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.. _sec-howto:

###################################
How To
###################################


.. _sec-howto-createrun:

******************************
Create and Run an Experiment
******************************

Experiments are defined in a declarative fashion inside Python modules using
classes. Basically, create a `.py` file and add a global variable
``experiments``, a list which contains multiple instances of the class
:class:`~simbricks.orchestration.experiments.Experiment`, each one describing a
standalone experiment. This is very helpful if you wish to evaluate your work in
different environments, for example, you may want to swap out some simulator or
investigate multiple topologies with different scale. 

The class :class:`~simbricks.orchestration.experiments.Experiment` provides
methods to add the simulators you wish to run. All available simulators can be
found in the module :mod-orchestration:`simulators.py`. Instantiating
:class:`~simbricks.orchestration.simulators.HostSim` requires you to specify a
:class:`~simbricks.orchestration.nodeconfig.NodeConfig`, which contains the
configuration for your host, for example, its networking settings, how much
system memory it should have, and most importantly, which applications to run by
assigning an :class:`~simbricks.orchestration.nodeconfig.AppConfig`. You can
find predefined classes for node and app configs in the module
:mod-orchestration:`nodeconfig.py`. Feel free to add new ones or just create a
new class locally in your experiment's module. For more details, see
:ref:`sec-orchestration`.

The last step to complete your virtual testbed is to specify which virtual
components connect to each other. You do this by invoking the respective methods
on the simulators you have instantiated. See the different simulator types' base
classes in the module :mod-orchestration:`simulators.py` for more information. A
simple and complete experiment module in which a client host pings a server can
be found :ref:`below <simple_ping_experiment>`.

If you plan to simulate a topology with multiple hosts, it may be helpful to
take a look at the module :mod-orchestration:`simulator_utils.py` in which we
provide some helper functions to reduce the amount of code you have to write.

Finally, to run your experiment, invoke ``experiments/run.py`` and provide the
path to your experiment module. In our docker containers, you can also just use
the following command from anywhere:

.. code-block:: bash

  $ simbricks-run --verbose --force <path_to_your_module.py>

``--verbose`` prints all simulators' output to the terminal and ``--force``
forces execution even if there already exist result files for the experiment. If
``simbricks-run`` is not available, you can always do 

.. code-block:: bash
  
  $ cd experiments
  $ python run.py --verbose --force <path_to_your_module.py>

While running, you can interrupt the experiment using Ctrl+C in your terminal.
This will cleanly stop all simulators and collect their output in a JSON file in
the directory ``experiments/out/<experiment_name>``. These are the necessary
basics to create and run your first experiment. Have fun.

.. literalinclude:: /../experiments/pyexps/simple_ping.py
  :linenos:
  :lines: 29-
  :language: python
  :name: simple_ping_experiment
  :caption: A simple experiment with a client host pinging a server, both are
    connected through a network switch. The setup of the two hosts could be
    simplified by using
    :func:`simbricks.orchestration.simulator_utils.create_basic_hosts`.

.. _sec-howto-nodeconfig:

********************************
Add a Node or Application Config
********************************

A host's configuration and the workload to run are defined via
:ref:`sec-node_app_config`. SimBricks already comes with a few examples in the
module :mod-orchestration:`nodeconfig.py`. If they don't fit your use-case, you
need to add your own by overriding the pre-defined member functions of
:class:`~simbricks.orchestration.nodeconfig.NodeConfig` and
:class:`~simbricks.orchestration.nodeconfig.AppConfig`. The most important one
is :meth:`~simbricks.orchestration.nodeconfig.AppConfig.run_cmds`, which defines
the commands to execute for your workload/application. Further information can
be found in the module :mod-orchestration:`nodeconfig.py` directly.

.. _sec-howto-custom_image:

******************************
Add a Custom Image
******************************

******************************
Integrate a New Simulator
******************************

The first step when integrating a new simulator into SimBricks is to implement a
SimBricks adapter for it. You can find the necessary information in the
:ref:`Simulator Adapters <Simulator Adapters>` section. To then make running
experiments and setting up the communication channels with other simulators more
convenient, add a class for the simulator to the orchestration framework that
inherits from :class:`~simbricks.orchestration.simulators.Simulator` or one of
the more specialized base classes in :mod-orchestration:`simulators.py`. In
this class you define the command to execute the simulator together with further
parameters, for example, to connect to the communication channels with other
simulators. Below is an example of what this looks like.

.. code-block:: python
  :linenos:
  :caption: Orchestration framework class for WireNet, a simple Ethernet wire
    that attaches to the SimBricks Ethernet adapters of two simulators.

  class WireNet(NetSim):

    def run_cmd(self, env):
        connects = self.connect_sockets(env)
        assert len(connects) == 2
        cmd = (
            f'{env.repodir}/sims/net/wire/net_wire {connects[0][1]}'
            f' {connects[1][1]} {self.sync_mode} {self.sync_period}'
            f' {self.eth_latency}'
        )
        if env.pcap_file:
            cmd += ' ' + env.pcap_file
        
        return cmd


******************************
Add a New Interface
******************************