Thomas Write Rule in DBMS

The Thomas Write Rule (TWR) is an extension of the Basic Timestamp Ordering (TO) Protocol used for concurrency control in Database Management Systems (DBMS).

• Developed by: R. H. Thomas (1979)
• Type: Timestamp-based concurrency control
• Goal: Reduce unnecessary rollbacks while maintaining view serializability

Note: It allows higher concurrency by ignoring obsolete writes instead of aborting transactions, as long as doing so doesn’t affect the final database state.

Basic Idea

In the Basic Timestamp Ordering (TO) protocol, if a transaction tries to write an outdated value, it is aborted. However, in the Thomas Write Rule, instead of aborting, the outdated write is ignored, allowing the transaction to continue.

Note: This helps avoid unnecessary aborts and increases system throughput.

Notations:

Write Operation Conditions in TWR

1. Abort the Transaction:

• If R_TS(X) > TS(T)
• A newer transaction has already read X, so writing now would violate serializability.
• Action: Abort and rollback T.

2. Ignore Outdated Writes (Allow Execution Without Writing):

• If W_TS(X) > TS(T)
• A newer write has already updated X. Writing now is obsolete.
• Action: Ignore the write; continue the transaction.

3. Execute Write Operation (Normal Execution):

• If neither condition above is true
• Action: Execute W_item(X) and update W_TS(X) = TS(T)

Example - Ignoring an Outdated Write

The main improvement in Thomas Write Rule is that it ignores obsolete (outdated) writes instead of rejecting transactions. This happens when a newer transaction has already updated a value, making the older transaction's write operation unnecessary. Let's consider two transactions:

• TS(T1) = 30, TS(T2) = 20
• T1 arrives after T2

Schedule Execution:

• In Basic Timestamp Ordering, T1 would abort because its write is considered outdated.
• In Thomas Write Rule, outdated writes are ignored instead of rollback, allowing T1 to continue execution.

Consider the partial schedule given below - Obsolete Writes are hence ignored in this rule which is in accordance with the 2^nd protocol.

Note: It seems to be more logical as users skip an unnecessary procedure of restarting the entire transaction. This protocol is just a modification to the Basic TO protocol.

Example - View-Serializable but Not Conflict-Serializable

• TWR allows such schedules because it ignores outdated writes.
• The schedule is view-serializable but not conflict-serializable.
• Hence, Thomas Write Rule guarantees view serializability, not conflict serializability.

Comparison between Basic TO and Thomas Write Rule