Durability is the property that makes sure that transactions are permanently stored and do not disappear or are erased by accident, even during a database crash. It is accomplished by storing all transactions into a non-volatile storage medium.
Durability is part of the ACID acronym, which was popularized by Theo Harder and Andreas Reuter in 1983. It stands for atomicity, consistency, isolation, and durability. ACID is the set of qualities that guarantees the reliable processing, handling, and storage of all database transactions.
ACID is a set of necessary properties so that a group of instructions is considered as a transaction in a database management system.
A transaction is a set of orders that are executed, forming a unit of work. An example of a complex transaction is the transfer of funds from one account to another, which involves multiple individual transactions. If a system passes the ACID test, it means that it is reliable.
It means that the system allows for atomic operations. An atomic operation is one that is formed by smaller operations, and are considered as an indivisible package. They must all be executed correctly, or in the event that some of them cannot do so, the effect of those that have already been executed must not be noted, it must be undone as if all the operations had not been performed.
Even though atomicity is closely linked to the concept of a transaction of database management systems, they are not synonyms. While atomicity is a property, the transaction is the mechanism that DBMS uses to achieve atomicity.
This property ensures that only what can be finished begins. Therefore, those operations that will not break the database integrity rules and guidelines are executed. It states that any transaction will take the database from a valid state to another valid one.
It is a property that ensures that one operation cannot affect others. This ensures that two transactions on the same information are independent and do not generate any type of error.
The last property ensures that once the operation has been carried out, it will persist and cannot be undone even if the system fails.
Fulfilling these 4 requirements a database management system can be considered ACID-Compliant.
However, implementing ACID features is not that simple. The process of a transaction often requires a number of small changes to be made, including updating the indexes that are used in the system to speed up searches. This sequence of operations may fail for a number of reasons; for example, the system may not have any available space on its disk drives, or it may have exceeded its assigned CPU time.
It is difficult to guarantee ACID features in a network environment. Network connections can fail, or two users can use the same part of the database at the same time.
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