6.2Migration approaches and planning

Migrating to a new system requires as much design attention on the migration itself as on feature development. A systems architect must choose the migration approach (big-bang, phased, or parallel) based on constraints such as the downtime the business can tolerate, risk appetite, and budget, and build a migration plan that includes a rollback procedure to safely revert if it fails. Getting the migration-approach choice wrong lets a failed cutover cause direct business damage.

6.2.1Big-bang, phased, and parallel migration

• Big-bang migration stops the old system on a set date and switches over to the new one all at once. It has the advantage of a short migration period and no cost or complexity from maintaining both systems simultaneously, but it is a high-risk, high-reward approach: if a problem surfaces right after cutover, the entire business is affected, and rollback is also difficult.
• Phased migration splits the target (by site, department, function, etc.) and switches over to the new system one portion at a time. Even if a problem occurs, its impact can be contained, and lessons learned in early stages can be fed into later ones, so risk is low—but old and new systems coexist for an extended period, incurring the cost of building and maintaining an interim link (data synchronization, etc.) between them.
• Parallel migration runs both the old and new systems live simultaneously for a set period, cross-checking their outputs to verify the new system's correctness before stopping the old one. It is the safest, lowest-risk approach, but the cost of operating the same processing twice (staffing, infrastructure) is the highest. It is often chosen for core systems in finance, accounting, and similar domains where errors cannot be tolerated.