Distributed Databases Questions Long
Distributed deadlock detection is a mechanism used in distributed databases to identify and resolve deadlocks that may occur when multiple transactions are accessing shared resources across different nodes or sites. A deadlock is a situation where two or more transactions are waiting indefinitely for each other to release resources, resulting in a system deadlock.
The concept of distributed deadlock detection involves the following steps:
1. Resource Allocation Graph (RAG): Each node in the distributed system maintains a local resource allocation graph that represents the current state of resource allocation and transaction dependencies. The graph consists of nodes representing transactions and resources, and edges representing the allocation and request relationships between them.
2. Local Deadlock Detection: Each node periodically checks its local resource allocation graph to detect any local deadlocks. This is done by searching for cycles in the graph. If a cycle is found, it indicates the presence of a local deadlock.
3. Distributed Deadlock Detection: Once a local deadlock is detected, the node initiates a distributed deadlock detection algorithm to determine if the deadlock is global or local. There are two commonly used distributed deadlock detection algorithms:
a. Wait-for Graph Algorithm: In this algorithm, each node sends its local wait-for graph to a central coordinator node. The coordinator then combines all the wait-for graphs received from different nodes to form a global wait-for graph. The global wait-for graph is then checked for cycles to identify global deadlocks. If a global deadlock is detected, the coordinator initiates a deadlock resolution process.
b. Chandy-Misra-Haas Algorithm: This algorithm is a distributed version of the resource allocation graph algorithm. Each node periodically sends probe messages to its neighboring nodes to collect information about their local resource allocation graphs. Based on the received information, each node updates its local resource allocation graph and performs a local deadlock detection. If a local deadlock is detected, the node sends an inquiry message to its neighboring nodes to determine if they are also part of the deadlock. If a node receives an inquiry message and determines that it is part of the deadlock, it sends a reply message to the inquiring node. Once all the nodes in the deadlock have replied, the initiating node can identify the global deadlock and initiate a deadlock resolution process.
4. Deadlock Resolution: Once a global deadlock is identified, a deadlock resolution process is initiated to break the deadlock. There are various deadlock resolution techniques, such as deadlock prevention, deadlock avoidance, and deadlock detection with resource preemption. The choice of deadlock resolution technique depends on the specific requirements and constraints of the distributed database system.
In conclusion, distributed deadlock detection is a crucial mechanism in distributed databases to identify and resolve deadlocks that may occur due to concurrent access to shared resources. It involves maintaining local resource allocation graphs, performing local deadlock detection, and using distributed deadlock detection algorithms to identify global deadlocks. Once a global deadlock is detected, a deadlock resolution process is initiated to break the deadlock and ensure the progress of transactions in the distributed system.