program execution is completed until we see the prompt, we can identify the transaction and submit it correctly, even if the database is at this time If a problem occurs, our transaction must be fully executed, otherwise we will see a major error that prompts the transaction to be completed, but the database did not execute the transaction due to a failure.

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