Difference between Recoverable Schedule, Cascadeless Schedule, and Strict Schedule

Recoverable Schedule:

A recoverable schedule is a type of schedule in which if a transaction fails or is aborted, it is still possible to recover the database to a consistent state. In other words, a recoverable schedule ensures that no updates are permanently stored in the database until all transactions have committed successfully.

Key points:
- In a recoverable schedule, the actions of an aborted transaction are undone by rolling back the changes made by that transaction.
- It guarantees that all committed transactions' updates are stored permanently in the database, even if a failure occurs.
- Recoverable schedules are achieved by utilizing two-phase locking or timestamp ordering techniques.

Cascadeless Schedule:

A cascadeless schedule is a type of schedule that avoids cascading rollback. Cascading rollback occurs when a transaction is rolled back, leading to other transactions being rolled back as well, which can result in a series of rollbacks throughout the system.

Key points:
- In a cascadeless schedule, if a transaction T1 reads a data item previously modified by an uncommitted transaction T2, T1 must wait until T2 commits or aborts before proceeding.
- Cascadeless schedules prevent the propagation of updates made by uncommitted transactions.
- These schedules are achieved by enforcing strict rules on the order in which transactions can read and write data items.

Strict Schedule:

A strict schedule is a type of schedule that ensures that the final state of the database after executing a set of transactions is the same as if the transactions were executed serially in some order.

Key points:
- In a strict schedule, the order in which transactions are executed does not affect the final state of the database.
- Strict schedules guarantee isolation and consistency by enforcing a strict order of execution among transactions.
- To achieve strict schedules, concurrency control mechanisms like serializability and strict two-phase locking are employed.

Summary:

- A recoverable schedule ensures that the database can be recovered to a consistent state even if a transaction fails or is aborted.
- A cascadeless schedule prevents cascading rollbacks by ensuring that a transaction waits until an uncommitted transaction finishes before reading its modified data.
- A strict schedule guarantees that the final state of the database is the same as if the transactions were executed serially, regardless of the order in which they are actually executed.