iPhone multitasking and webservice calls

Within my iPhone application I periodically make calls to a webservice, providing the endpoint with a list of numeric IDs. The webservice then returns information relating to the IDs it receives.

This is all well and good. However, I would like to be able to provide functionality whereby the user will receive a local/push notification when these changes occur, regardless of whether the application is open or not.

I'm just looking for guidance on my 开发者_如何学编程options in this scenario. As I see it, there are two main approaches: calculate any data changes on my webserver and send a push notification to all devices, or query the webservice from the device itself.

The second option seems ideal, as not all devices will need each push notification, but I'm unsure as to whether this is possible with the current state of iOS' multitasking APIs. Any advice would be appreciated.

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Bad news: it's not possible. Apps can only run in the background for a short period of time after the user has exited unless it fits into a small number of categories (GPS, VoIP, etc).

Web services, unfortunately, do not count. So this would have to be performed on the server side and with push notifications.