Caching

Core Principles

1. HybridCache is the default — .NET 9+ introduced HybridCache as the unified caching abstraction. It combines in-memory (L1) and distributed (L2) caching with stampede protection. See ADR-004.
2. Cache reads, not writes — Cache GET operations. Invalidate on mutations. Never cache POST/PUT/DELETE responses.
3. Output caching for entire responses — When the full HTTP response can be cached (public APIs, static data), use output caching middleware.
4. Set explicit TTLs — Every cached item needs an expiration. No unbounded caches.

Patterns

HybridCache (Recommended Default)

// Program.cs
builder.Services.AddHybridCache(options =>
{
    options.DefaultEntryOptions = new HybridCacheEntryOptions
    {
        Expiration = TimeSpan.FromMinutes(5),
        LocalCacheExpiration = TimeSpan.FromMinutes(2)
    };
});

// Optional: Add Redis as the L2 distributed cache
builder.Services.AddStackExchangeRedisCache(options =>
{
    options.Configuration = builder.Configuration.GetConnectionString("Redis");
});

// Usage in a handler
public class GetProduct
{
    public record Query(Guid Id);
    public record Response(Guid Id, string Name, decimal Price);

    internal class Handler(AppDbContext db, HybridCache cache)
    {
        public async Task<Response?> Handle(Query query, CancellationToken ct)
        {
            return await cache.GetOrCreateAsync(
                $"products:{query.Id}",
                async token => await db.Products
                    .Where(p => p.Id == query.Id)
                    .Select(p => new Response(p.Id, p.Name, p.Price))
                    .FirstOrDefaultAsync(token),
                new HybridCacheEntryOptions
                {
                    Expiration = TimeSpan.FromMinutes(10)
                },
                cancellationToken: ct);
        }
    }
}

Cache Invalidation

// Invalidate on mutation
public class UpdateProduct
{
    internal class Handler(AppDbContext db, HybridCache cache)
    {
        public async Task<Result> Handle(Command command, CancellationToken ct)
        {
            var product = await db.Products.FindAsync([command.Id], ct);
            if (product is null) return Result.Failure("Product not found");

            product.Update(command.Name, command.Price);
            await db.SaveChangesAsync(ct);

            // Invalidate the cached entry
            await cache.RemoveAsync($"products:{command.Id}", ct);

            return Result.Success();
        }
    }
}

Output Caching (Full Response Caching)

// Program.cs
builder.Services.AddOutputCache(options =>
{
    options.AddBasePolicy(b => b.NoCache()); // Don't cache by default

    options.AddPolicy("ProductList", b => b
        .Expire(TimeSpan.FromMinutes(5))
        .Tag("products"));

    options.AddPolicy("ProductById", b => b
        .Expire(TimeSpan.FromMinutes(10))
        .SetVaryByRouteValue("id")
        .Tag("products"));
});

app.UseOutputCache();

// Apply to endpoints
group.MapGet("/", ListProducts).CacheOutput("ProductList");
group.MapGet("/{id:guid}", GetProduct).CacheOutput("ProductById");

// Invalidate by tag on mutations
group.MapPut("/{id:guid}", async (Guid id, UpdateProductRequest request,
    IOutputCacheStore store, CancellationToken ct) =>
{
    // ... update logic ...
    await store.EvictByTagAsync("products", ct);
    return TypedResults.NoContent();
});

Cache-Aside Pattern (Legacy)

Prefer HybridCache for all new code. Manual IDistributedCache cache-aside lacks stampede protection, requires manual serialization, and has no L1/L2 layering. Use only when integrating with existing code that already uses IDistributedCache directly.

Anti-patterns

Don't Cache Without Expiration

// BAD — cache lives forever, stale data guaranteed
await cache.SetStringAsync(key, value);

// GOOD — always set TTL
await cache.SetStringAsync(key, value, new DistributedCacheEntryOptions
{
    AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(10)
});

Don't Cache Mutable User-Specific Data

// BAD — caching user's cart with a global key
await cache.GetOrCreateAsync("shopping-cart", ...);

// GOOD — include user ID in key
await cache.GetOrCreateAsync($"shopping-cart:{userId}", ...);

Don't Build Your Own Stampede Protection

// BAD — manual lock to prevent cache stampede
private static readonly SemaphoreSlim Lock = new(1, 1);
await Lock.WaitAsync();
try { /* check cache, populate if missing */ }
finally { Lock.Release(); }

// GOOD — HybridCache has built-in stampede protection
await hybridCache.GetOrCreateAsync(key, factory);

Decision Guide

Scenario	Recommendation
General data caching	HybridCache (GetOrCreateAsync)
Full HTTP response	Output caching with .CacheOutput()
Frequently read, rarely written	HybridCache with longer TTL
User-specific data	HybridCache with user-scoped key
Cache invalidation on write	cache.RemoveAsync() or output cache tags
Distributed deployment	HybridCache + Redis L2 backend
Single-server deployment	HybridCache with in-memory only