Distributed Databases Questions Long
Distributed database recovery refers to the process of restoring a distributed database system to a consistent and correct state after a failure or crash occurs. In a distributed database system, data is stored across multiple nodes or sites, and failures can happen at any of these sites. Therefore, it is crucial to have mechanisms in place to ensure data integrity and availability in the event of failures.
The concept of distributed database recovery involves two main aspects: failure detection and failure recovery. Failure detection involves identifying when a failure has occurred, while failure recovery focuses on restoring the system to a consistent state after the failure.
There are several techniques used in distributed database recovery, including:
1. Centralized Recovery: In this technique, a central site or node is responsible for coordinating the recovery process. When a failure is detected, the central site collects information about the failed site and initiates the recovery process. It may use techniques like shadow paging or write-ahead logging to restore the database to a consistent state.
2. Distributed Recovery: In this technique, each site in the distributed database system is responsible for its own recovery. When a failure occurs, the failed site initiates its recovery process independently. The recovery process may involve techniques like checkpointing, where the site periodically saves its state, and undo/redo logging, where the site logs its transactions to ensure atomicity and durability.
3. Two-Phase Commit (2PC): The 2PC protocol is a widely used technique for distributed database recovery. It ensures that all sites in a distributed transaction either commit or abort the transaction. The protocol involves a coordinator site that coordinates the commit or abort decision among the participating sites. If a failure occurs during the protocol, the coordinator can use techniques like timeout or participant failure detection to handle the failure and ensure the transaction's consistency.
4. Three-Phase Commit (3PC): The 3PC protocol is an extension of the 2PC protocol that addresses some of its limitations. It adds an extra phase called the pre-commit phase, which allows the coordinator to gather acknowledgments from all participating sites before making the final commit or abort decision. This additional phase improves the protocol's fault tolerance and reduces the chances of blocking due to failures.
5. Quorum-Based Techniques: Quorum-based techniques ensure data consistency and availability in a distributed database system. They involve dividing the system into multiple groups or quorums, where each quorum has a subset of nodes. These techniques use voting mechanisms to determine the correct state of the database after a failure. For example, a majority quorum-based technique requires a majority of nodes to agree on the state before committing a transaction.
Overall, distributed database recovery is a complex and critical aspect of distributed database systems. It requires careful planning, coordination, and the use of various techniques to ensure data integrity and availability in the face of failures.