When Google launched In-app Billing last year on Google Play, developers were able to find more ways to sell and engage users over the lifetime of their apps. In-app Billing now accounts for 23 of the 24 top-grossing apps in Google Play by allowing users to buy virtual goods and upgrades. The total revenue generated from in-app purchases now exceeds revenue from traditional app purchases. Now Google is allowing app developers to set up subscriptions with In-app Billing. This includes monthly or annual subscriptions from inside the apps. All subscriptions are auto-renewing. Developers can set the price and billing interval. After the transaction, Google Play manages the recurring billing and keeps the users informed about their charges by sending them an e-mail at each renewal. Google introduced an HTTP-based publisher API through enterprise-scale backend servers to validate or cancel subscriptions. Glu Mobile is one of the first games to utilize Google Play subscriptions for their Android title Frontline Commando.