.. include:: <mmlalias.txt>


Datalogger: A Semi-Realistic Project (Threaded Version)
=======================================================

.. topic:: Documentation

   * :doc:`../why-os/mqtt`

.. topic:: Trainer's note

   * Continue from :doc:`../messagequeue/index`

Add Second Sensor (Message Queue)
---------------------------------

* Add second sensor
* |longrightarrow| From interrupt, communicate sensor name

.. attention::

   ``std::string_view`` does not *own* |longrightarrow| Lifetime!

.. image:: datalogger-two-sensors.svg
   :scale: 60%

.. literalinclude:: code/datalogger-two-sensors.cpp
   :caption: :download:`code/datalogger-two-sensors.cpp`
   :language: c++

Add Watchdog
------------

.. topic:: Documentation

   * :doc:`../why-os/watchdog`

* Shift sensor data consumption into a dedicated thread
* Feed Watchdog in main thread (which has highest priority)

.. image:: datalogger-watchdog.svg
   :scale: 60%

.. literalinclude:: code/datalogger-watchdog.cpp
   :caption: :download:`code/datalogger-watchdog.cpp`
   :language: c++

Add CSV And MQTT
----------------

.. topic:: Documentation

   * :doc:`../why-os/file`
   * :doc:`../why-os/mqtt`

* Add datalogger functionality to consumer thread
* Write CSV: ``Why::File``
* Publish on an MQTT topic: ``Why::MQTTPublisher``

.. note:: 

   Both File IO and network IO are notoriously blocking!

.. image:: datalogger-csv-mqtt.svg
   :scale: 60%

.. literalinclude:: code/datalogger-csv-mqtt.cpp
   :caption: :download:`code/datalogger-csv-mqtt.cpp`
   :language: c++

.. _why-datalogger-snapshot-button:

Add Snapshot Button (Message Queue)
-----------------------------------

.. image:: datalogger-snapshot-button.svg
   :scale: 60%

* Interrupt from edge triggered GPIO (with hardware debouncing, say
  |:innocent:|): "create CSV snapshot"
* Dedicated handler thread

  * Use a :doc:`mutex <../why-os/mutex>` to guard CSV file rotation
    against concurrent CSV writes
  * Closes and renames file
  * Creates a new file to use from now on

.. note::

   As button presses are enqueued

.. literalinclude:: code/datalogger-snapshot-button.cpp
   :caption: :download:`code/datalogger-snapshot-button.cpp`
   :language: c++