Understanding ACID Properties
It is important to realize that there is more to transactions than simply coordinating all database updates in a single logical unit of work. When dealing with databases and transaction, you will frequently hear the word "ACID". No wonder no one gets any work done! In our industry ACID is an acronym that stands for "Atomicity, Consistency, Isolation, and Durability". These four words describe the basic properties of a transaction. However, what they really refer to is the level of consistency and integrity of your database. Atomicity means that all database update operations will be performed in a single unit of work; either all updates will all be applied, or no updates will be applied. This is one of the ways you achieve database consistency. However, atomicity is only part of the story. Consistency means that database referential integrity rules will be applied during the course of the transaction, meaning that you cannot insert a detail row without first inserting the header row. Isolation means that no one else can see (or use) the data you are adding or changing until the transaction is complete. This property is just as important as atomicity - you can seriously affect the overall level of database consistency and integrity without transaction isolation. Durability, the last property, means that once you commit a transaction, you are guaranteed that the database updates you applied will survive a system crash, regardless of the technology or methods used by the database vendor to manage updates.
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