Concurrency Control

DML processing converts interaction with sets of tuples to a sequence of operations that read and write physical objects in the database

• Database objects
  • Individual record files
  • Records
  • Physical pages
  • Entire files
  • Predicates qualifying one or more records, parts of records, etc
• Transaction and recovery management
  • Ensuring correct and concurrent execution of operations for reading and writing physical objects originating from DML commands of client transactions
  • Guaranteeing that acknowledged client transaction commits are reliably recorded and that acknowledged client transaction aborts are reliably undone

Recoverable Schedules

• Allow committing only in order of the read-from dependency between transactions
  • Conflicting operations must be committed in the same order they are performed

Cascadeless Schedules

• Do not allow transactions to read uncommitted objects
  • Can only read a write if it is committed