Modernizing a bank's mainframe is crucial for creating modern customer experiences, as it allows developers to focus on building high-quality applications tailored to the client, rather than managing outdated systems. A five-stage approach using an Operational Data Layer (ODL) enables banks to modernize their mainframes iteratively, while maintaining performance and risk control. An ODL acts as a bridge between existing and new systems, allowing for faster improvements and minimizing disruption to critical business operations. By leveraging MongoDB's capabilities, banks can efficiently connect mainframes and traditional databases to modern architectures, such as data mesh, and unlock the full potential of their data. The benefits of an ODL include improved data processing, enhanced security, and reduced costs, making it an ideal solution for banks looking to transform their digital experience.