Add In-Memory Cache — Cut DB Calls on Hot Endpoints

// Program.cs
builder.Services.AddMemoryCache(options =>
{
    options.SizeLimit = 1024;           // optional: cap total cache size
    options.CompactionPercentage = 0.25; // remove 25% when limit hit
});

public class ProductService(IMemoryCache cache, IProductRepository repo)
{
    // ...
}

public async Task<Product?> GetProductAsync(int id, CancellationToken ct = default)
{
    return await cache.GetOrCreateAsync($"product:{id}", async entry =>
    {
        entry.AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(5);
        entry.SlidingExpiration = TimeSpan.FromMinutes(2);
        entry.Size = 1; // required if SizeLimit is set

        return await repo.FindByIdAsync(id, ct);
    });
}

var options = new MemoryCacheEntryOptions
{
    // Absolute: expires at this point regardless of access
    AbsoluteExpiration = DateTimeOffset.UtcNow.AddHours(1),

    // Or relative to when the entry was created
    AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(10),

    // Sliding: resets timer on each access; entry lives at most AbsoluteExpiration
    SlidingExpiration = TimeSpan.FromMinutes(2),

    // Priority: controls what gets evicted first under memory pressure
    Priority = CacheItemPriority.Normal,  // Low / Normal / High / NeverRemove

    // Size: arbitrary unit counted against SizeLimit
    Size = 1,
};

public class OrderService(IMemoryCache cache, IOrderRepository repo, ILogger<OrderService> logger)
{
    private static readonly TimeSpan CacheDuration = TimeSpan.FromMinutes(3);

    public async Task<Order?> GetOrderAsync(Guid id, CancellationToken ct = default)
    {
        var cacheKey = $"order:{id}";

        // 1. Try cache first
        if (cache.TryGetValue(cacheKey, out Order? cached))
        {
            logger.LogDebug("Cache hit for order {Id}", id);
            return cached;
        }

        // 2. Miss — load from source
        logger.LogDebug("Cache miss for order {Id}", id);
        var order = await repo.FindByIdAsync(id, ct);

        // 3. Populate cache (don't cache nulls unless you want negative caching)
        if (order is not null)
        {
            cache.Set(cacheKey, order, CacheDuration);
        }

        return order;
    }

    public async Task UpdateOrderAsync(Order order, CancellationToken ct = default)
    {
        await repo.UpdateAsync(order, ct);

        // Invalidate on write
        cache.Remove($"order:{order.Id}");
    }
}

var options = new MemoryCacheEntryOptions
{
    AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(5)
};

options.RegisterPostEvictionCallback((key, value, reason, state) =>
{
    var logger = (ILogger)state!;
    logger.LogInformation(
        "Cache entry {Key} evicted. Reason: {Reason}",
        key, reason);
    // reason: Expired | Capacity | Removed | Replaced | TokenExpired
}, logger);

cache.Set("config:feature-flags", featureFlags, options);

// Create a shared cancellation token source
var cts = new CancellationTokenSource();
var expiryToken = new CancellationChangeToken(cts.Token);

// Apply to multiple entries
cache.Set("product:1", product1, new MemoryCacheEntryOptions()
    .AddExpirationToken(expiryToken));
cache.Set("product:2", product2, new MemoryCacheEntryOptions()
    .AddExpirationToken(expiryToken));

// Expire all at once
cts.Cancel(); // both entries evicted simultaneously

Thread 1: TryGetValue → miss → calling factory...
Thread 2: TryGetValue → miss → calling factory...  ← same key, concurrent!
Thread 3: TryGetValue → miss → calling factory...  ← all hitting the DB

public class StampedeResistantCache(IMemoryCache cache)
{
    // One semaphore per cache key
    private readonly ConcurrentDictionary<string, SemaphoreSlim> _locks = new();

    public async Task<T?> GetOrCreateAsync<T>(
        string key,
        Func<CancellationToken, Task<T?>> factory,
        TimeSpan duration,
        CancellationToken ct = default)
    {
        // Fast path — cache hit, no lock needed
        if (cache.TryGetValue(key, out T? cached))
            return cached;

        var semaphore = _locks.GetOrAdd(key, _ => new SemaphoreSlim(1, 1));

        await semaphore.WaitAsync(ct);
        try
        {
            // Double-check after acquiring lock
            if (cache.TryGetValue(key, out cached))
                return cached;

            var value = await factory(ct);

            if (value is not null)
                cache.Set(key, value, duration);

            return value;
        }
        finally
        {
            semaphore.Release();
            _locks.TryRemove(key, out _);
        }
    }
}

// Sentinel object to represent a known miss
private static readonly object NullSentinel = new();

public async Task<Product?> GetProductAsync(int id, CancellationToken ct = default)
{
    var key = $"product:{id}";

    if (cache.TryGetValue(key, out object? raw))
    {
        return raw == NullSentinel ? null : (Product?)raw;
    }

    var product = await repo.FindByIdAsync(id, ct);

    var options = new MemoryCacheEntryOptions
    {
        AbsoluteExpirationRelativeToNow = product is null
            ? TimeSpan.FromSeconds(30)  // short TTL for negative cache
            : TimeSpan.FromMinutes(5)
    };

    cache.Set(key, product ?? NullSentinel, options);

    return product;
}