File uploads

I have mentioned briefly work that I was doing to wrap file uploading in AJAX for a proper experience. Browser-based file uploads have been downtrodden over the past few years.

In client terms, file uploads work in almost exactly the same way as they have always done: the page blocks while the data is posted, and a very small progress bar shows up in the status bar. This is a user interface disaster for big files.

On the server side, the situation is more varied, but there is often little support for streaming of file uploads. In PHP, file uploads are read wholly into memory, parsed and saved out to a temporary folder before a script even gets called. The request must fit within both PHP's file upload size limit and its memory limit. As far as I can tell, something similar happens in Zope although you can argue that Zope allows other standards for upload such as DAV and FTP natively. In plain CGI, of course, there is no handling of the uploads, so if you're using a CGI wrapper, it can do whatever you want to handle this. Perl's module allows a hook, at least. Python's cgi module doesn't, nor is it easy to subclass.

All in all, the situation of binding file uploads to form submissions, and processing of those in common server-side languages is wholly inadequate as file size gets large. File uploads are convenient because they are a commonly-supported fall-back, but the workarounds, although solving some of these problems, don't have the simplicity of a browser-native solution.

In my recent project I looked at ways of working around these limitations. The best workaround for the client-side problems I have found so far is to perform the upload in an <iframe>, using AJAX queries to present a progress bar. This still has problems, notably that it's one file at a time, both on the choosing and the uploading. In Firefox I can actually perform two concurrent uploads in different <iframes>, but the AJAX progress bar doesn't then update.

Server-side, I wrote the whole thing as a webserver so that the AJAX queries could talk directly to the thread streaming the upload. Additionally I wrote my own parser to parse on-the-fly the data uploaded, so that the daemon knows what is uploading at any given stage. It works quite well, and the system is extensible in that it could combine a daemon that allows other forms of upload; feedback for these would also appear in the browser windows.

Even so, I wish that file uploading was something people were thinking about more. It's central to so many web applications now.

There are numerous problems:

  • File uploads are synchronous. Downloads can happen in the background in their own, but uploads can't.
  • File uploads don't have a proper UI. Current browsers appear to show a tiny upload bar that isn't really very accurate and doesn't give data rates or estimated time remaining.
  • Uploads are chosen one at a time.
  • Javascript can't be used for polish. The model that has empowered Web 2.0 improvements is that of taking an existing HTML/HTTP model and allowing it to be controlled by Javascript. However, there is no way into the uploading or the file selection processes with Javascript.

The most general solution I can see would provide a Javascript API for uploading. This would allow Javascript to show a (native) file chooser dialog, and instruct the browser on what to do with the files it returns. POST or PUT to the origin server seem useful, as does FTP upload. Clearly there are security concerns, but I fail to see how, as long as Javascript may instigate an operation, read upload statistics, but not read the filesystem, this presents a problem.

Perhaps an AJAX-style API could be along these lines:

//configure a native dialog to present to the user

var ufc=new UploadFileChooser();

ufc.setAcceptableFileTypes(['image/jpeg', 'image/png']);

var uploads=ufc.chooseFiles();

for each (var u in uploads)


u.onreadystatechange=doSomething; //callback

//this URL is constrained the origin server to prevent XSS



After this, the user could close the tab or leave the page, and the browser would upload the files in the background, perhaps with a progress bar appearing within the Downloads window. Note that it could queue the files rather than uploading them all at once, depending on user settings. The Javascript, and indeed the user, should be able to request that an upload is aborted. The Javascript should also be able to query the upload, using the object reference provided.


Comments powered by Disqus