Modernizing a Core Banking System Without Downtime
25 years of core banking, migrated with zero missed transactions.
Overview
Ashgrove Federal Bank was running account management, ledgers, and payments on a mainframe core system installed in the late 1990s. It worked, but every new product or integration required specialist contractors and months of lead time.
The Challenge
The bank needed to migrate core account and ledger data to a modern platform without a single missed transaction or a maintenance window customers would notice.
- ›Mainframe held 25 years of account and transaction history with no clean schema
- ›Regulators required a full audit trail through every stage of migration
- ›No acceptable customer-facing downtime window given 24/7 card and ACH activity
- ›Existing integrations (cards, ATMs, wire transfer) all depended on the legacy core
The Solution
We ran a parallel-core migration, mirroring transactions to the new platform in real time for months before cutover, so we could validate correctness under real load before switching over.
- ›Built a dual-write reconciliation layer to run old and new cores in parallel
- ›Migrated 25 years of ledger history with full regulatory audit trails preserved
- ›Ran 4 months of shadow-mode validation comparing live transactions across both cores
- ›Executed cutover in a phased rollout by account type, starting with lowest-risk segments
Results
The bank cut over to the new core with zero missed transactions and zero unplanned customer-facing downtime, and can now ship new products in weeks instead of quarters.
0
Missed transactions during cutover
0
Unplanned downtime minutes
4 months
Shadow-mode validation period
Weeks
New product lead time, down from quarters
“Everyone told us a core migration meant a bad weekend for our customers. It didn't. Nobody noticed, which was the entire point.”
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