Apple's iPhone is renowned for being the sole mobile platform that runs only one application at a time. If you want to write an instant messaging client for iPhone, knock yourself out, but recipients will be reachable only while your IM software has control of the screen. As soon as an iPhone user presses Home, the running application quits, voluntarily or otherwise. It's not allowed to leave so much as a thread behind to listen for connections from the network, do periodic GPS logging, or run anything else in the background. Its competitors, such as the Palm Pre and RIM BlackBerry, have no such limitation.
Apple's not likely to give ground on background apps, but it does realize the competitive and functional gap that its one-app-at-a-time policy creates. So the iPhone 3.0 OS offers the APNS (Apple Push Notification Service) to provide a workaround. Is it a limited workaround or enough of a bridge to overcome the multitasking advantage of its competitors?
[ Should you buy an iPhone 3G S or just upgrade to the iPhone 3.0 OS? | See who wins InfoWorld's mobile deathmatch: BlackBerry or iPhone. ]
APNS does run in the background and listens for communications from a single server, and vendors or organizations are free to use APNS as a gateway to deliver short alerts and data messages to individual iPhones. Google, Yahoo, and AOL still can't run background tasks on your iPhone, but as long as they link to APNS, they can push chat invitations that pop up on the invitee's iPhone no matter what he's running. If the user taps Accept, the IM client launches and the session begins.
IM is neither the only nor the best usage scenario for APNS. Most users will experience it in a familiar way: A small badge that appears on an app's icon, such as the familiar unread-message counter in the iPhone's Mail app or the new new-invitation counter in its Calendar app. (The user has to see the Home page in which the app resides to see such indicators, however, which means leaving whatever app you may be in and seeing if other apps have any new notifications.)
If the app is running, it gets the notification immediately. If the app isn't running, the notification is held in the phone to be consumed at the app's next launch. If the iPhone is offline when the sender attempts delivery, APNS attempts to send the notification for 28 days. (The notification itself is a small [256-byte], arbitrary encrypted payload sent from a server-side app to a specific application running on a specific iPhone. So APNS is a general mechanism for shooting structured data to iPhone applications.)
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