Pushing near real time position updates to a number of clients

Question

I'm currently in the early stages of designing a cross-platform mobile app (iOS, Android, WP8). At its core, the application will allow proximity based interaction between an unknown number of client devices.

Could someone recommend a framework or technology that would allow me to push server generated location based proximity alerts to the clients that even scales well? Could push notifications (cross platform using PushSharp) be a viable option for this, or will Push notifications always go hand in hand with some kind of toast notification on the device?

Update: 30 seconds or even a minute delay should do.

Answer 1

Push Notifications have way to much latency to give you anything near real-time. Also Push Notifications are not guaranteed to reach a device.

You really need to think about how much time constrained the location updates should be. You should also make some tests with how fast the GPS on various devices updates. I have tested on a Nexus 4 and it is not near real-time either. So throughout your entire application whether it is server side or client side you will have things blowing your real-time wishes.

However IF you can live with a delay of a couple of seconds to maybe 30 seconds or more, Push Notifications my work well for you, they also scales quite nicely.

EDIT:

Push notifications always go hand in hand with some kind of toast notification on the device?

There is a concept called RAW notifications, which allow you to send arbitrary information with the Notifications. However personally I would just notify the client about updates ready on a server, where it then can get all the information the application needs. This is because as I said Push Notifications are not guaranteed to ever reach the device, but also because you are limited to how much information you can embed in the Notifications.

So my suggestion for a flow would be:

1. Client A updates its location and sends it to a web service
2. Web Service receives info from Client A and prepares notifications for all the other Clients, which need information about Client A and pushes information about they need to update their info.
3. Client B receives a Push Notification telling it to refresh data from the Web Service and does that.

That would work well if the application is in a background state. When showing in the foreground I would simply poll a server every second or so; Still receiving Notifications just forcing the client to update.

Answer 2

For the "scales well" part, you might also want to take a look at Windows Azure Service Bus Notification Hubs. It's currently in preview and supports only Windows 8 Store apps and iOS but support for Android and Windows Phone is on the way.