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NRP Backend Architecture
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Purpose
-------

The :term:`NRP` Backend is responsible for providing a :ref:`backend-rest-api` to launch and control simulated experiments.
Such experiments leverage the nrp-core framework for orchestrating and synchronize different simulators (see TODO LINK).

Experiments are run by executing user-defined python scripts (usually named :code:`main_script.py`) using nrp-core python client (TODO link) to advance the simulation.

Using the REST API provided by NRP Backend, the user can request an experiment to be:

* launched
* paused
* resumed
* stopped


Architecture of the NRP
-------------------------------

.. _architecture-components:

.. figure:: img/architecture_components_backend.png
   :width: 100%

   Architecture of NRP Backend with NRP Frontend and nrp-core.

The component diagram in :numref:`architecture-components` depicts the architecture of the NRP, showing NRP Backend alongside :code:`NRP Frontend` and :code:`nrp-core` and the :code:`Simulation Server`.
The REST API server takes requests, coming from NRP Frontend, and fulfils them interacting with the :code:`Storage Client` to fetch the experiment data, and the :code:`Simulation Server` to run it.

The :code:`Storage Client` access the storage, where the experiment's data are kept, via the :code:`Storage Proxy`'s REST interface.

The :code:`Simulation Server` manages the execution of the experiment's :code:`main_script.py` that drives :code:`nrp-core`'s simulation via its python client.

A MQTT broker is used to send runtime information (e.g. status of the simulation, errors) to the Frontend and to exchange messages between backend's internal components.

Details on the REST API can be found here: :doc:`REST-API`.


Architecture of NRP Backend
-------------------------------

.. _backend-components:

.. figure:: img/components_backend.png
   :width: 100%

   Architecture of NRP Backend

The component diagram in :numref:`backend-components` details the structure of NRP Backend and its packages  :code:`hbp_nrp_backend` and  :code:`hbp_nrp_simserver`.
The first implements the :ref:`rest-server` and the second the :ref:`simulation-server` that runs nrp-core simulation scripts.