FastAPI Fetch error Failed to load .../openapi.json: 404 Error When Deployed in Kubernetes with a Path Prefix

Question

I'm having trouble with my FastAPI application when deployed in a Kubernetes pod. My application is exposed at https://api-gateway.example.com/myapp (myapp is part of the host), but OpenAPI was trying to fetch the JSON from https://api-gateway.example.com/api/v1/openapi.json (without myapp), resulting in a 404 error: Failed to load API definition.

Here's the relevant part of my main.py configuration:

    from fastapi import FastAPI
    from fastapi.middleware.cors import CORSMiddleware
    
    app = FastAPI(
        title="My Project",
        openapi_url=f"/api/v1/openapi.json",
        docs_url=f"/api/v1/docs",
        redoc_url=f"/api/v1/redoc",
        root_path="/myapp" # I added this to fix the error <-----------------
    )
    
    app.add_middleware(
        CORSMiddleware,
        allow_origins=["*"],
        allow_credentials=True,
        allow_methods=["*"],
        allow_headers=["*"],
    )
    
    @app.get("/ping")
    def ping():
        return {"message": "pong"}
    
    # Include other routers and setup

This is my dockerfile if it is useful:

FROM python:3.12.3-slim
ENV PYTHONUNBUFFERED 1
ENV APP_HOME=/home/app/web
ENV APP_DEPS=/home/app

RUN mkdir -p $APP_HOME

RUN apt-get update && apt-get install -y \
    gcc \
    g++ \
    make \
    python3-dev \
    tzdata \
    libpq-dev \
    libffi-dev \
    cmake \
    wget \
    && apt-get clean \
    && rm -rf /var/lib/apt/lists/*

WORKDIR $APP_DEPS
COPY requirements.txt $APP_DEPS
RUN python -m pip install --upgrade pip
RUN pip install -r requirements.txt

WORKDIR $APP_HOME
COPY /src/ $APP_HOME

CMD ["uvicorn", "main:app", "--reload", "--host", "0.0.0.0", "--port", "8000"]

Answer 1

It seems the issue arises because FastAPI needs to know the root path for all routes when deployed behind a reverse proxy or API Gateway that adds a prefix to the URL.

Here's how I resolved it:

Set the root_path in FastAPI configuration: This tells FastAPI to prepend the given root path to all routes, ensuring that the OpenAPI documentation and other paths are correctly resolved.

app = FastAPI(
    title="My Project",
    openapi_url=f"/api/v1/openapi.json",
    docs_url=f"/api/v1/docs",
    redoc_url=f"/api/v1/redoc",
    root_path="/myapp" # <------ This root_path fix the problem
)