Reporting upload progress from node.js - node.js

Question

I'm writing a small node.js application that receives a multipart POST from an HTML form and pipes the incoming data to Amazon S3. The formidable module provides the multipart parsing, exposing each part as a node Stream. The knox module handles the PUT to s3.
var form = new formidable.IncomingForm()
, s3 = knox.createClient(conf);
form.onPart = function(part) {
var put = s3.putStream(part, filename, headers, handleResponse);
put.on('progress', handleProgress);
};
form.parse(req);
I'm reporting the upload progress to the browser client via socket.io, but am having difficulty getting these numbers to reflect the real progress of the node to s3 upload.
When the browser to node upload happens near instantaneously, as it does when the node process is running on the local network, the progress indicator reaches 100% immediately. If the file is large, i.e. 300MB, the progress indicator rises slowly, but still faster than our upstream bandwidth would allow. After hitting 100% progress, the client then hangs, presumably waiting for the s3 upload to finish.
I know putStream uses Node's stream.pipe method internally, but I don't understand the detail of how this really works. My assumption is that node gobbles up the incoming data as fast as it can, throwing it into memory. If the write stream can take the data fast enough, little data is kept in memory at once, since it can be written and discarded. If the write stream is slow though, as it is here, we presumably have to keep all that incoming data in memory until it can be written. Since we're listening for data events on the read stream in order to emit progress, we end up reporting the upload as going faster than it really is.
Is my understanding of this problem anywhere close to the mark? How might I go about fixing it? Do I need to get down and dirty with write, drain and pause?

Answer 1

Your problem is that stream.pause isn't implemented on the part, which is a very simple readstream of the output from the multipart form parser.
Knox instructs the s3 request to emit "progress" events whenever the part emits "data". However since the part stream ignores pause, the progress events are emitted as fast as the form data is uploaded and parsed.
The formidable form, however, does know how to both pause and resume (it proxies the calls to the request it's parsing).
Something like this should fix your problem:
form.onPart = function(part) {
// once pause is implemented, the part will be able to throttle the speed
// of the incoming request
part.pause = function() {
form.pause();
};
// resume is the counterpart to pause, and will fire after the `put` emits
// "drain", letting us know that it's ok to start emitting "data" again
part.resume = function() {
form.resume();
};
var put = s3.putStream(part, filename, headers, handleResponse);
put.on('progress', handleProgress);
};