The ACID properties are one of the cornerstones of database theory. ACID defines four properties that must be present if a database is considered reliable: Atomicity, Consistency, Isolation, and Durability. While all four properties are important, isolation in particular is interpreted with the most flexibility. Most databases provide a number of isolation levels to choose from, and many libraries today add additional layers which create even more fine-grained degrees of isolation. The main reason for this wide range of isolation levels is that relaxing isolation can often result in scalability and performance increases of several orders of magnitude.
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