• A database transaction consists of one or more statements.

Beginning of transaction

End od transaction

commit statement

Rollback statement

•A COMMIT or ROLLBACK statement is issued •A DDL statement, such as CREATE is issued • The user exits the DBMS •A machine fails or the system crashes

• Process of managing simultaneous operations on the database without having them interfere with one another

allow multiple access

no interference

consistency

Lost update problem

Uncommitted data problem

Inconsistent analysis problem

Special DBMS program: establishes order of operations within which concurrent transactions are executed

Lock Granularity , Lock Types Two phase Locking to ensure serializability, Deadlocks

1. Database‐level lock 2. Table‐level lock 3. Page‐level lock 4. Row‐level lock 5. Field‐level lock

Lock type: Binary lock & Shared/exclusive lock

Transactions are accumulated over a period and processed as a single unit, or batch.

Transactions are processed immediately as they occur without any delay (the records in the system always reflect the current status.

This system features random and rapid input of transaction and user defined/ demanded direct access to databases/content when needed.

All changes to data are performed as if they are a single operation. That is, all the changes are performed, or none of them are.

Data is in a consistent state when a transaction starts and when it ends.

The intermediate state of a transaction is invisible to other transactions. As a result, transactions that run concurrently appear to be serialized.

The intermediate state of a transaction is invisible to other transactions. As a result, transactions that run concurrently appear to be serialized.

means: A transaction is essentially a single event that changes something.

Main operations: • Read(A): reads the value of A from the database and stores it in a buffer in main memory. Read(A) ‐> R(A • Write (A): writes the value back to the database from buffer. Write(A) ‐> W(A)