Strangling the Monolith: Zero-Downtime Migration for a Tier-1 Bank

A tier-1 bank needed to decommission a 30-year-old COBOL mainframe that processed $5 billion in daily transactions. A hard cutover was out of the question. Any downtime window long enough to migrate cleanly would violate regulatory SLAs and trigger contractual penalties. The migration had to be invisible — to users, to auditors, and to the core banking system itself.

• 01

  The COBOL mainframe had no API surface — all integration was via batch file exchange and proprietary MQ queues

• 02

  Transaction semantics varied by product line with undocumented edge cases accumulated over 30 years

• 03

  Dual-write consistency had to be maintained across two fundamentally different transaction models simultaneously

• 04

  Regulatory requirement: zero transaction loss with a full audit trail across both systems during transition

• 01

  Shadow routing with continuous reconciliation is the only safe approach when you cannot tolerate a single lost transaction

• 02

  Migrate by product line, not by technical layer — it gives you a blast radius you can reason about

• 03

  Undocumented business logic is the primary risk in legacy migration; instrument everything before you move anything

• 04

  Keep the old system authoritative until you have 99.999% reconciliation confidence — not 99%