Is it possible to stream files through a Spring Boot backend and up to S3, chunk by chunk?

Question

I have an application in which I'm trying to stream files up to S3, but I can't make it stream the file(s) chunk by chunk.

So the application basically consists of a frontend (Node.js), a backend (Java, Spring Boot) and then I have Cloudflare where I store the files.

When a client uploads one or multiple files, I want to stream the file up to cloudflare through the backend. The frontend streams the file(s) in chunks, and I want each chunk to go from the frontend all the way up to Cloudflare, before next chunk is streamed by the frontend. There are multiple reasons why I want to do this, one of them is to ensure that only a chunk is stored in the backend memory at a time. Another reason could be that the frontend will know how much of the file that has been streamed and give feedback to the client.

I've tried for weeks and I've not been able to achieve this, as the frontend seems to just do it's own thing and stream the full file to the backend completely disregarding the stream from backend to Cloudflare. I want the streams to be synchronized and work like one stream rather than two separate streams.

How can I achieve this?

private final S3Client s3Client;

@PostMapping(value = "/file/upload", consumes = "application/octet-stream")
public ResponseEntity<?> upload(MultipartUpload multipartUpload) {

    //Upload chunks
    
    return ResponseEntity.ok().body("File uploaded successfully");
}

I've tried to use BlockingQueue, Future, CompleteMultipartUploadRequest, but no matter what I do, the frontend just keeps pouring all the chunks into the backend without any synchronization with the stream to the S3. Anyone who can help with this?