Applying Domain-Driven Design (DDD) to eCommerce microservice architecture allows for clean service boundaries by aligning microservices with core business domains like cart, catalog, checkout, and payments, thereby enhancing autonomy and scalability. Event-driven architecture (EDA) further decouples services through asynchronous communication, using event brokers like Apache Kafka and RabbitMQ to handle core domain events and ensure elasticity and fault tolerance. Saga patterns manage long-running transactions across services to maintain data consistency, while API gateways and service meshes enhance security and observability. Scalability is achieved with inventory and pricing engines using technologies like Redis and DynamoDB, while search and recommendation engines leverage Elasticsearch and ML APIs for personalized experiences. Resilience strategies, including circuit breakers and Kubernetes health checks, ensure system robustness during high traffic. Observability tools like OpenTelemetry and Prometheus provide crucial insights into system performance, while security measures ensure compliance with standards such as PCI-DSS and GDPR. CI/CD pipelines with canary and blue-green deployments facilitate rapid, safe updates, supporting agile development and operational excellence in high-traffic eCommerce environments.