Database transactions (i)

Overview

A database transaction, which is a series of operations performed as a single logical unit of work (Transaction). Transaction processing ensures that data-oriented resources are not permanently updated unless all operations within the transactional unit are completed successfully. By combining a set of related actions into a single unit that either succeeds or all fails, you can simplify error recovery and make your application more reliable. To become a transaction, a logical unit of work must satisfy the so-called acid (atomicity, consistency, isolation, and persistence) attributes.

Operation Flow

Imagine a transaction of online shopping where the payment process includes at least the following steps of database operations:
· Update the inventory information of the products purchased by customers
· Save customer payment information-may include interactions with the banking system
· Generate orders and save to database
· Update user-related information, such as number of purchases, etc.
Under normal circumstances, these operations will proceed smoothly, the final transaction succeeds, and all database information related to the transaction is also successfully updated. However, if there is a mistake in any part of the process, such as an exception when updating commodity inventory information, the customer's bank account is under-deposited, and so on, will cause the transaction to fail. Once the transaction fails, all the information in the database must remain unchanged before the transaction, such as failure to update the user information in the last step, resulting in the failure of the transaction, then it must be ensured that the failed transaction does not affect the state of the database-the inventory information has not been updated, the user has not paid, and the order has not been generated. Otherwise, the database information will be chaotic and unpredictable.
Database transactions are the technology used to ensure the smoothness and predictability of transactions in this situation.

ACID Properties of database transactions

Atomicity (Atomic)
The transaction must be an atomic unit of work, either all executed or not executed for its data modification. Typically, the operations associated with a transaction have a common purpose and are interdependent. If the system only performs a subset of these operations, it may break the overall goal of the transaction. Atomicity eliminates the possibility of a system processing a subset of operations.
Consistency (consistent)
When a transaction is complete, you must keep all data in a consistent state. In a related database, all rules must be applied to transaction modifications to maintain the integrity of all data. At the end of the transaction, all internal data structures, such as B-tree indexes or doubly linked lists, must be correct. Some of the responsibility for maintaining consistency is borne by application developers who must ensure that all known integrity constraints are enforced by the application. For example, when developing an application for transfer, avoid arbitrarily moving the decimal point during the transfer process.
Isolation (insulation)
Modifications made by concurrent transactions must be isolated from modifications made by any other concurrent transaction. The state in which the data is located when the transaction is viewing the data, either when another concurrent transaction modifies its state or after another transaction modifies it, and the transaction does not view the data in the middle state. This is called serializable because it is able to reload the starting data and replay a series of transactions so that the state at the end of the data is the same state as the original transaction execution. The highest isolation level is obtained when the transaction is serializable. At this level, the results obtained from a set of transactions that can be executed in parallel are the same as those obtained by running each transaction continuously. Because high isolation limits the number of transactions that can be executed in parallel, some applications reduce the isolation level in exchange for greater throughput. Prevent data loss
Persistence (Durability)
After the transaction is complete, its effect on the system is permanent. This modification will persist even if a fatal system failure occurs.

Database transactions (i)