In parallel query execution engines, deadlocks can occur when multiple queries or transactions compete for resources such as locks on shared data structures. Deadlocks can lead to performance degradation or even system failures. To mitigate this issue, parallel query execution engines employ deadlock avoidance techniques.
What is Deadlock Avoidance?
Deadlock avoidance is a technique used to prevent or minimize the occurrence of deadlocks in a system. It involves careful resource allocation and scheduling to ensure that the system can progress without getting into a deadlock state.
Techniques for Deadlock Avoidance in Parallel Query Execution Engines
1. Resource Ordering
One approach to deadlock avoidance is to establish a fixed order for acquiring resources. By enforcing a consistent order for acquiring locks on shared data structures, the likelihood of deadlocks occurring can be reduced. This can be achieved by assigning unique identifiers to resources and ensuring that resources are acquired in ascending order of their identifiers. However, this technique may limit the parallelism in query execution as some queries may need to wait for resources to become available.
2. Two-Phase Locking
Two-phase locking is a widely used technique in database systems to prevent deadlocks. It ensures serializability of transactions by dividing the execution into two phases - the growing phase and the shrinking phase. During the growing phase, transactions can acquire new locks, but cannot release any locks. During the shrinking phase, transactions can release locks, but cannot acquire new locks. This technique can be extended to parallel query execution engines to avoid deadlocks between concurrent queries.
3. Wait-Die and Wound-Wait
Wait-Die and Wound-Wait are two deadlock avoidance techniques based on transaction ordering. In Wait-Die, if a transaction requests a resource held by another transaction, it is allowed to wait only if its timestamp is smaller than the other transaction’s timestamp. If the requesting transaction has a higher timestamp, it aborts itself and retries later. Wound-Wait, on the other hand, allows the requesting transaction to steal the resource from the other transaction by aborting it if its timestamp is smaller. These techniques assign priorities to transactions based on their timestamps, ensuring a strict ordering of resource requests.
Conclusion
Deadlock avoidance is crucial in parallel query execution engines to ensure efficient resource utilization and prevent system failures. Techniques such as resource ordering, two-phase locking, and transaction ordering based on time stamps can be used to achieve deadlock avoidance. Each technique has its own advantages and considerations, and the choice of technique depends on the specific requirements and characteristics of the system.
References:
- Concurrency Control and Recovery in Database Systems by Philip A. Bernstein, Vassos Hadzilacos, Nathan Goodman
- Database Systems: The Complete Book by Hector Garcia-Molina, Jeffrey D. Ullman, Jennifer Widom
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