Which testing method/implementation strategy involves running both the old system and the new system simultaneously?

When deploying updates to enterprise software systems, organizations utilize safe transition strategies to mitigate operational risks.

• Parallel Testing (or parallel conversion) involves running both the old system and the new system simultaneously for a set period to compare outputs and ensure the new infrastructure is stable and accurate.
• By contrast, Direct Changeover switches instantly, Pilot Testing introduces the system to a single localized department first, and Phased Conversion replaces modules step-by-step.