At work we’ve been having some discussions about sharing content between two websites: the natural first option was an XML solution, in this case RSS. Site A would subscribe to the RSS feeds of the site B, periodically retrieving the updated feeds, caching the contents of each feed for a specified period of time all the while displaying the resulting content on various parts of site A.

A couple months ago (December 2005 to be exact), Yahoo started supporting JSON (a lightweight data interchange format which stands for JavaScript Object Notation), as optional result format for some of it’s web services. The most common thing said about JSON is that it’s better than XML, usually meaning that it’s easier to parse and not as verbose, here’s a well written comparison of XML and JSON if you don’t believe me. While the comparisons of simplicity, openness and interoperability are useful, I think JSON really stands out when you’re working in a browser. Going back to the example I used above where site A needs to display content from site B, as I see it, this a sample runtime / flow that bits travel through in order to make the syndication work:
every_n_seconds() --> retrieve_feed() --> store_feed_entries() and then per request to site A:
make_page() --> get_feed_entries() --> parse_entries() --> display_entries(). There are a number of libraries built in Java for creating and parsing RSS, some for fetching RSS and you there’s even a JSP taglib for displaying RSS. But even with all the libraries, there’s still a good amount of code to write and a number of moving parts you’ll need to maintain. If you do the syndication on the client side using JSON, there are no moving parts. To display just the title of each one of my del.icio.us posts as an example, you would end up with something like this:

<script type="text/javascript" src="http://del.icio.us/feeds/json/ajohnson1200"></script>
<script type="text/javascript">
for (var i=0, post; post = Delicious.posts[i]; i++) {
  document.write(post.d + '<br />');
}
</script>
I’m comparing apples to oranges (server side RSS retrieval, storage, parse and display against client side JSON include) but there are a couple of non obvious advantages and disadvantages:

1. Caching: If used on a number of pages, syndicated JSON content can reduce the number of bits a browser has to download to fully render a page. For example, let’s say (for arguments sake) that we have an RSS feed that is 17k in size and a corresponding JSON feed of the same size (even though RSS would inevitably be bigger). Using the server side RSS syndication, the browser will have to download the rendered syndicated content (again let’s say it’s 17k). Using the JSON syndicated feed across a number of page views, the browser would download the 17k JSON feed once and then not again (assuming the server has been configured to send a 304) until the feed has a new item. Winner: JSON / client
2. Rendering: Of course, having the browser parse and render a 17K JSON feed wouldn’t be trivial. From a pure speed standpoint, the server could do the parse / generate once and then used an HTML rendering of the feed from cache from then on. Winner: RSS / server
3. Searching: Using JSON on the client, site A (which is syndicating content from site B), wouldn’t have any way of searching the content, outside of retrieving / parsing/ storing on the server. Also, spiders wouldn’t see the syndicated content from site B on site A unlike the server side RSS syndication where the syndicated content would look no different to a spider than the other content on site A. Winner: RSS / server
4. Ubiquity: JSON ‘only’ works if the browser has JavaScript enabled, which I’m guessing the large majority of users do have JavaScript enabled. But certain environments won’t and phones, set top boxes and anything else that runs in a browser but not on a PC may not have JavaScript, which means they won’t see the syndicated content. Server side generated content will be available across any platform that understands HTML. Winner: RSS / server

So wrapping up, when should you use JSON on the client and when should you use RSS on the server? If you need to syndicate a small amount of content to non programmers who can cut and paste (or programmers who are adept at JavaScript), JSON seems like the way to go. It’s trivial to get something up and running, the browser will cache the feed you create and your users will see the new content as soon as it becomes available in your JSON feed.

If you’ve read this far, you should go on and check out the examples on developer.yahoo.com and on del.icio.us. Also, if you’re a Java developer, you should head on over to sourceforge.net to take a look at the JSON-lib, which makes it wicked easy to create JSON from lists, arrays and beans.