Log Requests and Responses in ASP.NET Core

As part of trying to do some debugging, I needed a way to log the requests and responses. Writing a piece of middleware seemed to be a good way to handle this problem. It also turned out to be more complicated that I had expected to deal with the request and response bodies.

Middleware

In ASP.NET Core middleware are the components that make up the pipeline that handles request and responses for the application. Each piece of middleware called has the option to do some processing on the request before calling next piece of middleware in line. After execution returns from the call to the next middleware, there is an opportunity to do processing on the response.

The pipeline for an application is set in the Configure function of the Startup class. Run, Map and Use are the three types of middleware. Run should only be used to terminate the pipeline. Map is used for pipeline branching. Use seems to be the most common type of middleware that does some processing and call the next middleware in line. For more detail see the official docs.

Creating Middleware

Middleware can be implemented as a lambda directly in the  Configure function, but more typically it is implemented as a class that is added to the pipeline using an extension method on IApplicationBuilder. This example will be going the class route.

This example is a piece of middleware that using ASP.NET Cores built-in logging to log requests and responses. Create a class called RequestResponseLoggingMiddleware.

The class will need a constructor that takes to arguments both will be provided by ASP.NET Core’s dependency injection system. The first is a RequestDelegate for the next piece of middleware to be called. The second is an ILoggerFactory which will be used to create a logger. The RequestDelegate is stored to the class level  _next variable and the loggerFactory is used to create a logger which is stored to the class level _logger variable.

Add an Invoke function which is the function that will be called when your middleware is run by the pipeline. The following is the function that does nothing other than call the next middleware in the pipeline.

Next, add a static class to simplify adding the middleware to the application’s pipeline. This is the same pattern the built-in middleware uses.

Adding to the pipeline

To add the new middleware to the pipeline open the  Startup to the Configure function and add the following line.

Keep in mind that the order in which middleware is added can make a difference in how the application behaves. Since the middleware this post is dealing with is logging I have placed it near the begging of the pipeline just before app.UseStaticFiles().

Logging requests and responses

Now that the setup work for our new middleware is done we will come back to its Invoke function. As I stated above this ended up being more complicated that I expected, but thankfully I found this by Sul Aga which really helped me work through the issues I was having.

I created a couple of helper functions that we will look at first. The following is the function call to create the string that will be logged for a request.

The key to getting this function to work and allow reading of the request body was request.EnableRewind() which allows us to read from the beginning of the stream. The rest of the function is pretty straight forward.

The next function is used to get the string to that will be used to log the response body. This function looks simpler than it is and only works because of how it is called from the Invoke function.

Finally, the Invoke which does the logging and jumps through some hoops to allow the response body to be read.

As you can see the trick to reading the response body is replacing the stream being used with a new MemoryStream and then copying the data back to the original body steam. This works and is a concept I found in Sul’s blog post. I don’t know how much the affect performance and would make sure to study how it scales or just avoid using it in production as much as possible.

Wrapping up

This entry didn’t turn out anything like I expected. I came into this looking to do a very simple post due to some time restrictions and it turned into something larger. I hope you find it helpful. I didn’t do a full repo for this week’s post, but you can a gist with the middleware classes here.

7 thoughts on “Log Requests and Responses in ASP.NET Core”

  1. Sadly this does not work for post request and somehow it corrupts the request body and make it null. Bob can you pass the blog you found please?

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