Timestamp Ordering

The Timestamp Ordering (TO) algorithm is a concurrency control technique used in database systems to ensure serializability of transactions based on their timestamps. It aims to maintain the order of transactions in a way that is equivalent to their timestamp values. Here's a detailed explanation of the key components and variations of the TO algorithm:

1. Basic Idea of TO:

   • TO orders transactions based on their timestamps.

   • A schedule is considered serializable if it is equivalent to the order of transactions sorted by their timestamps.

2. Timestamps for Database Items:

   • TO associates two timestamp (TS) values with each database item X:

     • read_TS(X): The read timestamp of X is the largest timestamp among all transactions that have successfully read X.

     • write_TS(X): The write timestamp of X is the largest timestamp among all transactions that have successfully written X.

3. Basic Timestamp Ordering (TO):

   • When a transaction T issues a read_item(X) or write_item(X) operation, the algorithm compares the timestamp of T with read_TS(X) and write_TS(X) to ensure that the timestamp order of execution is not violated.

   • If the timestamp order is violated, transaction T is aborted and resubmitted as a new transaction with a new timestamp.

   • Cascading Rollback: If T is aborted, any transaction T1 that used a value written by T must also be rolled back, and this process continues, leading to cascading rollbacks.

4. Conditions for Conflict Detection:

   • The algorithm checks conflicting operations in two cases:

     a. For Write Operations (write_item(X)):

     • If read_TS(X) > TS(T) or write_TS(X) > TS(T), T is aborted and the operation is rejected.

     • This ensures that a younger transaction with a higher timestamp has not already read or written X, violating the timestamp ordering.

     • If the conditions are not met, T executes the write_item(X) operation and updates write_TS(X).

     b. For Read Operations (read_item(X)):

     • If write_TS(X) > TS(T), T is aborted and the operation is rejected.

     • This ensures that a younger transaction has not already written X, making T's read operation invalid.

     • If the condition is met, T executes the read_item(X) operation and updates read_TS(X).

   • These checks ensure that transactions follow the timestamp order, and if not, the violating transaction is aborted.

5. Conflict Serializability:

   • Schedules produced by basic TO are guaranteed to be conflict serializable, similar to the Two-Phase Locking (2PL) protocol.

   • However, some schedules may not be recoverable due to cascading rollbacks.

6. Strict Timestamp Ordering (TO):

   • This variant ensures both strict (for easy recoverability) and conflict serializability.

   • If TS(T) > write_TS(X), a read or write operation by T on X is delayed until the transaction that wrote X has committed or aborted.

   • This prevents anomalies in the schedule and ensures strictness.

7. Thomas's Write Rule:

   • A modification of basic TO that does not enforce conflict serializability strictly.

   • It rejects fewer write operations:

     1. If read_TS(X) > TS(T), T is aborted.

     2. If write_TS(X) > TS(T), the write operation is ignored, but processing continues. This is because the write operation is considered outdated.

     3. If neither condition occurs, T executes the write_item(X) operation and updates write_TS(X).

In summary, the Timestamp Ordering (TO) algorithm orders transactions based on timestamps, ensuring that the order of execution is consistent with these timestamps. Basic TO enforces conflict serializability but can lead to cascading rollbacks. Strict TO prevents anomalies, while Thomas's Write Rule provides flexibility but sacrifices strict conflict serializability. TO is effective in avoiding deadlocks but may lead to cyclic restart (starvation) if transactions are continually aborted and restarted. The choice of TO variant depends on specific database system requirements and trade-offs.