Google Shutting Down Cost-per-action Advertising

Google have advised they are closing down the "Google Affiliate Network". This Cost-per-action (CPA) advertising service, allowed web publishers, such as myself, to earn revenue when someone carried out a specified action (such as booking a trip). This is separate to the Google AdSense service, where the viewer just has to click on the ad for the publisher to receive a payment. Google purchased DoubleClick, which had a controversial history. Renamed "Affiliate Network", the system was never fully integrated into their other advertising system (AdWords) and never quite worked for me. In theory you could get a lot of revenue if someone carried out the specified action, perhaps a hundred times as much as for them just clicking on an ad. But hardly anyone ever did and the system required more manual intervention than Adsense (which essentially runs itself). I expect Google are working on expanding Adsense with an integrated CPA function.