Timestamp-based concurrency control (TBCC) is a technique used in database systems to manage
concurrent access to data. The goal is to ensure that transactions are executed in a way that maintains
consistency and isolation without causing conflicts. Here’s a simplified breakdown of its role:
1. Timestamps: Each transaction is assigned a unique timestamp when it begins. This timestamp
determines the order in which transactions will be processed.
2. Ordering Transactions: Transactions are ordered by their timestamps. A transaction with an
earlier timestamp will be processed before one with a later timestamp.
3. Validation: When a transaction wants to read or write data, the system checks its timestamp
against the timestamps of other transactions. This ensures that transactions don’t interfere with
each other in a way that would cause inconsistencies.
4. Conflict Resolution: If two transactions conflict (e.g., they both want to write to the same piece
of data), the system uses the timestamps to decide which transaction should proceed. Typically,
the transaction with the earlier timestamp is allowed to proceed, while the other transaction
may be rolled back and restarted with a new timestamp.
5. Ensuring Consistency: By carefully managing the order of transactions and resolving conflicts
based on timestamps, the system ensures that the database remains in a consistent state.
Handling Conflicts in Distributed Transactions
1. Global Timestamp Management: Each node in a distributed system must maintain a consistent
method for generating timestamps. Often, a centralized timestamp server or synchronized clocks
across nodes are used to ensure unique and globally consistent timestamps.
2. Distributed Validation: When a distributed transaction wants to commit, each participating
node validates the transaction against its own data. This involves checking timestamps to ensure
that the transaction does not violate the global order of operations.
3. Conflict Resolution:
o If a transaction at one node conflicts with another transaction at a different node (e.g.,
both want to modify the same piece of data), the system uses timestamps to decide
which transaction should proceed.
o Typically, the transaction with the earlier timestamp is allowed to proceed, and the
other transaction might be rolled back and restarted with a new timestamp.
4. Two-Phase Commit (2PC): TBCC is often used in conjunction with the two-phase commit
protocol:
o Phase 1 (Prepare phase): All involved nodes prepare to commit the transaction by
validating its operations.
o Phase 2 (Commit phase): If all nodes agree, the transaction is committed. Otherwise, it
is aborted. This ensures atomicity across the distributed system.
Limitations
1. Complex Timestamp Management: Ensuring unique and globally consistent timestamps can be
challenging in a distributed environment. It requires additional synchronization mechanisms,
which can introduce latency.
2. Increased Rollbacks: The likelihood of conflicts and rollbacks can be higher in distributed
transactions, particularly when multiple nodes are involved. This can impact performance as
transactions might need to be retried multiple times.
3. Latency and Overhead: The validation process across multiple nodes and the need for global
timestamp management can introduce significant latency and overhead, affecting the overall
system performance.
4. Scalability Challenges: As the number of nodes and transactions increases, managing
timestamps and resolving conflicts efficiently can become more difficult. This can limit the
scalability of the system.
5. Coordination Costs: Distributed systems often incur higher coordination costs for maintaining
consistency, which can further impact performance and throughput.
Hence, Timestamp-based concurrency control helps maintain transaction order and consistency but can lead to more rollbacks and delays in distributed systems. It’s effective, though, with challenges in scalability and coordination.