How to serve queued data via BLE?

Let's say I have a Puck/Pixl/MDBT42Q collecting data in a queue.
Before any overflow happens this data should be polled via BLE by another system as BLE central.

So as BLE peripheral I use NRF.setServices() to create some Service/Characteristics for pushing out data.

The thing is: How can I know if the central actually read the data so I can remove it from my queue?

For the other direction onWrite is a synchronous API for writing data back, but for reading there is only:

• notify: true and/or
  
• indicate: true which is documented as notify + ACK.
  
  

But how does it work?
As peripheral I am supposed to call NRF.updateServices() and set a new value.
But how can I detect if this value was successfully read by the connected central?
How often should I call updateService() in case the queue got filled up a bit and there are N items waiting?

My current idea is a multipart API:

• use onWrite so the central can request the next item
  
• in return the requested item is presented via updateServices()
  
  

If the queue entries have some kind of timestamp or sequence number I can assume that all previous entries are read already and safely remove them.

What do you think?

As peripheral I am supposed to call NRF.updateServices() and set a new value.

Yes, that's right, and you specify notify:true: http://www.espruino.com/Reference#l_NRF_updateServices

But how can I detect if this value was successfully read by the connected central?

Unfortunately I don't think there's any way of doing that at the moment. You could try just calling NRF.updateServices until you get an exception?

My current idea is a multipart API:

I think that's probably the best idea. On newer firmwares the device should have been able to negotiate a higher MTU so you can send 50 bytes each time (not 20) as well, which should make things easier.

I guess on your onWrite event you could have different message types:

• request a packet of data
  
• acknowledge receipt of last packet of data, request a new one
  
• acknowledge receipt of last packet of data and request a new one