Timestamp-based protocols in DBMS Theory - Step-by-Step Execution

Concept Flow - Timestamp-based protocols

Transaction requests data

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Check timestamps of transaction and data

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Compare transaction TS with read/write TS

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TS < readTS?

↓

Abort Tx

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Update read/write timestamps

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Transaction continues or restarts

The protocol checks timestamps to decide if a transaction can read or write data without conflicts, aborting if it violates timestamp order.

Execution Sample

DBMS Theory

Transaction T with TS=5 wants to write data X
Check X's readTS=3 and writeTS=4
If TS < readTS or TS < writeTS, abort
Else write and update writeTS=5

This shows how a transaction's timestamp is compared with data timestamps to decide if it can write.

Analysis Table

Step	Transaction TS	Data X readTS	Data X writeTS	Condition Checked	Result	Action
1	5	3	4	Is TS < readTS? (5 < 3)	False	Continue
2	5	3	4	Is TS < writeTS? (5 < 4)	False	Continue
3	5	3	4	No conflicts	True	Write allowed, update writeTS=5
4	5	3	5	End of operation	-	Transaction proceeds

💡 Transaction allowed to write because its TS is not less than readTS or writeTS

State Tracker

Variable	Start	After Step 3	Final
Transaction TS	5	5	5
Data X readTS	3	3	3
Data X writeTS	4	5	5

Key Insights - 2 Insights

Why does the transaction abort if its timestamp is less than the data's read timestamp?

Because it means the transaction is trying to write older data after a newer transaction has already read it, violating timestamp order (see execution_table step 1).

What happens if the transaction's timestamp is greater than both readTS and writeTS?

The transaction can safely perform the operation and update the writeTS or readTS accordingly (see execution_table step 3).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the condition checked?

A Is transaction TS less than data's writeTS?

B Is transaction TS less than data's readTS?

C Is transaction TS equal to data's writeTS?

D Is transaction TS greater than data's readTS?

💡 Hint

Check the 'Condition Checked' column in execution_table row for step 2.

At which step does the transaction update the data's writeTS?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the 'Action' column mentioning updating writeTS.

If the transaction TS was 2 instead of 5, what would happen at step 1?

A Transaction would proceed normally

B Condition TS < readTS would be False

C Transaction would abort because TS < readTS is True

D Data's writeTS would update to 2

💡 Hint

Compare transaction TS=2 with readTS=3 in execution_table step 1.

Concept Snapshot

Timestamp-based protocols use timestamps to order transactions.
Each data item tracks read and write timestamps.
Transactions abort if they violate timestamp order.
If allowed, timestamps update to reflect latest access.
This prevents conflicts and ensures serializability.

Full Transcript

Timestamp-based protocols in databases use timestamps to control the order of transactions accessing data. Each transaction has a unique timestamp. Data items keep track of the timestamps of the last read and write operations. When a transaction wants to read or write data, the protocol compares its timestamp with the data's read and write timestamps. If the transaction's timestamp is less than the data's read or write timestamp, it means the transaction is too old and must abort to avoid conflicts. Otherwise, the transaction proceeds and updates the data's timestamps accordingly. This method ensures transactions appear to execute in timestamp order, preventing conflicts and maintaining consistency.