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Multiversion Timestamp Ordering
Multiversion Timestamp Ordering (MVTO) is a concurrency control technique that maintains multiple versions of each data item. Each version has a unique timestamp, and transactions access the appropriate version based on their own timestamp, preventing conflicts and deadlocks.
Components
Each version Xi of a data item maintains
- Value The data content of that version.
- Read_TS(Xi) Largest timestamp of any transaction that successfully read Xi.
- Write_TS(Xi) Timestamp of the transaction that created Xi.
How MVTO Works
Serializability Rules
Rule 1 (Read) If transaction T reads X, find version Xi where Write_TS(Xi) ≤ TS(T). Return Xi's value and update Read_TS(Xi) = max(Read_TS(Xi), TS(T)).
Rule 2 (Write) If transaction T writes X
- If TS(T) < Read_TS(X) → abort T (a newer transaction already read it).
- If TS(T) = Write_TS(X) → overwrite the contents.
- If TS(T) > Write_TS(X) → create a new version.
Example
T1 (timestamp=5) and T2 (timestamp=10) operating on data item X ?
| Step | Operation | Result |
|---|---|---|
| 1 | T1: Read(X) | Reads X0, Read_TS(X0)=5 |
| 2 | T1: Write(X) | Creates X1, Write_TS(X1)=5 |
| 3 | T2: Read(X) | Reads X1, Read_TS(X1)=10 |
| 4 | T2: Write(X) | Creates X2, Write_TS(X2)=10 |
| 5 | T1: Read(X) | Reads X1 (appropriate version for TS=5) |
| 6 | T1: Write(X) | Abort T1 T2 already read X1 (Read_TS=10 > TS(T1)=5) |
Advantages and Disadvantages
| Advantages | Disadvantages |
|---|---|
| High concurrency | Storage overhead (multiple versions) |
| No deadlocks | Increased computational complexity |
| Data consistency guaranteed | Garbage collection needed for old versions |
| Supports long-running transactions | Cascading aborts possible |
Conclusion
MVTO maintains multiple timestamped versions of data items so transactions access the version appropriate for their timestamp. This avoids unnecessary aborts and reduces contention, but requires additional storage and garbage collection for obsolete versions.
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