Transactions

The solution to concurrency control is to treat the user’s queries as a single transaction. A transaction (or Xact) is a batch of queries that are extracted to line by line instructions.

• A sequence of read and writes to the DBMS

• Transactions are applied to the database once commited and ended or else they are aborted.

• Any logic (e.g., arithmetic) are computed within the transaction and not known to the DBMS.

Properties

The basic properties of a transaction are acronymed to be ACID:

• Atomicity

  All actions in the transaction happen or none of it happen.

  • Solved by requiring every transaction to have a COMMIT action at the end of their transaction. This line must be ran otherwise the transaction aborted.

• Consistency

  If the DB starts out consistent, it ends up consistent at the end of the transaction.

  • You can intepret consistency using the schema.

• Isolation

  Execution of each transaction is isolated from that of the others

  • Solved with a transaction manager/scheduler

• Durability

  If a transaction commits, then its effects persist.

  • Transactions are logged, if the transaction is not commited then use the log to undo. If the transaction is commited then use the log to redo.