Act as a Senior Solutions Architect. Design a highly available microservices architecture for a global payment processing platform handling 100k transactions per second. Your design must define service boundaries, data consistency models (strong vs. eventual), inter-service communication protocols (gRPC, Kafka, REST), and database sharding strategies. Specifically address how you will handle the 'Thundering Herd' problem during flash sales and ensure ACID compliance for financial ledger updates across geographically distributed regions.