fastapi_http_router.py

from fastapi import FastAPI, Request

app = FastAPI()

@app.middleware("http")
async def change_path(request: Request, call_next):
    # get mutable copy of request 
    request = dict(request)

    # extract host from headers
    _, host = request['headers'][0]
    host = host.decode()

    if ':' in host:
        host = host.split(':')[0]

    # route based on host
    if host == 'sub-a.domain.com':
        request['path'] = f"/router1{request['path']}"
    if host == 'sub-b.domain.com':
        request['path'] = f"/router2{request['path']}"
    
    request = Request(request)
    response = await call_next(request)
    return response

# not separate routers, but you get the point
@app.get('/router1/{name}')
async def get_name(name: str):
    return {'router1': name}

@app.get('/router2/{name}')
async def get_name(name: str):
    return {'router2': name}