Legacy modernisation projects fail more often than they succeed. The reasons are predictable: scope creep, underestimated complexity, big-bang cutovers, and insufficient testing. Here is the framework we use at Aaitek that consistently delivers.
Start with the Strangler Fig Pattern
Do not replace your legacy system all at once. Instead, route new functionality to the modern system while the legacy continues to serve existing requests. Over time, you strangle the old system by migrating functionality piece by piece until it handles no traffic.
This approach keeps production running, lets you test incrementally, and creates natural rollback points at every stage.
Step 1: Map the Dependency Graph
Before writing a line of code, document every system that touches your legacy application. How many integrations does it have? Which are synchronous? Which use batch jobs? Surprises in the dependency graph cause most modernisation failures.
Use tools like dependency-cruiser for code-level analysis and Mermaid diagrams for business process flows.
Step 2: Define the Strangler Seam
Find the right boundary. This is typically an API gateway, a message queue, or a shared database. Everything new goes through this seam. The legacy system operates behind it until it is retired.
Step 3: Database Migration Strategy
Option A: Shared database — New and old system write to the same database. Simple, but creates tight coupling that limits your architectural choices.
Option B: Synchronised dual writes — Write to both systems, compare outputs, then cut over. More complex, but lets you run old and new systems independently.
Option C: Event-driven sync — Publish change events from the legacy system. New system consumes them and builds its own read models. Best for eventual consistency use cases.
Step 4: Run Parallel Validation
Before cutting over, run both systems in parallel and compare outputs for 2–4 weeks. Automate the comparison. Fix every discrepancy. Only cut over when the new system produces identical results.
Step 5: Deprecate, Do Not Delete
Once the legacy system handles zero traffic, leave it running (but frozen) for 90 days. This gives you a cold standby if a production issue surfaces that was not caught in testing.
Aaitek has modernised 40+ legacy platforms. Our typical engagement is 6–18 months depending on complexity.
