Modernizing Critical Financial Infrastructure

The Mission

FinanceFlow S.p.A. operated their core ledger on a 30-year-old COBOL mainframe system. It was stable but rigid—new feature development took months, and COBOL developers were retiring. The mission was to migrate to a modern microservices architecture without a single second of downtime or a penny of data discrepancy.

Strategy: The Strangler Fig Pattern

Instead of a high-risk "big bang" rewrite, we adopted the Strangler Fig pattern. We built a modern Anti-Corruption Layer (ACL) that gradually intercepted traffic.

Identify: Mapped capabilities (Accounts, Transactions, Loans).
Strangle: Built the "Accounts" microservice in Python.
Route: Updated the API Gateway to route "read" traffic to the new service while keeping "writes" synchronized to both systems.
Verify: Ran a "Shadow Mode" for 3 months where outputs were compared byte-for-byte.
Switch: Once 100% parity was confirmed, the mainframe module was decommissioned.

Automated Validation Pipeline

We built a custom verification tool that replayed 1 year of production transaction logs against the new system, verifying interest calculations to the 10th decimal place.

Technology Stack

Languages: Python (FastAPI), Java (Spring Boot for some services)
Database: PostgreSQL (Aurora) replacing DB2
Event Bus: Kafka for eventual consistency
Infrastructure: AWS Fargate, API Gateway

Achievements

Zero Downtime migration of 2.3 million daily transactions.
Feature Velocity: New financial products can now be launched in 2 weeks (previously 6 months).
Cost Reduction: 60% savings on mainframe MIPS costs.