ASP.NET Core Metrics with Prometheus

How to use solid metrics to know how your app performs in the wild

Photo by Chris Liverani on Unsplash

When it comes to running production software, you’ll hardly ever find yourself in a situation where you know too much about how your application is performing. More often than not, the opposite is true, and we don’t nearly have enough information available about our application’s performance in the wild.

Fortunately, there are a variety of free, open source tools available that provide time series databases and allow us to accrue and store all sorts of metrics. While individual metrics at an instant in time may not mean much on their own, having access to metrics over an extended period of time allows for in-depth trend analysis. One of the most commonly used tools for aggregating metrics is Prometheus.

Prometheus utilizes a pull model to obtain its metrics. That is, you point it at an URL exposed by your application, Prometheus grabs metrics from that URL every once in a while, and stores them. The power of this model is in its simplicity — the only thing your application needs to do is expect a metrics endpoint in a specific format, and let Prometheus deal with the rest.

Unfortunately, ASP.NET Core doesn’t come with a metrics endpoint by default — we’ll have to create one for ourselves. In this article, we’ll create a simple ASP.NET Core Web API that collects and exposes metrics for Prometheus to interpret. We’ll then obtain those metrics in Prometheus, and plot a graph from them.

Setting up our Application

We’ll be starting from scratch, so the first thing we’ll do is create an ASP.NET Core Web API project. At the time of writing, .NET Core 3.1 is the most recent version of .NET, so we’ll go with that.

Next up, we want to expose a basic set of metrics. There is a library available named prometheus-net that comes with a companion ASP.NET Core Package. We’ll need them both in our example. Run the following commands in the Package Manager Console:

Install-Package prometheus-net
Install-Package prometheus-net.AspNetCore

We can now start exposing a default set of metrics using one simple line in Startup.cs: