Conservation Protocol and Multiversion Timestamp Protocol in Distributed Databases
In distributed databases, concurrency control mechanisms ensure that multiple transactions can execute simultaneously without violating the integrity of the database. Timestamp-based concurrency control protocols are often used to order transactions based on timestamps to avoid conflicts like dirty reads, non-repeatable reads, and lost updates.
Two of the most widely used timestamp-based protocols are the Conservation Protocol and the Multiversion Timestamp Protocol.
1. Conservation Protocol
The Conservation Protocol is designed to handle concurrent access to shared data while ensuring the serializability of transactions. The key idea of this protocol is to assign a timestamp to each transaction and use these timestamps to determine the validity of read and write operations.
Key Points of Conservation Protocol:
- Transaction Timestamp: Every transaction is assigned a unique timestamp when it starts, and the transactions are ordered based on these timestamps.
- Read and Write Operations: The protocol ensures that only transactions with valid timestamps are allowed to read or write data, preventing conflicts and maintaining database consistency.
- Conflict Resolution: If two transactions access the same data, their timestamps determine the order in which operations are allowed to proceed. The transaction with the lower timestamp is given priority.
Example:
Suppose we have two transactions, T1 and T2, with timestamps TS(T1) = 1 and TS(T2) = 2.
- If T1 reads or writes to a data item, it will be allowed to do so, as long as no other transaction with a timestamp earlier than T1 is currently modifying the data.
- If T2 tries to read or write the same data item after T1 has performed an operation, T2 will need to wait or be rolled back if there is a conflict.
This ensures that the database will not enter an inconsistent state due to concurrent transactions, and the transactions will follow a serializable order.
2. Multiversion Timestamp Protocol (MVTP)
The Multiversion Timestamp Protocol (MVTP) is a more advanced timestamp-based concurrency control mechanism that allows for multiple versions of the same data item to exist simultaneously. This is particularly useful in systems with high concurrency, as it allows transactions to read consistent versions of data even if other transactions are modifying it.
Key Points of Multiversion Timestamp Protocol:
- Multiple Versions of Data: In MVTP, each data item can have multiple versions, each associated with a timestamp. Each transaction reads the version of the data that was committed before its timestamp.
- Transaction Timestamps: Similar to the conservation protocol, each transaction is assigned a unique timestamp. However, in MVTP, transactions can read the version of data that was valid at the time of their start.
- Read and Write Operations:
- When a transaction performs a write operation, a new version of the data item is created with the timestamp of the transaction.
- When a transaction performs a read operation, it can read the most recent version of the data item that was committed before its timestamp.
- Version Control: The system maintains a history of all versions of each data item, and the versioning process prevents conflicts between transactions trying to access the same data.
Example:
Assume the following scenario:
- T1 starts at time 1 and writes a value to data item X.
- T2 starts at time 2 and tries to read data item X.
- Under MVTP, T2 will read the version of X that was valid at the start of T2 (timestamp 2), even if T1 modified it after T2 started.
This ensures that T2 does not see uncommitted changes made by T1 and maintains data consistency without blocking T2.
In this way, MVTP helps to avoid blocking transactions by providing read consistency even in the presence of concurrent writes, allowing high concurrency while maintaining serializability.
Comparison Between Conservation Protocol and Multiversion Timestamp Protocol
| Aspect | Conservation Protocol | Multiversion Timestamp Protocol (MVTP) |
|---|---|---|
| Concurrency | Prevents conflicts by strict timestamp ordering. | Allows high concurrency with multiple versions. |
| Data Versioning | Only the latest version of data is available. | Multiple versions of the data exist, enabling readers to access past states. |
| Blocking | Transactions may block each other to avoid conflicts. | Transactions do not block because they can read different versions of data. |
| Complexity | Simpler to implement, but can cause waiting. | More complex due to version management, but provides better concurrency. |
| Use Cases | Suitable for systems with low concurrency. | Suitable for systems with high concurrency and where read consistency is critical. |
Hence, the conservation Protocol is a simple, efficient way to handle concurrency by enforcing strict timestamp ordering. It works well for applications with low concurrency but may cause blocking when multiple transactions need to access the same data item. Multiversion Timestamp Protocol (MVTP) is designed to provide high concurrency by allowing multiple versions of the same data item. It helps avoid blocking and ensures that transactions read the data consistent with their timestamp, making it a better choice for systems with high read/write operations.
Both protocols rely on timestamps to enforce transaction ordering, but MVTP’s ability to maintain multiple versions of data makes it better suited for environments with high concurrency and complex data access patterns.