What is the strangler fig migration pattern?

Strangler fig is a migration pattern where the new application is built alongside the old application instead of as a single big bang rebuild. Traffic is gradually routed from old to new, module by module or route by route, over weeks or months. Both applications run in production throughout. The pattern lets you ship incremental value, validate each migrated piece against real production traffic, and reverse course on any module that does not work without rolling back the whole migration. For large legacy CodeIgniter codebases the strangler fig pattern typically delivers safer migrations than big bang cutover.