Traditional retail banks need to modernize their data estate to build high-quality customer-facing applications and fend off challenger banks. To achieve this, they can follow a 5-phase approach using an operational data layer (ODL) that acts as a bridge between existing systems and new ones. This allows for iterative modernization while preserving legacy components and protecting business-critical operations. An ODL enables banks to switch off legacy infrastructure piece by piece, making database operations more efficient and reducing the need for expensive queries against mainframe data. With an ODL, banks can unify data from multiple systems into a single real-time platform, relieve the mainframe of workloads, and support an evolutionary approach to legacy modernization.