Use Built-in Mods#

Note: This tutorial covers experimental features. The functionality and interfaces may change in future versions.

In this tutorial, we will learn how to utilize built-in mods to augment the behavior of a ClientApp. Mods (sometimes also called Modifiers) allow us to perform operations before and after a task is processed in the ClientApp.

What are Mods?#

A Mod is a callable that wraps around a ClientApp. It can manipulate or inspect the incoming Message and the resulting outgoing Message. The signature for a Mod is as follows:

ClientApp = Callable[[Message, Context], Message]
Mod = Callable[[Message, Context, ClientApp], Message]

A typical mod function might look something like this:

def example_mod(msg: Message, ctx: Context, nxt: ClientApp) -> Message:
    # Do something with incoming Message (or Context)
    # before passing to the inner ``ClientApp``
    msg = nxt(msg, ctx)
    # Do something with outgoing Message (or Context)
    # before returning
    return msg

Using Mods#

To use mods in your ClientApp, you can follow these steps:

1. Import the required mods#

First, import the built-in mod you intend to use:

import flwr as fl
from flwr.client.mod import example_mod_1, example_mod_2

2. Define your client function#

Define your client function (client_fn) that will be wrapped by the mod(s):

def client_fn(cid):
    # Your client code goes here.
    return # your client

3. Create the ClientApp with mods#

Create your ClientApp and pass the mods as a list to the mods argument. The order in which you provide the mods matters:

app = fl.client.ClientApp(
        example_mod_1,  # Mod 1
        example_mod_2,  # Mod 2

Order of execution#

When the ClientApp runs, the mods are executed in the order they are provided in the list:

  1. example_mod_1 (outermost mod)

  2. example_mod_2 (next mod)

  3. Message handler (core function that handles the incoming Message and returns the outgoing Message)

  4. example_mod_2 (on the way back)

  5. example_mod_1 (outermost mod on the way back)

Each mod has a chance to inspect and modify the incoming Message before passing it to the next mod, and likewise with the outgoing Message before returning it up the stack.


By following this guide, you have learned how to effectively use mods to enhance your ClientApp’s functionality. Remember that the order of mods is crucial and affects how the input and output are processed.

Enjoy building a more robust and flexible ClientApp with mods!