End-to-End API Testing for a German Investment Platform

Challenge

A German web platform for investing in securities — providing access to financial instruments via API — had no QA process to speak of. Testing was done manually by a developer, using basic Postman checks. API documentation existed in Swagger, but there were no test cases, no automation, and no structured approach.

For a platform handling financial transactions and securities data, this was a significant reliability and compliance risk. The client needed a complete API testing operation built from scratch, with the specific requirement that everything be transferable to their own team at the end of the engagement.

Solution

One QA specialist was brought in for a focused four-month engagement (Sep–Dec 2024). The mandate: build a professional API testing process from zero, covering user creation, transaction flows, and edge cases — then hand it all over.

Test design and coverage

• Developed structured test cases for all API endpoints documented in Swagger
• Built a full E2E API testing suite covering up to 3000 combinations of user creation parameters — every permutation of input scenarios and edge cases
• CRUD testing via Postman Flows, usable by any team member without deep API knowledge
• API response validation schema implemented — catching incorrect data structure at the integration boundary
• Data validation testing using Faker.js and JSON Schema Validator

Automation and CI/CD

• Automated API test collection built in Postman
• Integrated with Bitbucket Pipelines — tests run automatically on every code push
• Parallel test execution configured to reduce total run time
• Newman used for headless CI execution

Security and transaction testing

• Security testing using Charles Proxy and Burp Suite — checking for exposure in financial endpoints
• Transaction processing automated — accuracy of calculations verified and error detection built in
• Load testing to validate stability under high data volumes

Documentation and handoff

• Full API testing documentation prepared for client team onboarding
• Defect tracking set up in Jira with all detected issues recorded and resolved
• Complete infrastructure transferred to the client at end of engagement

Tools used: Postman, Postman Flows, Newman, Bitbucket Pipelines, Faker.js, JSON Schema Validator, Swagger, Charles Proxy, Burp Suite, Jira, TestRail, Confluence

Results

In four months, the platform went from developer-run manual Postman checks to a professional, automated API testing infrastructure:

• 3000+ user creation parameter combinations covered end-to-end
• Critical errors in transaction processing identified and fixed — direct impact on financial reliability
• Full E2E API testing cycle automated and integrated into CI/CD
• Complete test infrastructure transferred to the client team — Postman Flows designed so any team member can run tests without API expertise
• API response validation in place — data structure errors caught at the integration level
• Security vulnerabilities assessed via Burp Suite and Charles Proxy
• Full test documentation and defect history handed over for ongoing maintenance