Timestamp-Based Protocol

Timestamp-Based Protocol is a concurrency control mechanism in database systems that uses timestamps to order transactions, ensuring serializability. Each transaction is assigned a unique timestamp when it begins, and the protocol uses these timestamps to determine the execution order of conflicting operations.

Key Concepts

• Timestamp: A unique identifier (usually based on the system clock or a counter) assigned to each transaction when it starts.
• Read and Write Timestamps:
  • Each data item is associated with:
    • Read Timestamp (RT): The largest timestamp of any transaction that successfully read the data item.
    • Write Timestamp (WT): The largest timestamp of any transaction that successfully wrote the data item.

Rules for Timestamp-Based Protocol

The protocol enforces the following rules to maintain serializability:

1. Write Operation:
   • If a transaction T1 attempts to write to a data item:
     • If T1's timestamp is less than the read timestamp of the data item, the write is rejected, and T1 is aborted.
     • If T1's timestamp is less than the write timestamp of the data item, the write is rejected, and T1 is aborted.
     • Otherwise, the write is allowed, and the write timestamp is updated.
2. Read Operation:
   • If a transaction T1 attempts to read a data item:
     • If T1's timestamp is less than the write timestamp of the data item, the read is rejected, and T1 is aborted.
     • Otherwise, the read is allowed, and the read timestamp is updated.

Example of Timestamp-Based Protocol

Consider two transactions T1 and T2 accessing a shared data item A:

Step	Transaction T1	Transaction T2	Explanation
1	BEGIN TRANSACTION (TS=1)	BEGIN TRANSACTION (TS=2)	T1 and T2 are assigned timestamps 1 and 2, respectively.
2	READ(A)	-	T1 reads A. RT(A) is updated to 1.
3	-	WRITE(A)	T2 attempts to write to A. WT(A) is updated to 2.
4	WRITE(A)	-	T1 attempts to write to A but is aborted because TS(T1) < WT(A).

In this example, T1 is aborted because it attempts to write to A after a newer transaction (T2) has already written to it.

Advantages

• Deadlock-Free: Timestamp-based protocols avoid deadlocks because transactions are never required to wait.
• Simple Implementation: The protocol relies only on timestamps, reducing complexity.
• Ensures Serializability: Transactions are executed in timestamp order, guaranteeing a serializable schedule.

Limitations

• Cascading Aborts: Transactions may be frequently aborted due to timestamp conflicts, leading to performance degradation.
• Overhead: Maintaining timestamps and metadata for each data item can be resource-intensive.
• Starvation: Older transactions may be aborted repeatedly in favor of newer transactions.

Applications

Timestamp-based protocols are commonly used in:

• Distributed Databases: Ensuring consistency across nodes by assigning global timestamps.
• Real-Time Systems: Prioritizing transactions based on their timestamps.
• Optimistic Concurrency Control: Validating transactions at commit time using timestamp-based techniques.

Variants

The basic timestamp-based protocol has several extensions to handle specific scenarios:

• Thomas' Write Rule:
  • Allows a transaction to ignore certain outdated writes, reducing unnecessary aborts.
• Multiversion Timestamp Ordering (MVTO):
  • Maintains multiple versions of data to improve concurrency and reduce conflicts.

See Also

• Concurrency Control
• Two-Phase Locking
• Thomas' Write Rule
• Multiversion Concurrency Control
• Transaction Management
• Serializability