ACID properties

Folie 1

• ACID Properties ACID properties are an important concept for databases. The acronym stands for Atomicity, Consistency, Isolation, and Durability.
• The ACID properties of a DBMS allow safe sharing of data. Without these ACID properties, everyday occurrences such using computer systems to buy products would be difficult and the potential for inaccuracy would be huge. Imagine more than one person trying to buy the same size and color of a sweater at the same time -- a regular occurrence. The ACID properties make it possible for the merchant to keep these sweater purchasing transactions from overlapping each other -- saving the merchant from erroneous inventory and account balances.

Folie 2

• Atomicity The phrase "all or nothing" succinctly describes the first ACID property of atomicity. When an update occurs to a database, either all or none of the update becomes available to anyone beyond the user or application performing the update. This update to the database is called a transaction and it either commits or aborts. This means that only a fragment of the update cannot be placed into the database, should a problem occur with either the hardware or the software involved. Features to consider for atomicity:
• a transaction is a unit of operation - either all the transaction's actions are completed or none are
• atomicity is maintained in the presence of deadlocks
• atomicity is maintained in the presence of database software failures
• atomicity is maintained in the presence of application software failures
• atomicity is maintained in the presence of CPU failures
• atomicity is maintained in the presence of disk failures
• atomicity can be turned off at the system level
• atomicity can be turned off at the session level