how does Auto-renewable in app purchase / subscription renew itself in iphone -

i trying understand how auto-renewal part works. far went through

verify receipt in app purchase

http://www.phpriot.com/articles/verifying-app-store-receipts-php-curl

and apple documentation. have got app purchasing part complete not able auto-renewal flow of it.

when purchase expires apple server response has status:21002 receipt valid has expired.

what trying understand triggers new purchase? app store in backend? how app know renewal has happened? or in other words, how renewal certificate reach application processing?

br, darshan

the app store charging when auto renewable subscription expired. application not informed when happens. when validate auto renewable receipt has been extended server receives additional receipt new expire date.

documented here: http://developer.apple.com/library/ios/documentation/networkinginternet/conceptual/storekitguide/renewablesubscriptions/renewablesubscriptions.html