What do database transactions refer to?

Database transactions refer to: a series of operations performed as a single logical unit of work, either completely executed or not executed at all; simply put, a transaction is a unit of concurrency control and is user-defined A sequence of operations.

You use Alipay to go to the supermarket to buy things, and transfer 100 yuan to the supermarket. In fact, this is a two-step process:

• The first step is to subtract 100 from your account in the Alipay database;

• The second step is to add 100 yuan to the supermarket’s Alipay account and the transaction is completed;

But if the first step is completed, but before the second step is executed, what will happen if there is a power outage?

It will happen that your account has been reduced by 100 yuan, but the amount in the supermarket account has not changed. Doesn't this cause a problem and there is no need to fight?

In order to solve this data consistency problem, database transactions came into being.

Transaction refers to a series of operations that are performed as a single logical unit of work, either completely or not at all. Simply put, a transaction is a unit of concurrency control and a user-defined sequence of operations.

It is the unit for the database to maintain data consistency. It changes the database from a consistent state to a new consistent state. To put it simply, if a set of processing steps either all occur or none of the steps are executed, we Call this set of processing steps a transaction. This ensures that the data is always in a consistent state without damaging the integrity and reliability of the data. After a transaction is executed, the DBMS will automatically check the consistency of the data in the database.

Transactions have 4 characteristics, namely atomicity, consistency, isolation and durability. These 4 characteristics are usually referred to as ACID for short.

1. Atomicity

A transaction is a complete operation. The elements of a transaction are indivisible (atomic). All elements in the transaction must be committed or rolled back as a whole. If any element in the transaction fails, the entire transaction fails.

Take the bank transfer transaction as an example. If the transaction is submitted, the data of the two accounts will be updated. If for some reason the transaction terminates before successfully updating both accounts, the balances of both accounts will not be updated, modifications to any account balances will be undone, and the transaction cannot be partially committed.

2. Consistency

When the transaction is completed, the data must be in a consistent state. That is, the data stored in the database is in a consistent state before the transaction begins. During an ongoing transaction, the data may be in an inconsistent state, for example, the data may be partially modified. However, when the transaction completes successfully, the data must be returned to a known consistent state again. Modifications made to data through transactions cannot damage the data, or transactions cannot leave the data storage in an unstable state.

Take bank transfer transactions as an example. Before the transaction begins, the total of all account balances is in a consistent state. During the course of the transaction, the balance of one account is reduced, while the balance of the other account has not been modified. Therefore, the total of all account balances is inconsistent. After the transaction is completed, the total account balance is restored to a consistent state again.

3. Isolation

All concurrent transactions that modify data are isolated from each other, which indicates that the transaction must be independent and it should not depend on or affect other transactions in any way. A transaction that modifies data can access the data before another transaction that uses the same data begins, or after another transaction that uses the same data ends.

In addition, when a transaction modifies data, if any other process is using the same data at the same time, the modifications to the data cannot take effect until the transaction is successfully committed. The transfer between Zhang San and Li Si and the transfer between Wang Wu and Zhao Er are always independent of each other.

4. Durability

The durability of a transaction means that regardless of whether the system fails, the results of the transaction are permanent.

After a transaction completes successfully, the changes it makes to the database are permanent, even if the system fails. That is to say, once the transaction is committed, any changes made to the data by the transaction will be permanently retained in the database.

The ACID principle of transactions ensures that a transaction is either successfully committed or failed and rolled back, one of the two. Therefore, its modifications to the transaction are recoverable. That is, when a transaction fails, its data modifications will be restored to the state before the transaction was executed.

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