Private Client – AWS Serverless FinTech API Platform

A FinTech founder needed an API layer built quickly, correctly, and in a way that a small engineering team could take ownership of after handover. The combination of a tight deadline and a requirement for long-term maintainability is exactly the kind of constraint that reveals whether a contractor is senior or just experienced.

Approach

The architecture conversation happened before a line of code was written. The client had a vague sense that they needed “an API” — the first task was to understand exactly what the API needed to do, what data it needed to serve, what clients would consume it, and what the growth trajectory looked like over the next 12 months.

From that conversation, several decisions were made that the client hadn’t explicitly asked for but immediately understood when explained: serverless over containerised (traffic was spiky, not sustained), DynamoDB over relational (access patterns were key-based, not relational), CDK over manual console configuration (reproducibility mattered from day one).

What Was Built

• REST API via AWS API Gateway — resource model, request validation, and response shaping all defined in CDK
• Java Lambda handlers using Spring Cloud Function, giving the team a familiar Spring programming model without the container overhead
• JWT-based authentication middleware applied at the API Gateway layer — clean separation between auth and business logic
• DynamoDB data model designed around the actual access patterns, with GSIs for the query shapes the product required
• Full CDK stack — every resource defined as infrastructure as code, deployable to a fresh account with a single command
• Documentation — OpenAPI spec generated from the code, not written separately

Outcome

Delivered ahead of schedule. The handover session took two hours rather than two days — because the architecture was documented, the code was tested, and the infrastructure was reproducible. No rework was required after handover.