In-Process vs. Isolated Worker Model in Azure Functions

Azure Functions lets you run event-driven, serverless code in the cloud.
When using .NET, you can choose between two hosting models:

1. In-Process Model – Functions run inside the Azure Functions runtime.

2. Isolated Worker Model – Functions run in a separate .NET process, communicating with the host via gRPC.

With the release of .NET 9 , the Isolated Worker Model is the recommended and future-proof option.

1. In-Process Model

In this model, your functions run within the Azure Functions host process.

• Functions share memory and runtime with the host.

• You get direct access to ILogger , triggers, and bindings.

• Best for legacy apps or when you need maximum binding compatibility.

Example: In-Process Queue Trigger (C#, .NET 6)

using Microsoft.Azure.WebJobs;
using Microsoft.Extensions.Logging;

public static class QueueExample
{
    [FunctionName("QueueExample")]
    public static void Run(
        [QueueTrigger("my-queue", Connection = "AzureWebJobsStorage")] string message,
        ILogger log)
    {
        log.LogInformation($"In-Process Function processed: {message}");
    }
}

Notice that there’s no Program.cs. Everything is handled by the runtime host.

2. Isolated Worker Model (Recommended with .NET 9)

In this model, your functions run in a separate .NET process.
This gives you full control over:

• Dependency Injection

• Middleware

• Hosting configuration

• Supported runtime (you can target .NET 7, 8, or 9 without waiting for host updates).

Example: HTTP Trigger with .NET 9 Isolated Worker

Project file: FunctionApp.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <AzureFunctionsVersion>v4</AzureFunctionsVersion>
    <OutputType>Exe</OutputType>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Azure.Functions.Worker" Version="2.0.0-preview.6" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.Sdk" Version="2.0.0-preview.6" OutputItemType="Analyzer" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Http" Version="3.1.0" />
  </ItemGroup>

</Project>

Program.cs

using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

var host = Host.CreateDefaultBuilder()
    .ConfigureFunctionsWorkerDefaults()
    .ConfigureServices(services =>
    {
        services.AddSingleton<IMyService, MyService>();
    })
    .ConfigureLogging(logging =>
    {
        logging.AddConsole();
    })
    .Build();

host.Run();

public interface IMyService
{
    string GetMessage(string name);
}

public class MyService : IMyService
{
    public string GetMessage(string name) => $"Hello {name}, from a .NET 9 Isolated Function!";
}

HelloFunction.cs