Modernizing core banking systems is critical for banks to stay competitive and adapt to new business models. According to McKinsey & Company, a "flexible back end" is essential for this. The current market offers two potential solutions: the core banking platform and suite, which are inflexible and result in vendor lock-in. A new approach, composable ecosystems, is being adopted by some financial institutions, consisting of distinct independent services and functions that can incorporate "best-of-breed" functionality without major integration challenges. MongoDB's developer data platform is well-suited for building a composable core banking ecosystem, comprising JSON, BIAN, MACH, and data domains. Companies such as Temenos and Current have utilized MongoDB's capabilities to deliver innovative services and improve performance, leveraging its flexibility, scalability, and strong security features. A composable ecosystem offers greater flexibility and adaptability than traditional legacy systems, making it a powerful choice for driving innovation in the financial sector.