Only run code on first Azure WebApp instance - azure

Question

I have a webapp runnin on Azure. The web site is built in asp net core 3, and is running in a Docker container.
There is a background worker doing a few things such as database cleanup and sending emails built into the application.
My question is how I should best handle this if I need to scale out the application. That is if I create multiple instances of it, whats the best way to make sure the background worker is only running on one of the instances.. And if another instance is removed that another takes over the job.
I realize one solution to this is to break the application apart and run the backgroundworker separately as an Azure function. But I would prefer to avoid this for cost (it's a hobby project) and complexity reasons.
So I'm intrested if there are more ways of solving this which keeps things in one docker container.
Is there for example an environment variable that I can query to get the current instance name and a list of all instances (then I can just say that the first instance in alphabetical order is the "primary" instance). And check this every so often to know if the current instance is the primary instance.

Answer 1

Sidenote: Azure Functions don't cost extra if you reuse the App Service Plan of your website. And complexitywise they are probably less complex than what you are currently thinking about. But if your main goal is to run everything in a single container, you can achieve that as well:
You can use the WebJobs SDK to basically run the "event handler side" of Azure Functions, including the coordination of the required work. Use the singleton attribute if you need additional limitation of concurrency. Infrastructure-wise, WebJobs require a storage account how they manage scale.
You can run WebJobs in the same process as the rest of your ASP.NET Core Application. Some code to get you started if you want to go that route:
var builder = Host.CreateDefaultBuilder(args);
builder.ConfigureWebHostDefaults(webBuilder =>
{
// webBuilder.UseStartup<Startup>();
webBuilder.Configure(app =>
{
app.UseRouting();
app.UseEndpoints(endpoints =>
{
endpoints.MapGet("/", async context =>
{
await context.Response.WriteAsync("Hello World!");
});
});
});
});
builder.ConfigureWebJobs(b =>
{
b.AddAzureStorageCoreServices();
b.AddAzureStorage();
b.AddTimers();
});
IHost host = builder.Build();
using (host)
{
await host.RunAsync();
}