View Serializability

View serializability is a concept in database management systems (DBMS) that ensures the correctness and consistency of concurrent transactions. It guarantees that the result of executing multiple transactions concurrently is equivalent to some serial execution of those transactions, maintaining the integrity of the database.

In a database system, multiple transactions can execute concurrently, meaning they overlap in terms of their execution time. However, concurrent execution can lead to various problems, such as data inconsistency or incorrect results, if not properly managed. View serializability provides a way to control the interleaving of transactions to maintain correctness.

A serial execution of transactions refers to executing them one after another in a specific order. However, in a concurrent environment, transactions can interleave their operations. View serializability allows concurrent transactions to be scheduled in such a way that their interleaved operations produce the same result as if they were executed serially.

To determine whether a given schedule of transactions is view serializable, the following two conditions need to be satisfied:

  1. Conflict Serializability: This condition ensures that any conflicting operations between transactions are preserved in the schedule. Conflicting operations are those that refer to the same database object (e.g., reading and writing the same data item). If the schedule satisfies conflict serializability, it means that it preserves the order of conflicting operations between transactions.
  2. Equivalent Serializability: This condition ensures that the result produced by the concurrent schedule is equivalent to some serial execution of the transactions. In other words, the final database state should be the same as if the transactions were executed one after another in some order.

If both conflict serializability and equivalent serializability conditions are met, the schedule is view serializable. DBMSs use concurrency control techniques, such as locking or timestamp ordering, to enforce view serializability and maintain consistency while allowing concurrent execution of transactions.

By ensuring view serializability, DBMSs can provide a balance between concurrency and data integrity, allowing multiple transactions to execute concurrently while preserving the correctness of the overall system.

View Equivalent:

Two schedules S1 and S2 are said to be view equivalent if they satisfy the following conditions:

“Initial Read” typically refers to the first read operation performed on a data item in a transaction. In database systems, transactions often involve reading and modifying data items stored in the database. The initial read refers to the first access of a data item’s value within a specific transaction.

During the execution of a transaction, the initial read operation retrieves the current value of a data item from the database. This initial value serves as a baseline for subsequent operations within the transaction, such as updates or further reads. The transaction can use this initial read value to perform computations, make decisions, or compare it with other data.

The initial read is essential for maintaining the consistency and correctness of the transaction. It ensures that the transaction operates on the most up-to-date value of the data item at the start of its execution. Any subsequent reads or updates within the transaction are based on this initial value, and changes made by other concurrent transactions are typically isolated until the transaction commits or rolls back.

By capturing the initial read value, the database system can enforce isolation and guarantee the consistency of transactions. This helps prevent conflicts and data inconsistencies that could arise due to concurrent access and modification of shared data.

2. Updated Read:

In schedule S1, if Ti is reading A which is updated by Tj then in S2 also, Ti should read A which is updated by Tj.

DBMS View Serializability

Above two schedules are not view equal because, in S1, T3 is reading A updated by T2 and in S2, T3 is reading A updated by T1.

3. Final Write:

In the context of databases, “final write” typically refers to the last write operation performed on a data item within a transaction before it commits.

During the execution of a transaction, there might be multiple write operations on various data items. The final write operation represents the last modification made to a particular data item within the transaction before it is finalized and the changes are committed to the database.

The final write operation is significant because it determines the updated value of the data item that will be visible to other transactions once the current transaction commits. The changes made by the final write operation become part of the consistent and persistent state of the database after the transaction completes.

Once the final write operation is performed, the transaction typically proceeds to commit its changes. The commit operation ensures that the modifications made by the transaction are made durable and visible to other concurrent transactions. After the commit, the updated value resulting from the final write operation becomes accessible to subsequent transactions that read the data item.

The final write operation is crucial for maintaining data integrity and ensuring that the changes made by a transaction are reflected consistently in the database. It marks the completion of the transaction’s write operations and signifies the point at which the transaction’s modifications become permanent and visible to other transactions.