Tuesday, July 2, 2013

When is "ACID" ACID? Rarely

Many databases today differentiate themselves from their NoSQL counterparts by claiming to support "100% ACID" transactions or "guaranteeing strong consistency (ACID)." In reality, few of these databases—including traditional “big iron” systems like Oracle—provide formal ACID guarantees, even when they claim to do so.

The textbook definition of ACID Isolation is serializability (e.g., Architecture of a Database System, Section 6.2), which states that the outcome of executing a set of transactions should be equivalent to some serial execution of those transactions. This means that each transaction gets to operate on the database as if it were running by itself, which ensures database correctness, or consistency. A database with serializability (”I” in ACID), provides arbitrary read/write transactions and guarantees consistency (”C” in ACID), or correctness, of the database. Without serializability, ACID, particularly consistency, is generally1 not guaranteed

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