Flower Architecture

This page explains the architecture of deployed Flower federated learning system.

In federated learning (FL), there is typically one server and a number of clients that are connected to the server. This is often called a federation.

The role of the server is to coordinate the training process. The role of each client is to receive tasks from the server, execute those tasks and return the results back to the server.

This is sometimes called a hub-and-spoke topology:

Hub-and-spoke topology in federated learning

Hub-and-spoke topology in federated learning (one server, multiple clients).

In a real-world deployment, we typically want to run different projects on such a federation. Each project could use different hyperparameters, different model architectures, different aggregation strategies, or even different machine learning frameworks like PyTorch and TensorFlow.

This is why, in Flower, both the server side and the client side are split into two parts. One part is long-lived and responsible for communicating across the network, the other part is short-lived and executes task-specific code.

A Flower server consists of SuperLink and ServerApp:

  • SuperLink: a long-running process that forwards task instructions to clients (SuperNodes) and receives task results back.

  • ServerApp: a short-lived process with project-spcific code that customizes all server-side aspects of federated learning systems (client selection, client configuration, result aggregation). This is what AI researchers and AI engineers write when they build Flower apps.

A Flower client consists of SuperNode and ClientApp:

  • SuperNode: a long-running process that connects to the SuperLink, asks for tasks, executes tasks (for example, “train this model on your local data”) and returns task results back to the SuperLink.

  • ClientApp: a short-lived process with project-specific code that customizes all client-side aspects of federated learning systems (local model training and evaluation, pre- and post-processing). This is what AI researchers and AI engineers write when they build Flower apps.

Why SuperNode and SuperLink? Well, in federated learning, the clients are the actual stars of the show. They hold the training data and they run the actual training. This is why Flower decided to name them SuperNode. The SuperLink is then responsible for acting as the missing link between all those SuperNodes.

Basic Flower architecture

The basic Flower architecture for federated learning.

In a Flower app project, users will typically develop the ServerApp and the ClientApp. All the network communication between server and clients is taken care of by the SuperLink and SuperNodes.

Tip

For more details, please refer to the ServerApp and ClientApp documentation.

With multi-run, multiple ServerApps and ClientApps are now capable of running on the same federation consisting of a single long-running SuperLink and multiple long-running SuperNodes. This is sometimes referred to as multi-tenancy or multi-job.

As shown in the figure below, two projects, each consisting of a ServerApp and a ClientApp, could share the same SuperLink and SuperNodes.

Multi-tenancy federated learning architecture

Multi-tenancy federated learning architecture with Flower

To illustrate how multi-run works, consider one federated learning training run where a ServerApp and a ClientApp are participating in [run 1]. Note that a SuperNode will only run a ClientApp if it is selected to participate in the training run.

In [run 1] below, all the SuperNodes are selected and therefore run their corresponding ClientApps:

Multi-tenancy federated learning architecture - Run 1

Run 1 in a multi-run federated learning architecture with Flower. All SuperNodes participate in the training round.

However, in [run 2], only the first and third SuperNodes are selected to participate in the training:

Multi-tenancy federated learning architecture - Run 2

Run 2 in a multi-run federated learning architecture with Flower. Only the first and third SuperNodes are selected to participate in the training round.

Therefore, with Flower multi-run, different projects (each consisting of a ServerApp and ClientApp) can run on different sets of clients.

To help you start and manage all of the concurrently executing training runs, Flower offers one additional long-running server-side service called SuperExec. When you type flwr run to start a new training run, the flwr CLI bundles your local project (mainly your ServerApp and ClientApp) and sends it to the SuperExec. The SuperExec will then take care of starting and managing your ServerApp, which in turn selects SuperNodes to execute your ClientApp.

This architecture allows many users to (concurrently) run their projects on the same federation, simply by typing flwr run on their local developer machine.

Flower Deployment Engine with SuperExec

The SuperExec service for managing concurrent training runs in Flower.

Note

This explanation covers the Flower Deployment Engine. An explanation covering the Flower Simulation Engine will follow.

Important

As we continue to enhance Flower at a rapid pace, we’ll periodically update this explainer document. Feel free to share any feedback with us.