You will need to code it with Python.

If you're using async you should use a HTTP client that is also async, for example aiohttp.

import aiohttp

@app.get("/")
async def slow_route():
    async with aiohttp.ClientSession() as session:
        async with session.get("http://validation_service.com") as resp:
            data = await resp.text()
            # do something with data

The accepted answer certainly works, but it is not an effective solution. With each request, the ClientSession is closed, so we lose the advantage [0] of ClientSession: connection pooling, keepalives, etc. etc.

We can use the startup and shutdown events [1] in FastAPI, which are triggered when the server starts and shuts down respectively. In these events it is possible to create a ClientSession instance and use it during the runtime of the whole application (and therefore utilize its full potential).

The ClientSession instance is stored in the application state. [2]

Here I answered a very similar question in the context of the aiohttp server: https://stackoverflow.com/a/60850857/752142

from __future__ import annotations

import asyncio
from typing import Final

from aiohttp import ClientSession
from fastapi import Depends, FastAPI
from starlette.requests import Request

app: Final = FastAPI()


@app.on_event("startup")
async def startup_event():
    setattr(app.state, "client_session", ClientSession(raise_for_status=True))


@app.on_event("shutdown")
async def shutdown_event():
    await asyncio.wait((app.state.client_session.close()), timeout=5.0)


def client_session_dep(request: Request) -> ClientSession:
    return request.app.state.client_session


@app.get("/")
async def root(
    client_session: ClientSession = Depends(client_session_dep),
) -> str:
    async with client_session.get(
        "https://example.com/", raise_for_status=True
    ) as the_response:
        return await the_response.text()

• [0] https://docs.aiohttp.org/en/stable/client_reference.html
• [1] https://fastapi.tiangolo.com/advanced/events/
• [2] https://www.starlette.io/applications/#storing-state-on-the-app-instance