Reading a subset of data on a BLE server

Question

I am definitely missing something here, but I can't figure out how to specify a subset of data to read from my peripheral. 
 Here's the setup: my nrf51822 device is a server, publishing a custom service with custom characteristics. 
 I have in memory is the nrf51822 a long (2048 bytes) list of historical data. 
 I want to allow a client to be able to read a specific portion of this data rather than having to transfer all 2048 bytes (which I can't anyway). 
 The SoftDevice handles characteristic reads automatically, but even if I could intercept them I don't see how I can pass parameters to a characteristic read operation. 
 What is the official BLE way to do what I'm trying to do?

Carles · Accepted Answer

In the meantime, is there a way to get an event when the server gets a read from a client? What you are looking for is called "Read Authorization". The attached MSC should help you out. To create a characteristic with read authorization set the following bit: ble_gatts_attr_md_t :: rd_auth when calling: sd_ble_gatts_characteristic_add() So, combining rd_auth on your side with usage of Read Long Characteristic Values (aka Read Blob) you could simply do this: <- Read_Blob_Req(offset = 0) -> Read_Blob_Resp(first 20 bytes of data) <- Read_Blob_req(offset = 20) -> Read_Blob_Resp(bytes 20 to 40) ...

Reading a subset of data on a BLE server

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