fix(examples) Update quickstart-cpp for Flower 1.31

[BESTTOOLBOX]

Issue

Description

The existing examples/quickstart-cpp example is marked as compatible with flwr<1.13.0 and still documents the older way of running Flower applications. As a result, the C++ quickstart does not provide a working path for recent Flower versions that use flwr run, ServerApp, SuperLink, and the grpc-rere Fleet API.

Related issues/PRs

No related issue.

Proposal

Explanation

This PR updates examples/quickstart-cpp for Flower 1.31 by replacing the old transport path with an external C++ client that connects to a Flower SuperLink through the current grpc-rere Fleet API.

The updated example:

• adds a Flower App configuration with a Python ServerApp and a placeholder ClientApp required by Flower App metadata;
• implements C++ client registration, activation, heartbeat, message polling, message replies, and object push/pull against the Fleet API;
• serializes and deserializes Flower RecordDict, ArrayRecord, ConfigRecord, and MetricRecord payloads for the synthetic linear-regression quickstart;
• regenerates C++ protobuf sources from framework/proto/flwr/proto/*.proto during the CMake build instead of relying on stale generated files;
• updates the README to document the current flower-superlink, external C++ client, and flwr run . --stream workflow;
• adds a small Python serialization test for the quickstart helper functions.

The linear-regression workload remains intentionally small so the example can be used as a minimal C++ quickstart.

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Address LLM-reviewer comments, if applicable (e.g., GitHub Copilot)
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

Manual validation was run with Flower 1.31.0, two external C++ clients, and a Python ServerApp for three rounds. The run completed with aggregate_fit and aggregate_evaluate receiving 2 results and 0 failures in each round.

Validation summary after addressing Copilot comments:

Flower version: 1.31.0
CMake build: passed
Python unittest: passed, 2 tests
Run finished: 3 rounds
aggregate_fit: 2 results, 0 failures
aggregate_evaluate: 2 results, 0 failures
loss: 4.319e-08 -> 5.192e-16 -> 6.000e-24

Copilot review comments were addressed in eabc8f1 and the corresponding review threads were resolved.

CI currently reports action_required for the fork PR workflows, so I cannot honestly mark Make CI checks pass until a maintainer approves/runs the workflows and they complete successfully. I also left the Slack item unchecked because it requires posting in the Flower Slack workspace outside GitHub.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the C++ quickstart to work with newer Flower deployments by switching the client to the gRPC grpc-rere Fleet API and aligning the Python app entrypoints with flwr run.

Changes:

• Added a C++ SuperLink/Fleet client implementation with object store (de)serialization and auth metadata signing.
• Added RecordDict ↔︎ local types serialization helpers and a message handler for train/evaluate RPC-style messages.
• Updated the Python ServerApp/strategy utilities and app metadata (pyproject) to run via flwr run.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
examples/quickstart-cpp/src/supernode_client.cc	Implements Fleet/SuperLink client, object store upload/download, and message loop
examples/quickstart-cpp/src/recorddict_serde.cc	Adds RecordDict serialization/deserialization utilities for the C++ client
examples/quickstart-cpp/src/message_handler.cc	Adds message dispatch translating message types to client method calls
examples/quickstart-cpp/src/main.cc	Switches entrypoint to the new SuperLink client
examples/quickstart-cpp/server.py	Converts to ServerApp server_fn for flwr run
examples/quickstart-cpp/pyproject.toml	Declares Flower app metadata and Python dependencies
examples/quickstart-cpp/include/supernode_client.h	Declares the new C++ client start function and gRPC message size constant
examples/quickstart-cpp/include/recorddict_serde.h	Declares RecordDict serde helper APIs
examples/quickstart-cpp/include/message_handler.h	Declares handle_message API
examples/quickstart-cpp/fedavg_cpp.py	Updates strategy helper functions and tensor byte conversions
examples/quickstart-cpp/client.py	Adds placeholder ClientApp required by newer Flower app metadata
examples/quickstart-cpp/README.md	Updates docs for SuperLink + flwr run workflow and build requirements
examples/quickstart-cpp/CMakeLists.txt	Reworks build to use system gRPC/Protobuf and generates protos from Flower sources
examples/quickstart-cpp/.gitignore	Expands ignore patterns for Python and build artifacts

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