Option 1 – Using Middleware

You could use a Middleware. A middleware takes each request that comes to your application, and hence, allows you to handle the request before it is processed by any specific endpoint, as well as the response, before it is returned to the client. To create a middleware, you use the decorator @app.middleware("http") on top of a function, as shown below. As you need to consume the request body from the stream inside the middleware—using either request.body() or request.stream(), as shown in this answer (behind the scenes, the former method actually calls the latter, see here)—then it won’t be available when you later pass the request to the corresponding endpoint. Thus, you can follow the approach described in this post to make the request body available down the line (i.e., using the set_body function below). As for the response body, you can use the same approach as described in this answer to consume the body and then return the response to the client. Either option described in the aforementioned linked answer would work; the below, however, uses Option 2, which stores the body in a bytes object and returns a custom Response directly (along with the status_code, headers and media_type of the original response).

To log the data, you could use a BackgroundTask, as described in this answer and this answer. A BackgroundTask will run only once the response has been sent (see Starlette documentation as well); thus, the client won’t have to be waiting for the logging to complete before receiving the response (and hence, the response time won’t be noticeably impacted).

Note

If you had a streaming request or response with a body that wouldn’t fit into your server’s RAM (for example, imagine a body of 100GB on a machine running 8GB RAM), it would become problematic, as you are storing the data to RAM, which wouldn’t have enough space available to accommodate the accumulated data. Also, in case of a large response (e.g., a large FileResponse or StreamingResponse), you may be faced with Timeout errors on client side (or on reverse proxy side, if you are using one), as you would not be able to respond back to the client, until you have read the entire response body (as you are looping over response.body_iterator). You mentioned that “the body size of both request and response JSON is about 1MB”; hence, that should normally be fine (however, it is always a good practice to consider beforehand matters, such as how many requests your API is expected to be serving concurrently, what other applications might be using the RAM, etc., in order to rule whether this is an issue or not). If you needed to, you could limit the number of requests to your API endpoints using, for example, SlowAPI (as shown in this answer).

Limiting the usage of the middleware to specific routes only

You could limit the usage of the middleware to specific endpoints by:

• checking the request.url.path inside the middleware against a
  pre-defined list of routes for which you would like to log the
  request and response, as described in this answer (see
  “Update” section),
• or using a sub application, as demonstrated in this
  answer
• or using a custom APIRoute class, as demonstrated in Option 2
  below.

Working Example

from fastapi import FastAPI, APIRouter, Response, Request
from starlette.background import BackgroundTask
from fastapi.routing import APIRoute
from starlette.types import Message
from typing import Dict, Any
import logging

app = FastAPI()
logging.basicConfig(filename="info.log", level=logging.DEBUG)
    
def log_info(req_body, res_body):
    logging.info(req_body)
    logging.info(res_body)

async def set_body(request: Request, body: bytes):
    async def receive() -> Message:
        return {'type': 'http.request', 'body': body}
    request._receive = receive
    
@app.middleware('http')
async def some_middleware(request: Request, call_next):
    req_body = await request.body()
    await set_body(request, req_body)
    response = await call_next(request)
    
    res_body = b''
    async for chunk in response.body_iterator:
        res_body += chunk
    
    task = BackgroundTask(log_info, req_body, res_body)
    return Response(content=res_body, status_code=response.status_code, 
        headers=dict(response.headers), media_type=response.media_type, background=task)

@app.post("https://stackoverflow.com/")
def main(payload: Dict[Any, Any]):
    return payload

In case you would like to perform some validation on the request body—for example, ensruing that the request body size is not exceeding a certain value—instead of using request.body(), you can process the body one chunk at a time using the .stream() method, as shown below (similar to this answer).

@app.middleware('http')
async def some_middleware(request: Request, call_next):    
    req_body = b''
    async for chunk in request.stream():
        req_body += chunk
    ...

Option 2 – Using custom APIRoute class

You can alternatively use a custom APIRoute class—similar to here and here—which, among other things, would allow you to manipulate the request body before it is processed by your application, as well as the response body before it is returned to the client. This option also allows you to limit the usage of this class to the routes you wish, as only the endpoints under the APIRouter (i.e., router in the example below) will use the custom APIRoute class .

It should be noted that the same comments mentioned in Option 1 above, under the “Note” section, apply to this option as well. For example, if your API returns a StreamingResponse—such as in /video route of the example below, which is streaming a video file from an online source (public videos to test this can be found here, and you can even use a longer video than the one used below to see the effect more clearly)—you may come across issues on server side, if your server’s RAM can’t handle it, as well as delays on client side due to the whole (streaming) response being read and stored in RAM, before is returned to the client (as explained earlier). In such cases, you could exclude such endpoints that return a StreamingResponse from the custom APIRoute class—especially, if it is a large video file, or even live video that wouldn’t likely make much sense to have it stored in the logs—simply by not using the @<name_of_router> decorator (i.e., @router in the example below) for such endpoints, but rather using the @<name_of_app> decorator (i.e., @app in the example below), or some other APIRouter or sub application.

Working Example

from fastapi import FastAPI, APIRouter, Response, Request
from starlette.background import BackgroundTask
from starlette.responses import StreamingResponse
from fastapi.routing import APIRoute
from starlette.types import Message
from typing import Callable, Dict, Any
import logging
import httpx


def log_info(req_body, res_body):
    logging.info(req_body)
    logging.info(res_body)

       
class LoggingRoute(APIRoute):
    def get_route_handler(self) -> Callable:
        original_route_handler = super().get_route_handler()

        async def custom_route_handler(request: Request) -> Response:
            req_body = await request.body()
            response = await original_route_handler(request)
            
            if isinstance(response, StreamingResponse):
                res_body = b''
                async for item in response.body_iterator:
                    res_body += item
                                    
                task = BackgroundTask(log_info, req_body, res_body)
                return Response(content=res_body, status_code=response.status_code, 
                        headers=dict(response.headers), media_type=response.media_type, background=task)
            else:
                res_body = response.body
                response.background = BackgroundTask(log_info, req_body, res_body)
                return response
            
        return custom_route_handler


app = FastAPI()
router = APIRouter(route_class=LoggingRoute)
logging.basicConfig(filename="info.log", level=logging.DEBUG)


@router.post("https://stackoverflow.com/")
def main(payload: Dict[Any, Any]):
    return payload
 
 
@router.get('/video')
def get_video():
    url="https://storage.googleapis.com/gtv-videos-bucket/sample/ForBiggerBlazes.mp4"
    
    def gen():
        with httpx.stream('GET', url) as r:
            for chunk in r.iter_raw():
                yield chunk

    return StreamingResponse(gen(), media_type="video/mp4")
    

app.include_router(router)