ASP.NET Core: Using Output Caching to Improve API Performance

API performance is not always improved by rewriting database queries or adding more servers. Sometimes the same expensive response is generated repeatedly even though its contents rarely change. Output caching can reduce that repeated work by storing eligible responses and serving them again.

Understand Output Caching

ASP.NET Core output caching stores HTTP responses according to configured policies. It differs from ordinary application-level caching because it can reuse the response generated by an endpoint rather than requiring the endpoint to rebuild the response on every request.

Start by registering the service and middleware:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();
builder.Services.AddOutputCache();

var app = builder.Build();

app.UseOutputCache();

app.MapControllers();

app.Run();

Middleware ordering and endpoint configuration matter. In a real application, integrate output caching with the rest of the middleware pipeline and verify the behavior of the specific endpoints you intend to cache.

Apply Caching Selectively

An endpoint returning public reference data may be a good candidate. A personalized endpoint returning a user’s account details usually needs a different strategy.

For example, a controller can opt into a named policy:

using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/catalog")]
public class CatalogController : ControllerBase
{
    [HttpGet]
    [OutputCache(PolicyName = "CatalogPolicy")]
    public IActionResult GetCatalog()
    {
        var items = new[]
        {
            new { Id = 1, Name = "Keyboard" },
            new { Id = 2, Name = "Monitor" }
        };

        return Ok(items);
    }
}

Register the named policy before using it:

builder.Services.AddOutputCache(options =>
{
    options.AddPolicy("CatalogPolicy", policy =>
        policy.Expire(TimeSpan.FromSeconds(60)));
});

This example illustrates a short-lived cache for catalog data. The expiration period should reflect how frequently the underlying data changes and how stale the response is allowed to become.

Avoid Caching Sensitive Responses

Caching decisions must account for authorization, identity, query parameters, and other factors that influence response content. A response intended for one user must never be served to another user because the cache key failed to distinguish them.

For private or personalized endpoints, use conservative policies and verify the framework’s cache eligibility rules. Do not assume that adding an authorization attribute automatically makes every caching configuration safe.

Plan for Invalidation

Expiration is simple but can leave stale data visible until the entry expires. For frequently updated information, decide how the application should handle invalidation or cache refreshes.

Also remember that in-memory caching is local to a process. If an application runs across multiple instances, each instance may maintain its own cache unless the architecture provides shared coordination or a distributed caching strategy.

Measure the Results

Before and after introducing output caching, compare response latency, server resource usage, and request volume reaching expensive dependencies. Test cache misses, expired entries, concurrent requests, and data updates.

A cache that makes one endpoint faster but creates stale or incorrect responses is not a successful optimization.

Conclusion

Output caching can significantly reduce repeated work for stable API responses. The key is to choose suitable endpoints, define expiration deliberately, protect personalized data, and verify the performance improvement through measurements.