Google AdSense is great, but at some time, it may not show ads in the ads slot. Instead it will leave blank space. This will make the site looks ugly and unprofessional. 
This is rare but it may happen. Anyway we don’t want to miss out on any potential income.

This is why we want to set backup ads. There are some AdSense Alternatives: please read Top 10 Highest Paying Google Adsense Alternatives in 2013

But when we set up ad, most of ads network just gave us some javascript code which we can paste to our website. 
But as described at Backup adsAdSense backup ads should be one url.

To fix this, we can create a html to hold the JavaScript: just put an empty page with the JavaScript:
AdSense Backup Ads: No Ad Space Left Behind
if(document.location.pathname.match(new RegExp(“/[0-9]{4}/[0-9]{2}/.*”))!=null){
document.write(“(adsbygoogle = window.adsbygoogle || []).push({});”)
Next, how/where to serve these ads pages?
Luckily Google App Engine comes to rescue. Google App Engine allow us to create free application, and gave us generous free quotas: 1 GB daily request limit. This is engough for my small blog.
This saves me the cost of renting a web server.

I created a GAE application: http://placeholdads.appspot.com/, and add some ads pages:
Chitika 160X600 Ads
Chitika 728X90 Ads

Now, I can put these urls as Adsense backup ads in Adsense website.

via Blogger http://lifelongprogrammer.blogspot.com/2013/10/use-gae-to-hold-adsense-backup-ads.html