One of the biggest challenges we faced was designing the architecture of our application in a scalable way. We knew from the beginning that this application would need to serve a large user base globally with high performance. To achieve this, we designed our application using a modular microservices architecture instead of a monolithic architecture. We broke down the application into separate independent services for each core functionality like authentication, payments, analytics etc. Each service was developed independently by different teams which added its own coordination challenges.

The services communicated with each other asynchronously using message queues like RabbitMQ. While this allowed independent deployments, it introduced additional complexity in maintaining transactional integrity across services. For example, completing an order involved writing to the inventory, payment and shipping databases located in different services. We had to implement sophisticated distributed transactions using protocols like Saga patterns to ensure consistency.

Apart from architecture, probably our biggest challenge was building a high performance, reliable and scalable cloud infrastructure to run this application globally. We chose AWS as our cloud provider and had to make important decisions around VPC design, load balancing, auto-scaling, database partitioning, caching, metrics and monitoring at a massive scale. Setting up the right patterns for deploying our Kubernetes architecture across multiple regions/availability zones on AWS with proper disaster recovery was a significant effort. Even small mistakes in our infrastructure design could lead to poor performance or outages impacting thousands of users.

Another major area of focus was security. As a financial application dealing with sensitive user data, we had to ensure highest levels of security and compliance from the beginning. Right from the ground up, we designed our application following security best practices around authentication, authorization, input validation, encryption, secrets management, vulnerability scanning, attack simulation etc. We conducted several external security audits to evaluate and strengthen our defenses. Still, security remains an ongoing effort as new vulnerabilities are continually discovered.

Building sophisticated and user-friendly UIs for a multi-platform experience was a creative challenge. Our application needed to serve clients on web, iOS and Android with consistency. We adopted a design system approach allowing our UI teams to collaborate effectively. Implementing similar features across platforms with their own limitations and paradigms was difficult. Testing UIs systematically for accessibility, localization and ensuring pixel-perfect alignment cross-platform further increased effort.

Next, developing APIs for the application exposed its own issues around API design, documentation, versioning, rate limiting and caching API responses optimally. Multiple client applications and third-party integrations were built on top of our APIs so stability and performance were critical. Advanced technologies like GraphQL helped us address some challenges with flexible APIs but training teams took effort.

Integrating and migrating to new tools and techniques during the development cycle was another hurdle. For examples, migrating from monoliths to microservices, adopting containers and managing sprawling deployments, moving to serverless architectures, implementing event-driven architectures, adopting latest frontend frameworks like React etc. required reshaping architectures, refactoring codebases and retraining teams ongoing.

Coordinating releases and deployments of our complex application infrastructure across multiple services, regions, datacenters at scale to hundreds of thousands of users globally was an orchestration challenge. We adopted GitOps, deployment pipelines and canary deployments to roll out changes safely. Still, deployment bugs and incidents impacted user experience requiring constant improvements.

Building an application of this scale involved overcoming numerous technical, process and organizational challenges around architecture, infrastructure, security, cross-platform development, APIs, tool adoption, releases and operations. It was a continuous learning experience applying the latest techniques at massive scale with high reliability requirements. Even after years of development, we are still optimizing and evolving to improve the application experience further.