The Strangler Fig Pattern: How to Modernize Legacy Systems Without a Big Bang Rewrite
ISHIR Blog, Wednesday, July 8th, 2026
The strangler fig pattern incrementally replaces legacy systems via a routing layer, avoiding risky big-bang rewrites.
The ISHIR blog explains the strangler fig pattern as a low-risk alternative to big-bang legacy rewrites, which fail or underperform in 68-79% of cases due to undocumented business logic - as with TSB Bank's disastrous 2018 migration.
The approach places a routing layer (facade, proxy, or API gateway) in front of the legacy system, initially sending all traffic to the old code, then incrementally shifting requests to new components as they are validated. This maintains continuous operations, enables safe rollbacks, and lets feature development continue during migration.
Success, however, requires organizational discipline to finish the migration and careful data synchronization between parallel systems.