Only the chosen may pass through APNS
Safety is paramount in Apple's approach to APNS. The service and the client API guard against misuse. The sender is validated using keys issued to registered iPhone Developer Program participants, and the API ensures that only the specifically targeted application can unpack the message data. Opportunities for abuse are quite limited because cooperation of the user -- only a user can launch an iPhone application -- and sender identity that's beyond repudiation are prerequisites for getting a message through. APNS has the advantage of being extremely simple in concept, server platform-agnostic, safe, and free to use for anybody who has software listed on iPhone's App Store.
But the delivery of an APNS notification is neither guaranteed nor acknowledged, even though Apple's experience running global infrastructure makes delivery awfully likely. APNS has some qualities of RIM's fire-and-forget push infrastructure, but APNS is strictly one-way: From an external server, through APNS, to an iPhone. There is no return path for a delivery receipt or other communication from the phone.
The worst-case scenario that APNS presents is one in which some flashlight or girly pix app from App Store is a Trojan horse for notification spam. Apple has wired in an easy solution: If the iPhone user has uninstalled the application targeted by a push notification, if the App Store has withdrawn the app, or if the server generating the notification messages is sending an excess of messages, the APNS blocks the sender. In the case of uninstalled apps, Apple adds the targeted iPhone's device ID to a list of IDs maintained on Apple's servers. Servers are required to consult this list -- Apple recommends hourly -- to avoid tying up the APNS servers with undeliverable notifications.
APNS is not a substitute for multitasking
APNS isn't a substitute for other platforms' background processes or remote execution capabilities, but with cooperative users who launch apps when notifications appear, many custom mobile applications and services that rely on background tasks can now be ported to the iPhone. Apple gets to stick to its guns and still give customers and developers some of what they want.
APNS does have one major flaw: It is strictly one-way. It's impossible for a sender to know whether a message has been delivered to the device until the recipient drops what he's doing to launch the targeted application (which must then acknowledge receipt manually). There is no way to determine even whether a given iPhone is present on a network.
But for active users who want to be reachable at a moment's notice, APNS fits the bill without breaking the platform.
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