Schedules and Recovery

In database systems, transaction schedules determine how and when operations from different transactions are executed. Different types of schedules have different implications for how the system handles recovery and rollback.

Here's an explanation for each:

Nonrecoverable Schedule: In a nonrecoverable schedule, a transaction that has made some changes and committed may not be able to be rolled back. This typically happens when another transaction has already accessed the changed data and made further changes to the database. Since the database state has moved ahead based on the changes made by the committed transaction, rolling back the committed transaction may lead to inconsistencies and thus is not possible. Nonrecoverable schedules are dangerous and are generally not allowed in real systems due to the inability to recover to a consistent state after a failure.
Cascading Schedule: In a cascading schedule, one transaction's rollback can trigger the rollback of one or more other transactions. This situation can occur in a schedule where transactions are allowed to read uncommitted data (known as "dirty read"). If a transaction that has modified some data items fails and needs to roll back, all transactions that have read those data items (that are 'dirty') must also be rolled back to ensure consistency. This leads to a cascade effect of rollbacks - hence the name 'cascading schedule'. Cascading rollbacks can be prevented by using stricter levels of isolation such as 'Repeatable Read' or 'Serializable'.
Strict Schedule: In a strict schedule, a transaction is not allowed to read or write an item of data that is being held by another transaction that has not committed yet. As a result, if a transaction needs to be rolled back, it won't affect any other transactions since no other transaction can have any sort of dependency on the rolled back transaction. This makes the rollback process straightforward and avoids cascading rollbacks. Strict schedules can be achieved by implementing 'Strict Two Phase Locking' protocol where a transaction holds all its locks till it commits or aborts, thus ensuring no other transaction can read or write its data.

Each of these types of schedules represents different trade-offs between performance (how quickly transactions can be executed and how much concurrency is allowed) and safety (the ability to recover from failures and maintain consistency).