Proxy Windows 8 notification image requests to Windows Azure Storage after interpreting query parameters resulting from addImageQuery

server.js

/*
Copyright (c) Microsoft
All rights reserved.
Licensed under the Microsoft Limited Public License (the “License”); you may not
use this file except in compliance with the License.
You may obtain a copy of the License at http://msdn.microsoft.com/en-us/cc300389.

THIS CODE IS PROVIDED ON AN *AS IS* BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
EITHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OR CONDITIONS
OF TITLE, FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABLITY OR NON-INFRINGEMENT.
win
See the Microsoft Limited Public License (Ms-LPL) for specific language governing
permissions and limitations under the License.
*/

var url = require('url');
var http = require('http');

// Azure Storage Account host
var AZURE_URI = "STORAGE_ACCOUNT.blob.core.windows.net";
var AZURE_PROTOCOL = "http";
var AZURE_PORT = 80;

// only build specific URLs for the following
var CUSTOM_LANGUAGES = []; //  ["en-US", "fr-FR"];

var constructImageUri = function(img, l, c, s) {

    var lastSlash = img.lastIndexOf("/");
    if(lastSlash == -1) return img;

    // interpret last segment in URL as file name
    var imagePath = img.substring(0, lastSlash);
    var imageFile = img.substring(lastSlash + 1);
    var imageName = imageFile;
    var imageExt;

    // add language only if it's one with distinct images
    if(CUSTOM_LANGUAGES.indexOf(l) != -1) imagePath = imagePath + "/" + l;

    // add constrast name
    if(c != null) imagePath = imagePath + "/" + c;

    // modify file name to include scale-XXX nomenclature
    if(s != null) {
        var lastDot = imageFile.lastIndexOf(".");
        if(lastDot == -1) lastDot = imageFile.length;
        imageName = imageFile.substring(0, lastDot);
        imageExt = imageFile.substring(lastDot) || "";

        imageName = imageName + ".scale-" + s + imageExt;
    }

    return imagePath + "/" + imageName;
}

var retrieveImage = function(res, uris) {

    // no more URIs to try; recursion fails and no image is served
    if(uris.length == 0) {
        res.writeHead(404);
        res.end();
    }

    // full URI to Azure storage
    var fullUri = AZURE_PROTOCOL + "://" + AZURE_URI + uris[0];

    // create HTTP/HTTPS request to storage
    var options = { host: AZURE_URI, port: AZURE_PORT, method: "HEAD", path: uris[0] };
    require(AZURE_PROTOCOL).request(options, function(r) {
        console.log("  checking: " + fullUri + " - " + r.statusCode + "\r\n");

        // recurse if candidate URI not found
        if(r.statusCode == 404) {
            retrieveImage(res, uris.slice(1));

        // if URI found, redirect
        } else if(r.statusCode == 200) {
            res.writeHead("307", { location: fullUri });
            res.end();

        // pass on any other response (likely error)
        } else {
            res.writeHead(r.statusCode, r.headers);
            r.on('data', function(c) {
                res.write(c);
            });
            r.on('end', function() {
                res.end();
            });
        }
    }).on('error', function(e) {
        console.log(" error: " + e.message + "\r\n");
        res.writeHead("400", null, e.message);
        res.end();
    }).end();
}

http.createServer(function(req, res) {
    console.log("Image request: " + req.url + "\r\n");

    // parse request URL
    var parsedUrl = url.parse(req.url, true);
    var imgPath = parsedUrl.pathname;

    // list of candidate image URLs
    var candidateUris = [];

    // if there's a query string addImageQuery must have been set in template
    if(Object.keys(parsedUrl.query).length > 0) {
        var scaleArg = parsedUrl.query["ms-scale"] || "100";
        var contrastArg = parsedUrl.query["ms-contrast"] || "standard";
        var localeArg = parsedUrl.query["ms-lang"] || "";

        if(CUSTOM_LANGUAGES.indexOf(localeArg) >= 0) {
            candidateUris.push(constructImageUri(imgPath, localeArg, contrastArg, scaleArg));
            candidateUris.push(constructImageUri(imgPath, localeArg, contrastArg, null));
            candidateUris.push(constructImageUri(imgPath, localeArg, null, scaleArg));
            candidateUris.push(constructImageUri(imgPath, localeArg, null, null));
        }
        candidateUris.push(constructImageUri(imgPath, null, contrastArg, scaleArg));
        candidateUris.push(constructImageUri(imgPath, null, contrastArg, null));
        candidateUris.push(constructImageUri(imgPath, null, null, scaleArg));
    }

    // push the default image URL
    candidateUris.push(imgPath);

    // get the image
    retrieveImage(res, candidateUris);

}).listen(process.env.PORT || 8080);