Background
The Client
Our client is a major UK financial services firm with a customer base built over 25 years of operation. With over 50 million customer records spanning accounts, transactions, policy data, and relationship histories, they were running on a legacy database platform that had become a significant operational and compliance risk.
The business had long understood that migration was inevitable — but the stakes were enormous. Any downtime would affect millions of active customers, expose the firm to regulatory censure from the FCA, and risk reputational damage in a trust-sensitive sector. Their internal team had assessed the project and concluded that a traditional migration approach carried unacceptable risk. They needed a specialist partner with both the technical expertise and the financial sector compliance credentials to do it differently.
Following a formal procurement process, UK Data Services was appointed as the lead migration partner. The engagement brief was precise: zero customer-facing downtime, full FCA and PCI DSS compliance at every stage, and a hard delivery deadline tied to the sunset date of the legacy platform's vendor support contract.
The Challenge
The Scale of the Problem
This was one of the most complex data migration projects we have undertaken. The challenges were not just technical — they were organisational, regulatory, and operational simultaneously:
- 50 million customer records across 140 interdependent database tables, many with complex relationships built up over two and a half decades of organic schema evolution.
- Absolute zero tolerance for customer-facing downtime — the system processed live transactions 24 hours a day, including direct debits, standing orders, and real-time card authorisations.
- FCA Operational Resilience requirements mandated a full audit trail and rollback capability at every stage, with documented evidence for post-migration regulatory review.
- PCI DSS compliance requirements applied to all cardholder data throughout transit, requiring specialist handling and encrypted channels at every handoff point.
- 17 third-party system integrations (core banking, CRM, fraud detection, regulatory reporting) all required simultaneous cutover without service interruption.
Our Solution
A Phased, Zero-Risk Architecture
We designed a migration architecture centred on a parallel-run model: rather than a single cutover event, we built a shadow system that ran alongside the live platform for eight weeks, with continuous reconciliation to validate data integrity before any traffic was moved.
- Full schema analysis and data lineage mapping across all 140 tables, producing a dependency graph used to sequence the migration safely.
- Automated reconciliation engine running 24/7, comparing record counts, checksums, and referential integrity between legacy and target systems at five-minute intervals.
- Phased traffic migration: read traffic moved first, then write traffic by product line, with automated rollback triggers if any reconciliation threshold was breached.
- All cardholder and sensitive personal data transmitted via encrypted channels with tokenisation applied prior to migration, satisfying PCI DSS Requirement 3 throughout.
- Comprehensive audit log delivered to the client's compliance team in real time, structured for direct submission to the FCA under the Operational Resilience framework.
- Third-party integration cutover managed via a dedicated integration middleware layer, allowing each system to be switched independently with zero impact on others.
The final cutover — moving the last 12% of write traffic to the new system — was completed during a Sunday low-traffic window, with no customer-facing incidents and all reconciliation checks passing within tolerance.
Results
Delivered on Every Metric
The project was completed six weeks ahead of the contractual deadline, giving the client additional runway before their legacy support contract expired. Not a single customer complaint was logged relating to the migration.
The £2M cost saving compared to the client's internal project estimate reflected both the efficiency of our phased approach and the avoidance of a planned 72-hour maintenance window that had been considered necessary under the original internal plan — a window that, at the client's transaction volumes, would have represented significant revenue risk.
All 17 third-party integrations were confirmed live and reconciled within four hours of the final cutover. The FCA audit package was submitted and acknowledged without queries.