Migrating off legacy line-of-business apps without a forklift

Every long-tenured organization has a 1998 line-of-business app. The vendor is gone. The original hardware is dying. The data is irreplaceable. The forklift migration is terrifying.

The strangler fig pattern

Named after the tree that grows around an existing tree until it eventually replaces it, the strangler fig pattern lets you replace a legacy system gradually. Build new functionality alongside the old. Route traffic to the new system one feature at a time. The old system shrinks until it can be retired without ceremony.

What works in practice

1. Stand up a read-only data feed from the legacy app first. Get fresh eyes on the data structure.
2. Build the new system around that feed. Validate that the new system can produce the same answers as the old one for known inputs.
3. Cut over one workflow at a time. Start with the lowest-risk one.
4. Decommission the old system only when zero workflows still depend on it.

What to plan for

Data quality issues you did not know existed. Workflows the original users have memorized but never documented. The institutional knowledge that left with the original developers.