Double-Check Locking

Definition

The Double-Check Locking pattern optimizes concurrent access to a shared resource by checking the condition before and after acquiring a lock. The first check (without lock) serves as a fast-path for the nominal case (cache hit). The second check (after lock) protects against races.

Diagram

flowchart TD
    REQ[GetOrAddAsync] --> C1{Check 1<br/>without lock}
    C1 -->|hit| RET[Return value]
    C1 -->|miss| ACQ[Acquire SemaphoreSlim]
    ACQ --> C2{Check 2<br/>after lock}
    C2 -->|hit| REL1[Release lock] --> RET
    C2 -->|miss| FAC[Execute factory]
    FAC --> SET[Store in cache]
    SET --> REL2[Release lock] --> RET

    style C1 fill:#2d5a27,color:#fff
    style ACQ fill:#ff6b6b,color:#fff
    style C2 fill:#4a9eff,color:#fff

Implementation in Granit

Component	File	Lines
DistributedCacheService.GetOrAddAsync()	src/Granit.Caching/DistributedCacheService.cs	74-92

Flow

1. Check 1 (line 74): cache read without lock — fast-path
2. Acquire (line 78): SemaphoreSlim.WaitAsync(cancellationToken)
3. Check 2 (line 82): re-read cache after lock
4. Factory (line 86): execute factory if still a miss
5. Set (line 89): store in cache
6. Release (line 92): SemaphoreSlim.Release() in a finally

Anti-stampede

If 100 simultaneous requests have a cache miss:

• All 100 pass Check 1 (miss)
• 1 acquires the lock, 99 wait
• The first executes the factory and populates the cache
• The 99 pass Check 2 — cache hit, no factory call

Result: 1 single DB query instead of 100.

Rationale

Double-check locking is essential for feature resolution in a high-concurrency environment. Without protection, a simultaneous cache miss (cache expiration, restart) could overwhelm the database.

Usage example

// Double-check locking is internal -- the API is simple
ICacheService<PatientDto> cache = provider.GetRequiredService<ICacheService<PatientDto>>();

// 100 simultaneous calls with the same cache key:
// -> 1 DB query (the first one)
// -> 99 responses from cache (after the lock)
PatientDto patient = await cache.GetOrAddAsync(
    $"patient:{patientId}",
    async ct => await db.Patients.FindAsync([patientId], ct),
    cancellationToken);