Logging

In modern application development, especially with APIs, knowing what’s happening under the hood isn’t just a “nice-to-have,” it’s a necessity. Audit logging provides a detailed record of every significant action, which is invaluable for security, compliance (like GDPR or HIPAA), and debugging complex issues.

While you could add logging calls in every controller action, that approach is repetitive and error-prone. A much cleaner, more powerful solution is to use ASP.NET Core’s middleware. In this post, we’ll build a piece of custom middleware from scratch to create a robust audit logging system for any API.

What is Middleware, Really?

Think of the ASP.NET Core request pipeline as an assembly line. When an HTTP request comes in, it passes through a series of components, or “middleware,” before it reaches your API controller. Each piece of middleware has a chance to inspect the request, modify it, or even short-circuit it. After the controller generates a response, it travels back down the same line.

This structure makes middleware the perfect place for cross-cutting concerns like authentication, caching, exception handling, and, of course, logging.

Designing Our Audit Logging Middleware

Before writing code, let’s define what information we want to capture for each API call. A good audit log should be comprehensive.

Key Data Points to Log:

• Request Info: HTTP Method, Path, Query String, Headers, and the Request Body. …

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In modern distributed systems, a single user action can trigger a cascade of requests across multiple microservices. When something goes wrong, tracing that single action through a sea of log entries can feel like finding a needle in a haystack. This is where correlation IDs become an indispensable tool for observability.

A correlation ID is a unique identifier that follows a request from start to finish, even as it hops between services. By including this ID in every log message related to that request, you can easily filter and piece together the entire transaction.

The most effective way to implement this in ASP.NET Core is with custom middleware. Let’s dive into how to build it.

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Why Middleware is the Perfect Solution

The ASP.NET Core request pipeline is a series of components, or middleware, that process an HTTP request. By creating our own middleware and placing it early in the pipeline, we can ensure that:

1. Every request is intercepted: No request gets processed without a correlation ID.
2. Logic is centralized: We avoid cluttering our controllers or services with repetitive logging code.
3. It’s set up once: Once the middleware is registered, the correlation ID is available for the entire scope of the request.

This approach is clean, efficient, and aligns perfectly with the ASP.NET Core design philosophy.

Building the Correlation ID Middleware

First, we’ll create the middleware component itself. This class will inspect incoming request headers for an …

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