STRANGLER-FIG MODERNIZATION: SLICE THE MONOLITH WITHOUT THE BIG-BANG WEEKEND
How strangler-fig migrations work in practice: seams, parallel runs, reconciliation, and cutovers decided by evidence - the default for COBOL and legacy ERP exits.
Strangler-fig modernization carves capabilities out of a legacy system behind stable interfaces, runs old and new in parallel with reconciliation, and cuts over when evidence says the numbers match - not when a calendar says 'weekend.' It is the default method for leaving mainframes and brittle ERPs without stopping the business. See /solutions/modernization and /work/cobol-logistics-modernization.
Find the seam
Pick a bounded capability with clear inputs/outputs (billing, a read model, a channel). Wrap it. Do not 'boil the ocean' with a full rewrite as plan A.
Characterize before you change
Golden-master tests pin current behavior - including bugs the business relies on. Without characterization, every migration is a guessing game.
Reconcile until bored
Parallel run until mismatches are explained and accepted. Cutover is a decision from evidence, reversible when possible.