Modernizing legacy systems without the big-bang rewrite

We have never seen a big-bang rewrite land on schedule. We have repeatedly seen incremental migrations succeed.

Start with a seam Put a routing layer in front of the legacy system. Every request flows through it, so you can move one endpoint at a time without the callers knowing.

Migrate by business capability Pick the capability with the highest change frequency and the lowest coupling. Ship it new, route traffic to it, delete the old path. Repeat.

Non-negotiables during migration - Characterization tests that pin current behaviour before you touch it - Dual-write with reconciliation while both systems are live - A rollback route at every step - Deletion of the old code path as part of the definition of done

Keep the lights on The legacy system pays the bills until the last endpoint moves. Treat its stability as a first-class requirement, not a distraction from the new build.