Flower Configuration¶

Note

The Flower Configuration is a new feature introduced in Flower 1.26.0. While its functionality will evolve over time, as a start it serves as a direct replacement of the federations section in the pyproject.toml file of Flower Apps in older versions.

Note

From Flower 1.28.0 onwards, it isn’t recommended to define Simulation Runtime settings (i.e. fields starting with options.) in the Flower Configuration. Check the Simulation Runtime documentation for the recommended way to configure simulations.

What is it?

The Flower Configuration is a system for managing SuperLink connections, stored in a TOML file (which we refer to as the Flower Configuration file) that lives in the directory specified by the FLWR_HOME environment variable (which defaults to $HOME/.flwr when unset). It is designed to facilitate the usage of Flower CLI commands.

Why use it?

The Flower Configuration allows you to define reusable connection configurations that you can reference by name when running flwr commands. For example, you can set up configurations for local testing, staging servers, and production deployments, then easily switch between them.

Most flwr commands (like flwr log, flwr list, and flwr stop) can use the Flower Configuration from anywhere on your system. The exception is flwr run, which must be executed from within a Flower App directory to access the app code.

Tip

First-time setup: If the Flower Configuration file doesn’t exist in your system, it will be automatically created for you the first time you run a Flower CLI command.

Upgrading from older versions: If you are upgrading from a previous version of Flower, the federations section in your pyproject.toml file will be automatically migrated to the Flower Configuration. The syntax remains the same with the exception of the name of the section and the superlink keyword instead of tool.flwr.federations.

Flower Configuration File Structure¶

Understanding the terminology

The Flower Configuration file uses the term “superlink” to refer to SuperLink connection configurations. Each connection configuration describes how to connect to a Flower SuperLink (the central server component that coordinates federated learning). You can define multiple SuperLink connection configurations for different scenarios, such as local simulations or remote deployments.

The configuration structure is similar to the older federations section in pyproject.toml before Flower 1.26.0, but now lives in a central location and uses clearer naming.

Basic example

[superlink]
default = "local"

[superlink.local]
address = ":local:"

[superlink.local-poc]
address = "127.0.0.1:9093"
insecure = true

Explanation

[superlink] section defines which connection configuration to use by default
default = "local" means the superlink.local configuration will be used when you don’t specify a connection explicitly in your flwr commands
[superlink.local] defines a managed local SuperLink profile. The special address value ":local:" tells Flower to launch or reuse the background local SuperLink on demand with its default on-disk state. The alternative value ":local-in-memory:" starts the managed local SuperLink with an in-memory database instead. The SuperLink will run simulations using the default configuration of the Simulation Runtime.
[superlink.local-poc] defines a configuration for connecting to a locally running SuperLink at address 127.0.0.1:9093

Connection configuration names must be unique and use the superlink. prefix.

Listing your connections¶

You can list all your connection configurations using the flwr config list command. Assuming the default configuration file shown earlier, the expected output will be:

$ flwr config list

Flower Config file: /path/to/your/.flwr/config.toml
SuperLink connections:
  local (default)
  local-poc

This shows you have two connection configurations available, with local set as the default.

Local Simulation Example¶

Local simulations allow you to test your Flower App on your own machine using virtual SuperNodes instead of real distributed SuperNodes. This is useful for development and testing before deploying to real distributed environments.

Basic simulation configuration

[superlink.local]
address = ":local:"

This creates a managed local SuperLink profile using the default simulation configuration, which involves running 10 simulated SuperNodes through the simulation runtime. Check the Simulation Runtime docs to learn how to customize the number of SuperNodes and resources (CPU/GPU) allocated to your simulations.

Simulation with in-memory local state

[superlink.local-in-memory]
address = ":local-in-memory:"

This creates a managed local SuperLink profile that keeps its database in memory instead of on disk. This can be useful on networked filesystems where SQLite locking or performance issues can occur. The tradeoff is that local run history and logs are lost when the managed local SuperLink is shut down.

When to use each

Use the basic configuration for most local development and testing scenarios, especially when you want to keep a history of your runs and logs on disk.
Use address = ":local-in-memory:" when the managed local SuperLink runs on a filesystem where SQLite performs poorly, such as some network drives or HPC storage

Learn more in the How to Run Simulations guide about other optional parameters you can use to configure your local simulation.

When you use a local simulation profile with address = ":local:" or address = ":local-in-memory:", Flower CLI commands that communicate with SuperLink use the Control API. Flower starts a managed local SuperLink automatically when needed and reuses it across flwr run, flwr list, flwr log, and flwr stop. See Run Flower Locally with a Managed SuperLink for the full local workflow, background process behavior, and runtime file locations.

Remote Deployment Example¶

When you’re ready to deploy your federated learning app to real distributed nodes, you configure connections that point to a remote SuperLink.

Example configuration

[superlink.remote-deployment]
address = "<SUPERLINK-ADDRESS>:<PORT>"
root-certificates = "/absolute/path/to/root/cert.crt"  # Optional, for TLS
# insecure = true  # Disable TLS (not recommended for production)

Explanation

address (required): The address of the SuperLink Control API to connect to (e.g., my-server.example.com:9093).
root-certificates: Path to the root certificate file for TLS encryption. This secures the communication between your CLI and the SuperLink. If omitted, Flower uses the default gRPC root certificate. This field is ignored if insecure is set to true.
insecure: Set to true to disable TLS encryption (only use this for local testing, never in production). Defaults to false if omitted, meaning TLS is enabled by default.

TLS (Transport Layer Security) explained

TLS encrypts the communication between your local machine and the remote SuperLink to prevent eavesdropping and tampering. In production, you should always use TLS by either:

Providing a root-certificates file (recommended for custom certificates)
Omitting both root-certificates and insecure to use default certificates

Only set insecure = true for local testing environments.

Refer to the deployment documentation for TLS setup and advanced configurations.

Upgrading from previous versions of Flower¶

If you’re new to Flower 1.26.0+, you can skip this section.

For users upgrading from versions before 1.26.0: The federations section in your pyproject.toml file will be automatically migrated to the Flower Configuration the first time you run a flwr command.

What happens during migration

The Flower Configuration file will be created at $HOME/.flwr/config.toml if not already existing
Your federation definitions are copied and renamed (federations → superlink)
The old [tool.flwr.federations] section in pyproject.toml is commented out for your reference

After migration

You can safely delete the commented-out federations section from your pyproject.toml file. All connection configurations now live in the Flower Configuration and work across all your Flower projects.