n database systems and multi-threaded applications, locks and deadlocks are critical concepts that ensure data integrity and prevent conflicts. Understanding these concepts helps in designing efficient and safe applications.
1. What is a Lock?
A lock is a mechanism that prevents multiple users or processes from accessing the same resource simultaneously in a way that could cause inconsistencies. Locks are used to maintain data consistency and isolation.
Types of Locks:
- Shared Lock (Read Lock): Multiple transactions can read a resource simultaneously but cannot modify it.
- Exclusive Lock (Write Lock): Only one transaction can read or modify the resource at a time.
Locking Modes:
- Pessimistic Locking: Locks the resource before performing operations to prevent conflicts.
- Optimistic Locking: Assumes conflicts are rare and checks for conflicts only before committing changes.
2. What is a Deadlock?
A deadlock occurs when two or more processes are waiting for each other to release resources, creating a cycle of dependency where none of the processes can proceed.
Example Scenario:
- Process A holds Resource 1 and waits for Resource 2.
- Process B holds Resource 2 and waits for Resource 1.
Neither process can continue, resulting in a deadlock.
3. Causes of Deadlocks
- Circular wait: Processes form a circular chain where each process waits for a resource held by the next.
- Mutual exclusion: Some resources can only be used by one process at a time.
- Hold and wait: A process holding one resource waits for another resource.
- No preemption: Resources cannot be forcibly taken from a process.
4. Deadlock Detection and Prevention
Prevention Techniques:
- Resource Ordering: Assign a global order to resources and acquire them in that order.
- Request All Resources at Once: Avoid partial acquisition of resources.
- Timeouts: Abort or rollback a transaction if it takes too long to acquire a resource.
Detection Techniques:
- Use a wait-for graph to identify cycles in resource allocation.
- Periodically check for processes that have been waiting longer than a threshold.
5. Handling Deadlocks
- Transaction Rollback: Abort one of the deadlocked processes to break the cycle.
- Deadlock Recovery: Identify the victim process based on priority, cost, or age and restart it.
- Avoidance Algorithms: Apply algorithms like Banker’s Algorithm to ensure safe allocation of resources.
6. Best Practices for Locks & Deadlocks
- Keep transactions short to reduce the time resources are held.
- Avoid nested locks whenever possible.
- Use consistent locking order across all processes.
- Monitor the system for long waits or repeated deadlocks.
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