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Timestamp-Based Protocol
Last Updated : 21 May, 2026
The Timestamp Ordering Protocol is an alternative to locking for concurrency control. Instead of using locks, each transaction is assigned a unique timestamp when it star
Unit 4248 words13 headingsTables includedExamples included
Overview
The Timestamp Ordering Protocol is an alternative to locking for concurrency control. Instead of using locks, each transaction is assigned a unique timestamp when it starts, and the protocol ensures that conflicting operations execute in timestamp order.
No locks → No deadlocks (a key advantage over 2PL).
Timestamps
Each transaction Ti is assigned a unique timestamp TS(Ti) when it begins:
- Typically the system clock or a logical counter
- Older transactions have smaller timestamps
- TS(Ti) < TS(Tj) means Ti started before Tj
Per Data Item Bookkeeping
For each data item X, the system maintains:
| Field | Description |
|---|---|
| R-timestamp(X) | Largest timestamp of any transaction that successfully read X |
| W-timestamp(X) | Largest timestamp of any transaction that successfully wrote X |
Timestamp Protocol Rules
Read Rule
Transaction Ti issues read(X):
IF TS(Ti) < W-timestamp(X):
Ti is trying to read a value that was ALREADY OVERWRITTEN
by a newer transaction → Ti is "too old"
→ REJECT: Abort Ti, restart with new timestamp
ELSE (TS(Ti) ≥ W-timestamp(X)):
Read is allowed ✓
R-timestamp(X) = MAX(R-timestamp(X), TS(Ti))
Write Rule
Transaction Ti issues write(X):
IF TS(Ti) < R-timestamp(X):
Ti is trying to write a value that was ALREADY READ
by a newer transaction (who expected to read the old value)
→ REJECT: Abort Ti, restart with new timestamp
ELSE IF TS(Ti) < W-timestamp(X):
Ti is trying to write a value that was ALREADY OVERWRITTEN
by a newer transaction → obsolete write
→ REJECT: Abort Ti, restart with new timestamp
ELSE:
Write is allowed ✓
W-timestamp(X) = TS(Ti)
Example
| Transactions | T1 (TS=1), T2 (TS=2) |
| Data items | X (R-ts=0, W-ts=0 initially) |
| Step 1 | T2 reads X |
| TS(T2)=2 ≥ W-ts(X)=0 | ALLOWED ✓ |
| Step 2 | T1 reads X |
| TS(T1)=1 ≥ W-ts(X)=0 | ALLOWED ✓ |
| Step 3 | T1 writes X |
| TS(T1)=1 < R-ts(X)=2 | REJECTED! ✗ |
| Step 4 | T2 writes X |
Thomas' Write Rule (Optimization)
The Thomas' Write Rule modifies the write rule to allow more concurrency:
IF TS(Ti) < W-timestamp(X):
This is an OBSOLETE WRITE — a newer transaction already wrote X.
Ti's write would be immediately overwritten anyway.
→ IGNORE the write (don't abort Ti, just skip the write)
→ Continue execution
This makes some non-conflict-serializable (but view-serializable) schedules acceptable.
Timestamp vs. 2PL Comparison
| Feature | 2PL | Timestamp Protocol |
|---|---|---|
| Mechanism | Locks | Timestamps |
| Deadlock | Possible | Not possible |
| Starvation | Possible | Possible (restarting) |
| Conflict Resolution | Block (wait) | Abort and restart |
| Overhead | Lock table | Timestamp per item |
| Serial order | Lock point order | Timestamp order |
Advantages and Disadvantages
Advantages
- No deadlocks — transactions never wait; they either proceed or restart
- Simple implementation (no lock table management)
- Guarantees serializability with order = timestamp order
Disadvantages
- High abort rate if many conflicting transactions restart repeatedly
- Starvation possible (a transaction keeps getting aborted by newer ones)
- Long-running transactions get aborted often (large gap between start and end)
Multiversion Timestamp Protocol
An extension that keeps multiple versions of each data item:
Each write creates a new version with the writer's timestamp.
Reads are served from the most recent version ≤ the reader's timestamp.
→ Readers never need to wait or abort for writes.
→ Used in MVCC (Multi-Version Concurrency Control) in PostgreSQL, Oracle.
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Timestamp-Based Protocol.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this DBMS topic.
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