Warm up .NET containers

Use readiness probe to warm up container before using it

Profile pictureToni Petrina
Published on 2023-09-192 min read
  • #csharp
  • #k8s

One of the apps I deployed to Kubernetes used Roslyn in its hot path. The first request would always be slow and I wanted to "warm it up" by calling it at least once. This also could serve as a nice way to trigger alerts if things start failing.

The first implementation followed this article from Mark Vincze Running ASP.NET Core in auto-scaling containers? Warm up!, but I had issues with the following code:

private string GetFullUrl(string relativeUrl) =>

Unfortunately, when running in the cluster, the host was wrong and it throw errors. I needed a way for the /ready endpoint to correctly call itself.

After scouring the web, I ended up with the following code:

public ReadyController(IServer server)
    this.server = server;

private string GetFullUrl(string relativeUrl)
    var addressFeature = server.Features.Get<IServerAddressesFeature>();
    var address = addressFeature
        ?.Replace("[::]", "localhost");

    if (address is null)
        return $"{Request.Scheme}://{Request.Host}{relativeUrl}";

    return $"{address}{relativeUrl}";

This worked like a charm. The replacement is because the address I got when running in the cluster was http://[::]:8080. Luckily, this worked for me, but it might not be bulletproof solution for your situation.

Alternatively, I could have just called the business logic directly which would initialize the same code path that the controller was using - but I insisted on warming up via HTTP calls.

Let me know in the comments below if there is a better way or if this has different limitations.

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