How to Effectively Upload Large Files (≥3GB) to a FastAPI Backend?

How to Upload a Large File (≥3GB) to FastAPI Backend?

FastAPI can process files exceeding 1MB by retrieving portions of the file at a time from the request body. This approach eliminates the need to load the entire file into memory, which is recommended when dealing with large files.

Client-Side Request:

m = MultipartEncoder(fields = {"upload_file":open(file_name,'rb')})
prefix = "http://xxx:5000"
url = "{}/v1/uploadfiles".format(prefix)
try:
    req = requests.post(
    url,
    data=m,
    verify=False,
            )

Server-Side Response:

HTTP 422 {"detail":[{"loc":["body","upload_file"],"msg":"field required","type":"value_error.missing"}]}

Reason for the Error:

The error occurs because the client-side request omits the Content-Type header. FastAPI expects the client to send multipart/form-data requests when uploading files. Without the Content-Type header, FastAPI cannot correctly parse the request body.

Solution 1 (Recommended): Using Streaming File Uploads and Chunk-Encoded Requests

HTTPX library supports streaming file uploads by default, allowing you to send files without loading them entirely into memory.

Example:

import httpx
import time

url = 'http://127.0.0.1:8000/upload'
files = {'file': open('bigFile.zip', 'rb')}
headers = {'Filename': 'bigFile.zip'}
data = {'data': 'Hello World!'}

with httpx.Client() as client:
    start = time.time()
    r = client.post(url, data=data, files=files, headers=headers)
    end = time.time()
    print(f'Time elapsed: {end - start}s')
    print(r.status_code, r.json(), sep=' ')

Solution 2: Using streaming_form_data Library

This library provides astreaming multipart/form-data parser for Python, allowing you to parse multipart/form-data requests without fully loading the entire request body into memory.

Example:

from streaming_form_data import StreamingFormDataParser
from streaming_form_data.targets import FileTarget, ValueTarget

app = FastAPI()
MAX_REQUEST_BODY_SIZE = 1024 * 1024 * 1024 * 4  # = 4GB
MAX_FILE_SIZE = 1024 * 1024 * 1024 * 3  # = 3GB

@app.post('/upload')
async def upload(request: Request):
    
    parser = StreamingFormDataParser(headers=request.headers)
    filename = request.headers.get('Filename')
    file_ = FileTarget('./' + filename)
    data = ValueTarget()
    parser.register('file', file_)
    parser.register('data', data)
    body_validator = MaxBodySizeValidator(MAX_REQUEST_BODY_SIZE)
    file_validator = MaxSizeValidator(MAX_FILE_SIZE)
    
    async for chunk in request.stream():
        body_validator(chunk)
        parser.data_received(chunk)

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