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In the Basic Timestamp Ordering (TO) protocol, if a transaction tries to write an outdated value, it is aborted. However, in the Thomas Write Rule, instead of aborting, the outdated write is ignored, allowing the transaction to continue.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:
| Symbol | Meaning |
|---|---|
| TS(T) | Timestamp of transaction T |
| R_TS(X) | Timestamp of the last read on data item X |
| W_TS(X) | Timestamp of the last write on data item X |
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 Ignore Outdated Writes (Allow Execution Without Writing):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- A newer write has already updated X. Writing now is obsolete.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Example - Ignoring an Outdated Write
The main improvement in Thomas Write Rule is that it ignores obsolete (outdated) writes instead of rejecting transactions.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:
| Step | Operation | Description |
|---|---|---|
| 1 | T2: W2(X) | T2 writes X (sets W_TS(X) = TS(T2)) |
| 2 | T1: W1(X) | T1 tries to write X (TS(T1) > W_TS(X)) |
| 3 | T1's write is ignored | Since T2 has already written X, T1’s write is obsolete and ignored instead of rollback |
- In Basic Timestamp Ordering, T1 would abort because its write is considered outdated.
- In Thomas Write Rule, outdated writes are ignored instead of rollback, allowing T1 to continue execution.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 2nd 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
| T1 | T2 |
|---|---|
| R(A) | |
| W(A) | |
| Commit | |
| W(A) | |
| Commit |
- 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
| Aspect | Basic Timestamp Ordering (TO) | Thomas Write Rule (TWR) |
|---|---|---|
| Basis | Strict timestamp checking for reads/writes | Modified TO that ignores outdated writes |
| Outdated Writes | Transaction aborted | Write ignored |
| Type of Serializability | Conflict serializable | View serializable (not always conflict serializable) |
| Concurrency Level | Low (more rollbacks) | Higher (fewer rollbacks) |
| System Throughput | Low | High |