Developer Guide

This file contains an overview of architecture, coding design/practices, testing and style.

Before submitting a PR

  • If you have general questions, feel free to reach out to the faucet-dev mailing list.

  • If you are new to FAUCET, or are contemplating a major change, it’s recommended to open a github issue with the proposed change. This will enable broad understanding of your work including being able to catch any potential snags very early (for example, adding new dependencies). Architectural and approach questions are best settled at this stage before any code is written.

  • Please send relatively small, tightly scoped PRs (approx 200-300 LOC or less). This makes review and analysis easier and lowers risk, including risk of merge conflicts with other PRs. Larger changes must be refactored into incremental changes.

  • You must add a test if FAUCET’s functionality changes (ie. a new feature, or correcting a bug).

  • All unit and integration tests must pass (please use the docker based tests; see Software switch testing with docker). Where hardware is available, please also run the hardware based integration tests also.

  • You must use the github feature branches (see, for your change and squash commits ( when creating the PR.

  • Please use the supplied git pre-commit hook (see ../git-hook/pre-commit), to automatically run the unit tests and pylint for you at git commit time.

  • pylint must show no new errors or warnings.

  • Code must conform to the style guide (see below).

PR handling guidelines

This section documents general guidelines for the maintainers in handling PRs. The overall intent is, to enable quality contributions with as low overhead as possible, maximizing the use of tools such as static analysis and unit/integration testing, and supporting rapid and safe advancement of the overall project.

In addition to the above PR submission guidelines, above:

  • PRs require a positive review per github’s built in gating feature. The approving reviewer executes the merge.

  • PRs that should not be merged until some other criteria are met (e.g. not until release day) must include DO NOT MERGE in the title, with the details in PR comments.

  • A typical PR review/adjust/merge cycle should be 2-3 days (timezones, weekends, etc permitting). If a PR upon review appears too complex or requires further discussion it is recommended it be refactored into smaller PRs or discussed in another higher bandwidth forum (e.g. a VC) as appropriate.

  • A PR can be submitted at any time, but to simplify release logistics PR merges might not be done before release, on release days.

Code style

Please use the coding style documented at Existing code not using this style will be incrementally migrated to comply with it. New code should comply.

Faucet Development Environment

A common way of developing faucet is inside a virtualenv with an IDE such as PyCharm.

Instructions on setting up PyCharm for developing faucet are below.

If you would rather develop on the command line directly, a short summary of the command line setup for development in a venv with Python 3.7+ is included after the PyCharm instructions.

Create a new project in PyCharm

Set the Location of the project to the directory where a checked out copy of the faucet code from git is, for this tutorial I will assume the path is /Dev/faucet/.

Ignore the Project Interpreter settings for now, we will set those up after the project is created.

Click Create when you have completed these steps.

When asked Would you like to create a project from existing sources instead? click Yes.

Create virtual environment

Now that the project is created and source code imported, click the File -> Settings menu. In the dialog box that opens click the Project: faucet -> Project Interpreter sub menu.

Click the cog and select Add...

Under Virtualenv Environment you want to select New environment and select a Location for the virtualenv (which can be inside the directory where the faucet code lives, e.g /Dev/faucet/venv).

The Base interpreter should be set to /usr/bin/python3.

Click Ok which will create the virtualenv.

Now while that virtualenv builds and we still have the settings dialog open we will tweak a few project settings to make them compatible with our code style. Click on the Tools -> Python Integrated Tools menu and change the Docstring format to Google.

Finally, click Ok again to get back to the main screen of PyCharm.

Install requirements

Inside the PyCharm editor window if we open one of the code files for faucet (e.g. faucet/ we should now get a bar at the top of the window telling us of missing package requirements, click the Install requirements option to install the dependencies for faucet.

Create log and configuration directories

Now we need to create a log and configuration directory so that faucet can start:

mkdir -p /Dev/faucet/venv/var/log/faucet/
mkdir -p /Dev/faucet/venv/etc/faucet/

Copy the sample faucet configuration file from /Dev/faucet/etc/faucet/faucet.yaml to /Dev/faucet/venv/etc/faucet/ and edit this configuration file as necessary.

Copy the sample gauge configuration file from /Dev/faucet/etc/faucet/gauge.yaml to /Dev/faucet/venv/etc/faucet/ and edit this configuration file as necessary.

If you are using the sample configuration “as is” you will also need to copy /Dev/faucet/etc/faucet/acls.yaml to /Dev/faucet/venv/etc/faucet/ as that included by the sample faucet.yaml file, and without it the sample faucet.yaml file cannot be loaded.

You may also wish to copy /Dev/faucet/etc/faucet/ryu.conf to /Dev/faucet/venv/etc/faucet/ as well so everything can be referenced in one directory inside the Python virtual environment.

Configure PyCharm to run faucet and gauge

Now we need to configure PyCharm to run faucet, gauge and the unit tests.

First, click the Run -> Run.. menu, then select the Edit Configurations... option to get to the build settings dialog.

We will now add run configuration for starting faucet and gauge. Click the + button in the top left hand corner of the window. First, change the name from Unnamed to faucet. Change the Script path to point to ryu-manager inside the virtualenv, for me this was ../venv/bin/ryu-manager. Then set the Parameters to faucet.faucet. Make sure the working directory is set to /Dev/faucet/faucet/.

We will use the same steps as above to add a run configuration for gauge. Changing the Script path to ../venv/bin/ryu-manager and setting the Parameters this time to faucet.gauge. Make sure the working directory is set to /Dev/faucet/faucet/.

Configure PyCharm to run unit tests

For running tests we need a few additional dependencies installed, I couldn’t work out how to do this through PyCharm so run this command from a terminal window to install the correct dependencies inside the virtualenv:

/Dev/faucet/venv/bin/pip3 install -r /Dev/faucet/test-requirements.txt

To add the test run configuration we will again click the + button in the top left hand corner, select Python tests -> Unittests. You can provide a Name of Faucet Unit Tests for the run configuration. For Target select Script path and enter the path /Dev/faucet/tests/unit/faucet. For Pattern enter test_*.py.

We will also add test run configuration for gauge using the same steps as above. Use Gauge Unit Tests as the Name and for Target select Script path and enter the path /Dev/faucet/tests/unit/gauge. For Pattern enter test_*.py.

You can click Apply and Close now that we’ve added all our new run configuration.

Now that everything is setup you can run either the faucet controller, gauge controller and test suite from the Run menu.

Developing with a Python 3.7+ venv

If you would prefer not to use PyCharm and are comfortable developing Python directly on the command line, these steps should get you started. They have been tested with Ubuntu 18.04 LTS, which includes Python 3.7, but similar instructions should work on other platforms that include Python 3.7+.

Install C/C++ compilers and Python development environment packages:

sudo apt-get install python3-venv libpython3.7-dev gcc g++ make

If you have not already, clone the faucet git repository:

git clone

Then create a Python venv environment within it:

cd faucet
python3 -m venv "${PWD}/venv"

and activate that virtual environment for all following steps:

. venv/bin/activate

Ensure that the faucet config is present within the virtual environment, copying from the default config files if required:

mkdir -p "${VIRTUAL_ENV}/var/log/faucet"
mkdir -p "${VIRTUAL_ENV}/etc/faucet"

for FILE in {acls,faucet,gauge}.yaml ryu.conf; do
  if [ -f "${VIRTUAL_ENV}/etc/faucet/${FILE}" ]; then
    echo "Preserving existing ${FILE}"
    echo "Installing template ${FILE}"
    cp -p "etc/faucet/${FILE}" "${VIRTUAL_ENV}/etc/faucet/${FILE}"

Then install the runtime and development requirements

"${VIRTUAL_ENV}/bin/pip3" install wheel   # For bdist_wheel targets
"${VIRTUAL_ENV}/bin/pip3" install -r "${VIRTUAL_ENV}/../test-requirements.txt"

Finally install faucet in an editable form:

pip install -e .

And then confirm that you can run the unit tests:

pytest tests/unit/faucet/
pytest tests/unit/gauge/


Makefile is provided at the top level of the directory. Output of make is normally stored in dist directory. The following are the targets that can be used:

  • uml: Uses pyreverse to provide code class diagrams.

  • codefmt: Provides command line usage to “Code Style” the Python file

  • codeerrors: Uses pylint on all Python files to generate a code error report and is placed in dist directory.

  • stats: Provides a list of all commits since the last release tag.

  • release: Used for releasing FAUCET to the next version, Requires version and next_version variables.

To directly install faucet from the cloned git repo, you could use sudo python install command from the root of the directory.

To build pip installable package, you could use python sdist command from the root of the directory.

To remove any temporarily created directories and files, you could use rm -rf dist *egg-info command.

Building Documentation

The documentation is built with Sphinx, from within the docs directory.

To be able to build the documentation ensure you have the relevant packages installed:

cd docs
sudo apt-get install librsvg2-bin make
pip3 install -r requirements.txt

and then you can build HTML documentation with:

cd docs
make html

and the documentation will be found under _build/html in the docs directory.

Key architectural concepts/assumptions:

FAUCET’s architecture depends on key assumptions, which must be kept in mind at all times.

  • FAUCET is the only controller for the switch, that can add or remove flows.

  • All supported dataplanes must implement OpenFlow functionally (hardware, software or both) identically. No TTP or switch specific drivers.

In addition:

  • FAUCET provisions default deny flows (all traffic not explicitly programmed is dropped).

  • Use of packet in is minimized.

FAUCET depends upon these assumptions to guarantee that the switch is always in a known and consistent state, which in turn is required to support high availability (FAUCET provides high availability, through multiple FAUCET controllers using the same version of configuration - any FAUCET can give the switch a consistent response - no state sharing between controllers is required). The FAUCET user can program customized flows to be added to the switch using FAUCET ACLs (see below).

FAUCET also programs the dataplane to do flooding (where configured). This minimizes the use of packet in. This is necessary to reduce competition between essential control plane messages (adding and removing flows), and traffic from the dataplane on the limited bandwidth OpenFlow control channel. Unconstrained packet in messages impact the switch CPU, may overwhelm the OpenFlow control channel, and will expose the FAUCET controller to unvalidated dataplane packets, all of which are security and reliability concerns. In future versions, packet in will be eliminated altogether. The FAUCET user is expected to use policy based forwarding (eg ACLs that redirect traffic of interest to high performance dataplane ports for NFV offload), not packet in.

FAUCET requires all supported dataplanes to implement OpenFlow (specifically, a subset of OpenFlow 1.3) in a functionally identical way. This means that there is no switch-specific driver layer - the exact same messages are sent, whether the switch is OVS or hardware. While this does prevent some earlier generation OpenFlow switches from being supported, commercially available current hardware does not have as many restrictions, and eliminating the need for a switch-specific (or TTP) layer greatly reduces implementation complexity and increases controller programmer productivity.