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h2zero / NimBLE-Arduino

A fork of the NimBLE library structured for compilation with Arduino, for use with ESP32, nRF5x.
https://h2zero.github.io/NimBLE-Arduino/
Apache License 2.0
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How to send data via a characteristic as fast as possible / detect lost information #728

Open h2zero opened 1 month ago

h2zero commented 1 month ago

Discussed in https://github.com/h2zero/NimBLE-Arduino/discussions/707

Originally posted by **Tirus42** August 11, 2024 Hello together, I am working on a BLE remote control interface with a flexible web interface (web bluetooth) to allow to control actions on the ESP via a browser. See https://github.com/Tirus42/Arduino-BLE-Remote for details, the send operation is located there: https://github.com/Tirus42/Arduino-BLE-Remote/blob/6832e6bfcbdc5fab2857ab38abc1bb5b38dc6e3b/arduino-library/src/BLELedController.cpp#L489. For this, i send some data (in my example about 1,5kb) to the connected client by using characteristic.notify() inside a loop with 128 byte chunks. This works, as long as the whole data block is not too large, but for example 5kb of data, there are many packets lost. Also i got problems, when multiple clients are connected. I assume that the BLE stack contains a send queue which will be overloaded by to many packets. Is there a way to detect this and to handle it properly? I saw in a example (https://github.com/h2zero/NimBLE-Arduino/blob/master/examples/Refactored_original_examples/BLE_server_multiconnect/BLE_server_multiconnect.ino Line 139) that a simple delay is used. But this seems not to be a reliable solution.
afpineda commented 4 days ago

Maybe this is already implemented.

User code may implement a NimBLECharacteristicCallbacks::onRead() callback (on the proper characteristic) to perform flow control using semaphores. Before doing so, user code must also keep track of how many clients are actually using the characteristic thanks to the onSubscribe() callback. You can have many different services in a GATT server, but clients are not forced to use all of them. As a result, the count of connected clients is not reliable for this purpose.