A detailed guide to the strangler fig pattern for modernizing legacy systems capability by capability rather than through a big-bang rewrite. It covers how the facade layer routes traffic, the tradeoffs between dual writes and change data capture for data ownership during migration overlap, how to choose which capability to strangle first, why migrations commonly stall around 80% completion, the real infrastructure and reconciliation costs of running two systems in parallel, when the pattern is a poor fit, and how to define when a migration is actually finished versus just feature-complete