How Can Asynchronous Key-Based Locking Improve Concurrent Object Access and Prevent Cache Errors?

Improving Concurrent Object Access with Asynchronous Key-Based Locking

This article tackles concurrent object locking using hashed URL keys within the ImageProcessor library. The library experiences intermittent file access errors during cache updates. A custom AsyncDuplicateLock class, designed for asynchronous key-based locking, proved insufficient.

Analyzing the AsyncDuplicateLock Class

The AsyncDuplicateLock class provides synchronous (Lock) and asynchronous (LockAsync) locking methods. Both utilize a key to identify the locked object.

Locking Mechanism

The locking process involves these key-based steps:

1. Semaphore Acquisition/Creation: A semaphore is retrieved for the given key or created if one doesn't exist.
2. Semaphore Wait: If the semaphore is acquired, Wait (synchronous) or WaitAsync (asynchronous) is called to acquire the lock.
3. Semaphore Release: After the object operation, the semaphore is released using Release (synchronous) or asynchronously.
4. Semaphore Disposal: When the lock is no longer needed, the semaphore is disposed to free resources.

Identifying the Problem

The original code's flaw lies in prematurely removing SemaphoreSlim instances from the ConcurrentDictionary. This premature disposal, before semaphore release, results in multiple semaphores associated with the same key, leading to cache addition errors.

Enhanced Implementation

Our solution introduces a reference count for each semaphore within the dictionary. A single lock ensures atomic decrement and removal of reference counts. This enhanced approach offers robust locking and prevents the previous error.

This improved implementation enables reliable asynchronous key-based locking, minimizing cache errors and ensuring the ImageProcessor library functions correctly.

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