Why is ProgressEvent.lengthComputable false?

Question

I am loading a JSON file using XMLHttpRequest in Google Chrome, Safari and Firefox. In all three browsers I am receiving ProgressEvents which correctly show the .loaded property. However the .lengthComputable property is false and the .total property is zero. I have checked that the Content-Length HTTP header is being sent and is correct - it is. The response is being gzip-encoded, but the Content-length correctly shows the encoded length (before decompression).

Why would the total length not be available in my ProgressEvents?

Here are the headers:

HTTP/1.1 200 OK
ETag: "hKXdZA"
Date: Wed, 20 Jun 2012 20:17:17 GMT
Expires: Wed, 20 Jun 2012 20:17:17 GMT
Cache-Control: private, max-age=3600
X-AppEngine-Estimated-CPM-US-Dollars: $0.000108
X-AppEngine-Resource-Usage: ms=2 cpu_ms=0 api_cpu_ms=0
Content-Type: application/json
Content-Encoding: gzip
Server: Google Frontend
Content-Length: 621606

Note: the file is being served via Google App Engine.

Here is the JavaScript:

var req;
if (window.XMLHttpRequest){
    req = new XMLHttpRequest();
    if(req.overrideMimeType){
        req.overrideMimeType( "text/json" );
    }
}else{
    req = new ActiveXObject('Microsoft.XMLHTTP');
}

// Listen for progress events
req.addEventListener("progress", function (event) {
    console.log(event, event.lengthComputable, event.total);
    if (event.lengthComputable) {
        self.progress = event.loaded / event.total;
    } else if (this.explicitTotal) {
        self.progress = Math.min(1, event.loaded / self.explicitTotal);
    } else {
        self.progress = 0;
    }
    self.dispatchEvent(Breel.Asset.ON_PROGRESS);
}, false);

req.open('GET', this.url);

Note: The console.log in that code is showing hundreds of events with up to date .loadeds but .lengthComputable is always false and .total is always zero. self refers to the object responsible for this XMLHttpRequest.

Answer 1

If lengthComputable is false within the XMLHttpRequestProgressEvent, that means the server never sent a Content-Length header in the response.

If you're using nginx as a proxy server, this might be the culprit, especially if it's not passing the Content-Length header from the upstream server through the proxy server to the browser.