How to prevent the Cache Stampede (Thundering Herd) problem when cache keys expire:

When a popular cache key expires, 100 concurrent requests all miss the cache simultaneously and query the database at once, overwhelming the database server.

Solution: Double-Checked Locking with SemaphoreSlim:

public async Task<string> GetCachedDataAsync(string key)
{
    if (_cache.TryGetValue(key, out string val)) return val;
    
    await _semaphore.WaitAsync();
    try
    {
        if (_cache.TryGetValue(key, out val)) return val; // Double check!
        val = await FetchFromDatabaseAsync(key);
        _cache.Set(key, val, TimeSpan.FromMinutes(10));
        return val;
    }
    finally { _semaphore.Release(); }
}

In .NET 9, HybridCache provides built-in multi-tier caching and stampede locking out of the box!

Double-checked locking on cache misses is mandatory for high-traffic API endpoints.