Serializability

In concurrency control of databases,transaction processing (transaction management), and various transactional applications (e.g., transactional memory and software transactional memory), both centralized and distributed, a transaction schedule is serializable if its outcome (e.g., the resulting database state) is equal to the outcome of its transactions executed serially, i.e. without overlapping in time. Transactions are normally executed concurrently (they overlap), since this is the most efficient way. Serializability is the major correctness criterion for concurrent transactions' executions. It is considered the highest level of isolation between transactions, and plays an essential role in concurrency control. As such it is supported in all general purpose database systems. Strong strict two-phase locking (SS2PL) is a popular serializability mechanism utilized in most of the database systems (in various variants) since their early days in the 1970s.

Serializability theory provides the formal framework to reason about and analyze serializability and its techniques. Though it is mathematical in nature, its fundamentals are informally (without mathematics notation) introduced below.

A database transaction is a specific intended run (with specific parameters, e.g., with transaction identification, at least) of a computer program (or programs) that accesses a database (or databases). Such a program is written with the assumption that it is running in isolation from other executing programs, i.e., when running, its accessed data (after the access) are not changed by other running programs. Without this assumption the transaction's results are unpredictable and can be wrong. The same transaction can be executed in different situations, e.g., in different times and locations, in parallel with different programs. A live transaction (i.e., exists in a computing environment with already allocated computing resources; to distinguish from a transaction request, waiting to get execution resources) can be in one of three states, or phases:

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