High concurrency architectures address the challenge of managing numerous simultaneous user activities, such as RSVPing or scanning tickets, by implementing distributed system principles like stateless services, event queues, horizontal scaling, and consistent data partitioning. A layered architecture featuring components such as reverse proxies, API gateways, and asynchronous event backbones enhances scalability and fault tolerance, while employing circuit breakers and retries with exponential backoff maintains stability under peak loads. Event-driven domain modeling employs Domain-Driven Design patterns to handle RSVP processes and real-time notifications, using event stores for consistency and debugging. The Command Query Responsibility Segregation (CQRS) pattern separates command and query operations to optimize performance for high-read traffic in event applications, while a hybrid API strategy leverages GraphQL for flexibility and REST for simplicity. Temporal data models, multi-tenant sharding, and audit trails support data management strategies, while OAuth2 and JWT provide authentication and authorization frameworks. Real-time capabilities are facilitated by WebSockets and push notifications, and mobile and web frontend applications benefit from offline support and progressive web app readiness. Observability is achieved through structured logging, distributed tracing, and incident-first monitoring, and infrastructure-as-code practices, such as Kubernetes deployments and secret management, ensure reproducibility. Third-party integrations include calendar sync, payment gateways, and ticket scanning SDKs, while compliance with regulations like GDPR and SOC2 involves data minimization and secure guest data workflows.