Tag: json

Varnish: No ESI processing, first char not ‘<‘

I’ve spent some time setting up varnish for a service at work where we deliver JSON-fragments from different resources as a unified document. We build the resulting JSON document by including the different JSON fragments through an ESI (Edge Side Includes) statement in the response. Varnish then fetches these fragments from the different services, allowing for independent caching duration for each of the services, combining then into a complete document.

The main “problem” is that of Varnish 2.0.4, varnish does a check to see if the content returned from the server actually is HTML-based content. It does this by check if the first character of the returned document is ‘<'. The goal is to avoid including binary data directly into a text document. Since we’re working with JSON instead of regular HTML/XML, the first character in our responses is not a ‘<', and the ESI statement did not get evaluated. You can however disable this check in varnishd by changing the esi_syntax configuration setting - just add the preferred value to the arguments when starting varnish. To disable the content check, add: 

varnishd … -p esi_syntax=0x1

I’m not sure if you can set this for just one handler / match in varnish as this solved our issue, but feel free to leave a comment if you have any experience with this.

jQuery, .getJSON and the Same-Origin Policy

When creating a simple mash-up with data from external sources, you usually want to read the data in a suitable format – such as JSON. The tool for the job tends to be javascript, running in your favourite browser. The only problem is that requests made with XHR (XMLHttpRequest) has to follow the same origin policy, meaning that the request cannot be made for a resource living on another host than the host serving the original request.

To get around this clients usually use JSONP – or a simple modification of the usual JSON output. The data is still JSON, but the output also includes a simple callback at the end of the request, triggering a javascript in the local browser. This way the creator of the data actually tells the browser (in so many hacky ways) that it’s OK, I’ve actually thought this through. Help yourself.

In jQuery you can trigger the usual handling of events by using “?” as the name of your callback function. jQuery will handle this transparently and then trigger the function you provided to .getJSON in the first place.

Example

url = "http://feeds.delicious.com/v2/json/recent?callback=?";

$.getJSON(url, function(data) { alert(data); });

There’s an article up at IBM’s developerWorks giving quite a few more examples and information about the issue.