Serializability

Serializability is a concept used to determine the correctness of non-serial schedules and ensure database consistency. It identifies which concurrent schedules are valid and preserve the integrity of the database.

1. Serializable Schedules

A non-serial schedule involving 'n' transactions is serializable if it is equivalent to some serial schedule of the same 'n' transactions.
Serializable schedules maintain:

• Consistency
• Recoverability
• Cascadelessness
• Strictness

Types of Serializable Schedules

Conflict Serializable

A non-serial schedule that can be transformed into a serial schedule by swapping non-conflicting operations.

View Serializable

A schedule is view serializable if it is view equivalent to a serial schedule, i.e., it results in the same final state and reads the same data values.

2. Non-Serializable Schedules

Schedules that cannot be transformed into a serial schedule. These may or may not be:

• Consistent
• Recoverable

Irrecoverable Schedules

If a transaction reads data modified by another uncommitted transaction and commits before the other transaction commits, it's called an Irrecoverable Schedule.

Recoverable Schedules

If a transaction delays its commit until the transaction from which it read uncommitted data either commits or rolls back, it's called a Recoverable Schedule.

Types of Recoverable Schedules

1. Cascading Schedule (Cascading Rollback / Cascading Abort)

Occurs when the failure of one transaction leads to rollback of several dependent transactions.

2. Cascadeless Schedule

Prevents cascading rollbacks by disallowing reads from uncommitted transactions.

3. Strict Schedule

A stricter form of cascadeless schedules — no transaction is allowed to read or write a data item until the last transaction that wrote it is committed or aborted.