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Virtual Nodes Can Help: Tackling Distribution Shifts in Federated Graph Learning
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Proposes FedVN, a federated graph learning framework that adds multiple shared learnable virtual nodes whose edges to local graphs are set by a personalized edge generator, so that graphs become client-invariant under distribution shift.
Problem
Graph property prediction in federated graph learning suffers from distribution shifts across clients, where label-irrelevant substructures differ per client and degrade GNN training.
Approach
Clients jointly learn a set of shared virtual nodes (VNs) while each client trains a personalized edge generator that decides how VNs connect its local graphs; a decoupling loss keeps VNs diverse and a score-contrastive loss makes edge patterns consistent within a client but distinct across clients.
Key results
- FedVN reaches 75.72% accuracy on Motif/Basis versus 67.12% for the best baseline (Self-training) and 63.62% for FedStar.
- On ZINC/Scaffold FedVN achieves 0.4947 MAE, lower than the best baseline FedBN at 0.5129.
- Removing the edge generator drops FedVN from 75.72% to 60.20% on Motif/Basis and from 43.67% to 40.15% on CMNIST/Color.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| use FedVN instead of Self-training | GIN for federated graph learning · Motif/Basis | accuracy: 67.12$\pm$0.89 -> 75.72$\pm$1.85 | Helped |
| use FedVN instead of GCFL+ | GIN for federated graph learning · Motif/Size | accuracy: 49.34$\pm$2.70 -> 50.41$\pm$1.17 | Helped |
| use FedVN instead of FedProx | GIN for federated graph learning · CMNIST/Color | accuracy: 39.78$\pm$0.68 -> 43.67$\pm$1.25 | Helped |
| use FedVN instead of FedBN | GIN for federated graph learning · ZINC/Scaffold | MAE: 0.5129$\pm$0.0119 -> 0.4947$\pm$0.0174 | Helped |
| use FedVN instead of FedAvg | GIN for federated graph learning · SST2/Length | accuracy: 81.79$\pm$0.27 -> 83.13$\pm$0.79 | Helped |
| add personalized edge generators (ablation) | FedVN · Motif/Basis | accuracy: 60.20$\pm$1.75 -> 75.72$\pm$1.85 | Helped |
| add personalized edge generators (ablation) | FedVN · CMNIST/Color | accuracy: 40.15$\pm$1.27 -> 43.67$\pm$1.25 | Helped |
| set the decoupling-loss weight to 0 (ablation) | FedVN · SST2/Length | accuracy | Hurt |
| set the score-contrastive-loss weight to 0 (ablation) | FedVN · SST2/Length | accuracy | Hurt |
| remove the decoupling loss (ablation) | FedVN · ZINC/Scaffold and SST2/Length | pairwise VN cosine similarity | Hurt |
| vary the number of virtual nodes; use only 1 or 5 (ablation) | FedVN · Motif/Size and CMNIST/Color | performance | Hurt |
Surprising findings
- FedStar degrades sharply on CMNIST/Color (28.10%) and SST2/Length (58.57%), which the authors attribute to sharing structure encoders when labels depend on node features.
Limitations
- Adds extra computation and communication cost from edge generators and multiple VNs.
- Shared VNs may leak private information and are vulnerable to malicious clients perturbing them; privacy not analyzed.
- Theoretical result (Theorem 1) is shown for a linear/single-layer GIN setting and extended to multi-layer by informal argument.
Experimental setup
Datasets: Motif CMNIST ZINC SST2
Benchmarks: Motif/Basis Motif/Size CMNIST/Color ZINC/Scaffold SST2/Length
Metrics: accuracy MAE
Baselines: Self-training FedAvg FedProx FedBN Ditto FedRep FedALA GCFL+ FedStar
Builds on: FedAvg GIN NT-Xent FedBN
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