PPT - Joseph Smarr

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Transcript PPT - Joseph Smarr

Cross-Site Ajax
Challenges and Techniques for
Building Rich Web 2.0 Mashups
Joseph Smarr
Plaxo, Inc.
[email protected]
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The promise of mashups
• Create new experiences by combining
components from different authors
– Each site focuses on what it does best
– Can wire up components in a novel way
• Google maps + Craigslist = HousingMaps
• Rich interaction often requires talking
back and forth between components
– House’s address  lat / long  map it
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Talking between web components
• Normal situation: all on the same web site
– Communicate across frames / iframes / popups
– Talk to server using AJAX (XMLHttpRequest)
• Problem: doesn’t work when components
live at different domains (mashup situation)
– Same-origin policy
• Can include JavaScript / images from another domain
• Can’t talk to frame / iframe popup or send an XHR
• Prevents snooping on secret web pages (e.g. intranet)
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•CAN include image / JavaScript / CSS from Domain B
•CAN send XMLHttpRequest to Domain A
•CAN talk to other frames / iframes / popups on Domain A
•CAN'T send XMLHttpRequest to Domain B
•CAN'T talk to other frames / iframes / popups on Domain B
•CAN talk to other pages
on Domain A
•CAN’T talk to any page
on Domain A
XML / Web Page
• So, how do mashups
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How do mashups communicate?
• Often, they don’t (Google maps 1.0)
– Single JavaScript  all processing done locally
• Plotting lat/long or fetching image tiles is deterministic
– Can’t get any additional data (e.g. geo-coding)
• Server-side proxy (HousingMaps, Flickr)
– Talk to your server  it talks to the foreign site
– Problem: bottleneck; barrier to mass deployment
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Server-side proxy
Domain A
Domain B
Web Page
Domain A
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How do mashups communicate?
• Often, they don’t (Google maps 1.0)
– Single JavaScript  all processing done locally
• Plotting lat/long or fetching image tiles is deterministic
– Can’t get any additional data (e.g. geo-coding)
• Server-side proxy (HousingMaps, Flickr)
– Talk to your server  it talks to the foreign site
– Problem: bottleneck; barrier to mass deployment
• Use flash for cross-domain communication
– Can use crossdomain.xml to allow foreign sites
– Yahoo maps uses this to do geo-coding
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Flash proxy
Domain B
Web Page
Domain A
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How do mashups communicate?
• JSON-P (Script injection + callback)
– Dynamically load JavaScript file with data
• Use DHTML to inject a <script> tag in the document
• Data is returned in JSON (JavaScript data format)
– Clever trick: specify callback function on URL
• foreign-site.com/json-api.js?callback=myFunc
• Loads myFunc({
data: “hello” });
 round-trip!
– Works well in practice, but some drawbacks
• Limited control vs. XHR (just loads or doesn’t)
• API provider can return malicious code (e.g. steal cookies)
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Domain B
JSON + callback
Web Page
Domain A
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Using JSON-P in OO web apps
• Problem: callback function has to be global
– Complex code maintains state / instances
– Would like to handle callback with instance function on
instantiated object
• Solution: bind the callback dynamically
– Create a globally-unique function name
• Usually global prefix + auto-incremented counter
– Use a closure to call your function in object-scope
– Send the global function name as callback 
it calls your scoped function
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Dynamically binding a global callback
var cbName = 'cb' + JSON.callbackCounter++;
var cbFunc = function(jsonData) {
myFunc.call(myObj, jsonData); // call bound callback
JSON.callbacks[cbName] = cbFunc;
var globalCallbackName = ‘JSON.callbacks.’ + cbName
// url: '/json-api.js?callback=' + globalCallbackName;
• Used by dojo (ScriptSrcIO), Google Maps 2.5, Plaxo, etc.
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Summary of cross-site techniques so far
• No cross-site communication
– Include partner’s JS file once (can’t talk again)
• Server-side proxy
– Talk on the backend (bottleneck, barrier to adoption)
• Flash proxy
– Talk through cross-domain flash (requires flash)
– Talk through script-injection with callback
(less control, partner could be malicious)
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What about updating another web page?
• Proxies / JSON-P let you access foreign data
– But still can’t touch a foreign frame/iframe/popup
• Many potential mashups would like to interact with
existing web pages
– Auto-fill a form with your contact info
– Highlight relevant search results / text snippets
• How can two sites that want to cooperate get
around the same-origin policy?
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What does the partner site have to do?
• Add the
button to your page
<a onclick="showPlaxoABChooser('textarea', '/cb.html'); return false"
<img src="http://www.plaxo.com/images/abc/buttons
/add_button.gif" alt="Add from my address book" /></a>
– Specify the ID of your e-mail <textarea>
– Specify the location of your hidden callback page
• Add a small callback page on your site
<html><head><script type="text/javascript"
• Full instructions and demo: http://www.plaxo.com/api
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What does Plaxo have to do?
• Remember: Plaxo filled in a textarea on zazzle!
– Need to get around same-origin policy
– Without server-side proxy (JS/HTML only)
• JSON-P won’t solve this problem
– Widget popup is hosted by Plaxo and goes thru several steps
– Zazzle doesn’t know when to request the contact data
• Solution: “The JavaScript Wormhole”
– Add hidden callback page on zazzle that includes Plaxo script
– Plaxo popup loads callback in an iframe when done
– Script is dynamically generated, and includes selected data
– IFrame is also on zazzle (and has the data), so it can tell
parent.opener to fill in the textfield
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Iframe: zazzle.com/cb.html
 Script: plaxo.com/ab_chooser/abc_comm.jsdyn
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Who’s using the Plaxo widget?
• See more at http://www.plaxo.com/api/gallery
• Using the widget? Let us know!
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Generalizing the JavaScript Wormhole
• Site has a generic callback page to give you access
– Site tells you the location of their callback page
– Callback page loads your domain’s JavaScript
• JavaScript is dynamically generated to include your data
• Can pass script url with query args to callback page
– /cb.html?http://foreign-site.com/json-api.js?name=val
– Access query string on cb page with location.search
• Site can restrict callback page to trusted hosts
– Only load script if it’s on a trusted domain
– Could further restrict to certain URL prefixes, etc.
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<html><head><title>Generalized JavaScript Wormhole</title>
<script type="text/javascript">
var trustedDomains = [
function isTrustedDomain(url) {
for (var i = 0; i < trustedDomains.length; i++)
if (url.indexOf(trustedDomains[i]) == 0)
return true;
return false;
function doWormhole() {
var url = location.search.substr(1); // chop off ?
if (isTrustedDomain(url)) {
var script = document.createElement('script');
script.type = "text/javascript";
script.src = url;
} else alert("ignoring untrusted url: " + url);
</script></head><body onload="doWormhole()"></body></html>
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Where do we go from here?
• Ajax is a “hack” on top of an old platform
• The platform can evolve
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Where do we go from here?
• Ajax is a “hack” on top of an old platform
• The platform is evolving
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Where do we go from here?
• Ajax is a “hack” on top of an old platform
• The platform is evolving
• What platform do we want to build?
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Should we open cross-domain XHR?
• After all, we can already include foreign JavaScript,
image, and CSS files
– So why not XML too? (“just another resource file”)
• HTML looks like XML (esp. XHTML)
– Hard to make sure you really meant to serve that page
as XML data (cf. js / img / css)
• Personal / private info more often in XML / HTML
– But increasingly a problem with JS too (JSON)
• Cookies / creds sent with XHR request
– Can’t easily distinguish direct vs. XHR access
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Trust relationships between sites
• Random site accessing this file vs. trusted partner
– Flash does this with crossdomain.xml
– Web services do this with certs / IP whitelists
– JavaScript wormhole does it (sort of)
• Should a trust relationship exist for mashups?
– Want to minimize barriers to adoption / innovation
– When sharing user info, should have prior agreement?
– How formal / technical should this trust be?
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Proposals for better cross-site tools
• ContextAgnosticXmlHttpRequest (Chris Holland)
– Alternative to normal XmlHttpRequest for cross-site
– Don’t send/receive any cookie or HTTP auth info
– Server must specify X-Allow-Foreign-Hosts in response
• JSONRequest (Douglas Crockford)
– New browser object for cross-site JSON (like XHR)
– Allows GET / POST / cancel of JSON request / response
– No cookies / auth info sent or received
– Requires special headers in request / response
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In conclusion…
• Cross-site communication is tricky but important
– Key enabling technology for building rich mashups
– Raises legitimate security issues that can’t be ignored
• Today: Use server-side proxy or JSON-P
– Proxy introduces bottleneck, but provides full access
– JSON-P is more scalable, but limited to JSON APIs
– JavaScript Wormhole lets you touch foreign pages
• Keep the discussion of better tools / protocols going!
– This problem is here to stay
– Browser developers are listening!
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For further reading…
• Cross-site limitations
– http://getahead.ltd.uk/ajax/cross-domain-xhr
– http://msdn.microsoft.com/library/default.asp?url=/workshop/a
• FlashXMLHttpRequest
– http://blog.monstuff.com/FlashXMLHttpRequest
• ContextAgnosticXMLHttpRequest
– http://chrisholland.blogspot.com/2005/03/contextagnosticxmlht
• JSONRequest
– http://json.org/JSONRequest.html
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