Composable Core Migration: A 90-Day Playbook for Banks
Nobody rips out a core banking system over a weekend anymore. The modern approach is strangler-fig: wrap the legacy core, route new capabilities to a composable stack, and migrate customer journeys one segment at a time.
Wrap before you replace
The first move is an integration layer that fronts the legacy core with clean APIs and events. This decouples channels from the core, so new experiences can ship against stable contracts while the old system keeps running underneath.
Migrate by journey, not by module
Slicing the migration by customer journey — onboarding, then payments, then lending — keeps every phase shippable and reversible. Each slice delivers value on its own and de-risks the next, which is what keeps executive sponsorship alive across a multi-quarter programme.
Run both cores, briefly
A controlled period of dual-running, with reconciliation between old and new, is the safety net that lets teams cut over confidently instead of hopefully. Plan the exit from day one so "temporary" does not become permanent.
- Front the legacy core with APIs and events first
- Migrate journey by journey to keep each phase shippable
- Dual-run with reconciliation, and plan the cut-over up front
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