Parity Testing With Feature Flags

Originally posted January 2023 on the Split Blog (with minor edits Jan 2024 by Joy specifically for this blog)

Feature Parity Testing, sometimes called TAP compare testing, ensures a new system behaves the same way as an old one. It is used when replacing part or all of an old system. At a high level, you mirror your traffic to both systems and compare the results, logging any that differ. Unlike many…