Audit before ambition
Map which modules produce revenue, which create risk, and which are merely annoying. Modernize along business value, not aesthetic dislike of old code.
Strangle, don’t shatter
Route new traffic to new services while the monolith continues serving the rest. Dual-write or CDC bridges keep data consistent during cutover windows.
Tests are the migration contract
Characterize existing behavior with tests before you change it. If you cannot describe current behavior, you cannot prove the rewrite is faithful.
Plan the decommission
A migration is unfinished until old paths are turned off. Schedule decommission milestones with owners, or zombie systems will quietly return.