“…Statistical heterogeneity [4] Meta-learning [12,18,21,30,36] Model personalization [34,35,37] User clustering [16,31,38] Adaptive update scheme Expensive communication [4,9,12,13,20,26,27,30,36] FedAvg [15,37,38] Flexible local updates [21,28,32,34,35] Compression schemes System heterogeneity [16] Active sampling [34,35] Fault tolerance Privacy leakage [28,38] Differential privacy [12,15,22,37] Homomorphic encryption [16,21,29] Strict information sharing scheme [33] 2-stage privacy-preserving scheme Real-time data stream [36] Incremental learning [38] Web-based learning [9,21,23] Repeat working with continuously emerging data…”