Running FastAPI Web Apps on IIS with HttpPlatformHandler

HttpPlatformHandler can help IIS host Java/Python/Node.js/Go applications, so in this post we wil see how to configure a Python/FastAPI web app on IIS and troubleshoot the common issue.

It becomes very important for Python developers to learn HttpPlatformHandler, because Microsoft no longer recommends FastCGI,

“We recommend using HttpPlatform to configure your apps, as the WFastCGI project is no longer maintained.”

Prerequisites

To follow this post, you need to have the following software installed,

• Windows 10 or Windows Server 2016 or later (IIS 10 or later)
• HttpPlatformHandler v1.2 (from Microsoft) or v2.0 (from LeXtudio)

Basic FastAPI Setup

No doubt we will start from a sample application as below,

from typing import Union

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}

If we save it as C:\test-fastapi\main.py, then on a Windows machine with Python and FastAPI installed, a simple command uvicorn main:app --reload in the directory of C:\test-fastapi\ can launch the application at port 8000,

$ C:\Users\lextudio\AppData\Local\Programs\Python\Python310\python.exe -m uvicorn main:app --reload  
INFO:     Will watch for changes in these directories: ['C:\\test-fastapi']
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [11128] using StatReload
INFO:     Started server process [10864]
INFO:     Waiting for application startup.
INFO:     Application startup complete.

If Python and FastAPI are not yet installed, you can search for guides.

HttpPlatformHandler Setup

Now let’s download and install HttpPlatformHandler on IIS, and add a web.config in C:\test-fastapi,

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <handlers>
            <add name="httpPlatformHandler" path="*" verb="*" modules="httpPlatformHandler" resourceType="Unspecified" requireAccess="Script" />
        </handlers>
        <httpPlatform stdoutLogEnabled="true" stdoutLogFile=".\python.log" startupTimeLimit="20" processPath="C:\Users\<user name>\AppData\Local\Programs\Python\Python310\python.exe" arguments="-m uvicorn --port %HTTP_PLATFORM_PORT% main:app">
        </httpPlatform>
    </system.webServer>
</configuration>

With all settings in place, I can go back to IIS Manager and create a site (I chose *:8013 as site binding) to point to C:\test-fastapi. By opening a web browser and navigate to http://localhost:8013/, I should now see {"Hello":"World"}.

Troubleshooting

Well if you hit any error instead of the JSON response, please refer to my old post for troubleshooting tips.

FastAPI on IIS Express

You can take a look at the new open source HttpPlatformHandler v2.0 from LeXtudio.

Other Languages on IIS?

If you want to learn more about HttpPlatformHandler and how to host other languages (Go/Node.js/Java) or frameworks (Django/Flask), you can read this post.