Which service to use for generic data?

[Frank Zhao]

I need to have an iOS device send my nRF51 some commands, some settings, and retrieve some data, not at regular intervals. The nRF51 should only-speak-when-spoken-to.

I'm looking at all of the possible Bluetooth "services" and none of them seems like they are designed for this.

Except maybe HID, where I can probably use custom input and output reports for my purposes. Will this have an problems working?

Or maybe "alert notification"? I see how to get a message to the nRF51, but I don't know how the nRF51 can send a message back.

Is there another better way?

[wlgrd]

You can create your own service, with an arbitrary UUID and send data to this service, creating your own characteristic and handlers to write data to the nRF51.

You can use the notification service to send data to the iOS-device whenever the nRF51 has some data to send. This can be a bit tricky, but by using the example projects provided you should be able to derive how to set up your own service.