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Build a strategy from scratch#

Welcome to the third part of the Flower federated learning tutorial. In previous parts of this tutorial, we introduced federated learning with PyTorch and Flower (part 1) and we learned how strategies can be used to customize the execution on both the server and the clients (part 2).

In this notebook, we’ll continue to customize the federated learning system we built previously by creating a custom version of FedAvg (again, using Flower and PyTorch).

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Let’s build a new Strategy from scratch!


Before we begin with the actual code, let’s make sure that we have everything we need.

Installing dependencies#

First, we install the necessary packages:

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!pip install -q flwr[simulation] torch torchvision

Now that we have all dependencies installed, we can import everything we need for this tutorial:

[ ]:
from collections import OrderedDict
from typing import Dict, List, Optional, Tuple

import numpy as np
import torch
import torch.nn as nn
import torch.nn.functional as F
import torchvision.transforms as transforms
from import DataLoader, random_split
from torchvision.datasets import CIFAR10

import flwr as fl

DEVICE = torch.device("cpu")  # Try "cuda" to train on GPU
    f"Training on {DEVICE} using PyTorch {torch.__version__} and Flower {fl.__version__}"

It is possible to switch to a runtime that has GPU acceleration enabled (on Google Colab: Runtime > Change runtime type > Hardware acclerator: GPU > Save). Note, however, that Google Colab is not always able to offer GPU acceleration. If you see an error related to GPU availability in one of the following sections, consider switching back to CPU-based execution by setting DEVICE = torch.device("cpu"). If the runtime has GPU acceleration enabled, you should see the output Training on cuda, otherwise it’ll say Training on cpu.

Data loading#

Let’s now load the CIFAR-10 training and test set, partition them into ten smaller datasets (each split into training and validation set), and wrap everything in their own DataLoader. We introduce a new parameter num_clients which allows us to call load_datasets with different numbers of clients.

[ ]:

def load_datasets(num_clients: int):
    # Download and transform CIFAR-10 (train and test)
    transform = transforms.Compose(
        [transforms.ToTensor(), transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5))]
    trainset = CIFAR10("./dataset", train=True, download=True, transform=transform)
    testset = CIFAR10("./dataset", train=False, download=True, transform=transform)

    # Split training set into `num_clients` partitions to simulate different local datasets
    partition_size = len(trainset) // num_clients
    lengths = [partition_size] * num_clients
    datasets = random_split(trainset, lengths, torch.Generator().manual_seed(42))

    # Split each partition into train/val and create DataLoader
    trainloaders = []
    valloaders = []
    for ds in datasets:
        len_val = len(ds) // 10  # 10 % validation set
        len_train = len(ds) - len_val
        lengths = [len_train, len_val]
        ds_train, ds_val = random_split(ds, lengths, torch.Generator().manual_seed(42))
        trainloaders.append(DataLoader(ds_train, batch_size=32, shuffle=True))
        valloaders.append(DataLoader(ds_val, batch_size=32))
    testloader = DataLoader(testset, batch_size=32)
    return trainloaders, valloaders, testloader

trainloaders, valloaders, testloader = load_datasets(NUM_CLIENTS)

Model training/evaluation#

Let’s continue with the usual model definition (including set_parameters and get_parameters), training and test functions:

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class Net(nn.Module):
    def __init__(self) -> None:
        super(Net, self).__init__()
        self.conv1 = nn.Conv2d(3, 6, 5)
        self.pool = nn.MaxPool2d(2, 2)
        self.conv2 = nn.Conv2d(6, 16, 5)
        self.fc1 = nn.Linear(16 * 5 * 5, 120)
        self.fc2 = nn.Linear(120, 84)
        self.fc3 = nn.Linear(84, 10)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        x = self.pool(F.relu(self.conv1(x)))
        x = self.pool(F.relu(self.conv2(x)))
        x = x.view(-1, 16 * 5 * 5)
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x

def get_parameters(net) -> List[np.ndarray]:
    return [val.cpu().numpy() for _, val in net.state_dict().items()]

def set_parameters(net, parameters: List[np.ndarray]):
    params_dict = zip(net.state_dict().keys(), parameters)
    state_dict = OrderedDict({k: torch.Tensor(v) for k, v in params_dict})
    net.load_state_dict(state_dict, strict=True)

def train(net, trainloader, epochs: int):
    """Train the network on the training set."""
    criterion = torch.nn.CrossEntropyLoss()
    optimizer = torch.optim.Adam(net.parameters())
    for epoch in range(epochs):
        correct, total, epoch_loss = 0, 0, 0.0
        for images, labels in trainloader:
            images, labels =,
            outputs = net(images)
            loss = criterion(net(images), labels)
            # Metrics
            epoch_loss += loss
            total += labels.size(0)
            correct += (torch.max(, 1)[1] == labels).sum().item()
        epoch_loss /= len(trainloader.dataset)
        epoch_acc = correct / total
        print(f"Epoch {epoch+1}: train loss {epoch_loss}, accuracy {epoch_acc}")

def test(net, testloader):
    """Evaluate the network on the entire test set."""
    criterion = torch.nn.CrossEntropyLoss()
    correct, total, loss = 0, 0, 0.0
    with torch.no_grad():
        for images, labels in testloader:
            images, labels =,
            outputs = net(images)
            loss += criterion(outputs, labels).item()
            _, predicted = torch.max(, 1)
            total += labels.size(0)
            correct += (predicted == labels).sum().item()
    loss /= len(testloader.dataset)
    accuracy = correct / total
    return loss, accuracy

Flower client#

To implement the Flower client, we (again) create a subclass of flwr.client.NumPyClient and implement the three methods get_parameters, fit, and evaluate. Here, we also pass the cid to the client and use it log additional details:

[ ]:
class FlowerClient(fl.client.NumPyClient):
    def __init__(self, cid, net, trainloader, valloader):
        self.cid = cid = net
        self.trainloader = trainloader
        self.valloader = valloader

    def get_parameters(self, config):
        print(f"[Client {self.cid}] get_parameters")
        return get_parameters(

    def fit(self, parameters, config):
        print(f"[Client {self.cid}] fit, config: {config}")
        set_parameters(, parameters)
        train(, self.trainloader, epochs=1)
        return get_parameters(, len(self.trainloader), {}

    def evaluate(self, parameters, config):
        print(f"[Client {self.cid}] evaluate, config: {config}")
        set_parameters(, parameters)
        loss, accuracy = test(, self.valloader)
        return float(loss), len(self.valloader), {"accuracy": float(accuracy)}

def client_fn(cid) -> FlowerClient:
    net = Net().to(DEVICE)
    trainloader = trainloaders[int(cid)]
    valloader = valloaders[int(cid)]
    return FlowerClient(cid, net, trainloader, valloader)

Let’s test what we have so far before we continue:

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# Specify client resources if you need GPU (defaults to 1 CPU and 0 GPU)
client_resources = None
if DEVICE.type == "cuda":
    client_resources = {"num_gpus": 1}


Build a Strategy from scratch#

Let’s overwrite the configure_fit method such that it passes a higher learning rate (potentially also other hyperparameters) to the optimizer of a fraction of the clients. We will keep the sampling of the clients as it is in FedAvg and then change the configuration dictionary (one of the FitIns attributes).

[ ]:
from typing import Callable, Union

from flwr.common import (
from flwr.server.client_manager import ClientManager
from flwr.server.client_proxy import ClientProxy
from flwr.server.strategy.aggregate import aggregate, weighted_loss_avg

class FedCustom(fl.server.strategy.Strategy):
    def __init__(
        fraction_fit: float = 1.0,
        fraction_evaluate: float = 1.0,
        min_fit_clients: int = 2,
        min_evaluate_clients: int = 2,
        min_available_clients: int = 2,
    ) -> None:
        self.fraction_fit = fraction_fit
        self.fraction_evaluate = fraction_evaluate
        self.min_fit_clients = min_fit_clients
        self.min_evaluate_clients = min_evaluate_clients
        self.min_available_clients = min_available_clients

    def __repr__(self) -> str:
        return "FedCustom"

    def initialize_parameters(
        self, client_manager: ClientManager
    ) -> Optional[Parameters]:
        """Initialize global model parameters."""
        net = Net()
        ndarrays = get_parameters(net)
        return fl.common.ndarrays_to_parameters(ndarrays)

    def configure_fit(
        self, server_round: int, parameters: Parameters, client_manager: ClientManager
    ) -> List[Tuple[ClientProxy, FitIns]]:
        """Configure the next round of training."""

        # Sample clients
        sample_size, min_num_clients = self.num_fit_clients(
        clients = client_manager.sample(
            num_clients=sample_size, min_num_clients=min_num_clients

        # Create custom configs
        n_clients = len(clients)
        half_clients = n_clients // 2
        standard_config = {"lr": 0.001}
        higher_lr_config = {"lr": 0.003}
        fit_configurations = []
        for idx, client in enumerate(clients):
            if idx < half_clients:
                fit_configurations.append((client, FitIns(parameters, standard_config)))
                    (client, FitIns(parameters, higher_lr_config))
        return fit_configurations

    def aggregate_fit(
        server_round: int,
        results: List[Tuple[ClientProxy, FitRes]],
        failures: List[Union[Tuple[ClientProxy, FitRes], BaseException]],
    ) -> Tuple[Optional[Parameters], Dict[str, Scalar]]:
        """Aggregate fit results using weighted average."""

        weights_results = [
            (parameters_to_ndarrays(fit_res.parameters), fit_res.num_examples)
            for _, fit_res in results
        parameters_aggregated = ndarrays_to_parameters(aggregate(weights_results))
        metrics_aggregated = {}
        return parameters_aggregated, metrics_aggregated

    def configure_evaluate(
        self, server_round: int, parameters: Parameters, client_manager: ClientManager
    ) -> List[Tuple[ClientProxy, EvaluateIns]]:
        """Configure the next round of evaluation."""
        if self.fraction_evaluate == 0.0:
            return []
        config = {}
        evaluate_ins = EvaluateIns(parameters, config)

        # Sample clients
        sample_size, min_num_clients = self.num_evaluation_clients(
        clients = client_manager.sample(
            num_clients=sample_size, min_num_clients=min_num_clients

        # Return client/config pairs
        return [(client, evaluate_ins) for client in clients]

    def aggregate_evaluate(
        server_round: int,
        results: List[Tuple[ClientProxy, EvaluateRes]],
        failures: List[Union[Tuple[ClientProxy, EvaluateRes], BaseException]],
    ) -> Tuple[Optional[float], Dict[str, Scalar]]:
        """Aggregate evaluation losses using weighted average."""

        if not results:
            return None, {}

        loss_aggregated = weighted_loss_avg(
                (evaluate_res.num_examples, evaluate_res.loss)
                for _, evaluate_res in results
        metrics_aggregated = {}
        return loss_aggregated, metrics_aggregated

    def evaluate(
        self, server_round: int, parameters: Parameters
    ) -> Optional[Tuple[float, Dict[str, Scalar]]]:
        """Evaluate global model parameters using an evaluation function."""

        # Let's assume we won't perform the global model evaluation on the server side.
        return None

    def num_fit_clients(self, num_available_clients: int) -> Tuple[int, int]:
        """Return sample size and required number of clients."""
        num_clients = int(num_available_clients * self.fraction_fit)
        return max(num_clients, self.min_fit_clients), self.min_available_clients

    def num_evaluation_clients(self, num_available_clients: int) -> Tuple[int, int]:
        """Use a fraction of available clients for evaluation."""
        num_clients = int(num_available_clients * self.fraction_evaluate)
        return max(num_clients, self.min_evaluate_clients), self.min_available_clients

The only thing left is to use the newly created custom Strategy FedCustom when starting the experiment:

[ ]:
    strategy=FedCustom(),  # <-- pass the new strategy here


In this notebook, we’ve seen how to implement a custom strategy. A custom strategy enables granular control over client node configuration, result aggregation, and more. To define a custom strategy, you only have to overwrite the abstract methods of the (abstract) base class Strategy. To make custom strategies even more powerful, you can pass custom functions to the constructor of your new class (__init__) and then call these functions whenever needed.

다음 단계#

Before you continue, make sure to join the Flower community on Slack: Join Slack

There’s a dedicated #questions channel if you need help, but we’d also love to hear who you are in #introductions!

The Flower Federated Learning Tutorial - Part 4 introduces Client, the flexible API underlying NumPyClient.

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