What is Output Caching in ASP.NET Core 8+?

Output caching stores the entire response (headers + body) keyed on request features, served from memory or a distributed store. Cheaper than re-running the handler.

builder.Services.AddOutputCache(options => {
    options.AddPolicy("ProductList", b => b
        .Expire(TimeSpan.FromSeconds(60))
        .SetVaryByQuery("category", "page"));
});

app.UseOutputCache();

app.MapGet("/api/products", async (IProductService svc, string? category, int page = 1) =>
    Results.Ok(await svc.ListAsync(category, page)))
   .CacheOutput("ProductList");

Tag-based invalidation is the killer feature:

options.AddPolicy("Product", b => b.Tag("product-list"));

// In your admin write path:
public async Task PublishAsync(IOutputCacheStore cache) {
    await db.SaveChangesAsync();
    await cache.EvictByTagAsync("product-list", default);
}

Publishing a new product evicts every cached response tagged product-list.

Different from response caching: response caching emits Cache-Control headers and relies on the client to cache. Output caching stores the response on the server — works regardless of client headers and lets you invalidate.

Don't cache: authenticated personalised content (key by user → cache explosion), POST results, anything with Set-Cookie you need to refresh.