The Legacy Software Trap
The most dangerous moment for a business is when the cost of maintaining legacy software exceeds the cost of replacing it — but the risk of replacement feels too high to take. Companies stay stuck in this trap for years, paying more and more to maintain systems that slow them down.
Why 'Big Bang' Rewrites Fail
- Netscape tried to rewrite their browser from scratch — it took 3 years and nearly killed the company
- Hershey's SAP big bang in 1999 disrupted $100M in orders
- FBI's Virtual Case File: $170M written off after big bang modernisation failure
- The pattern: parallel systems, extended timelines, budget overruns, integration failures
The Strangler Fig Pattern: The Right Approach
- Identify a discrete piece of functionality in the legacy system
- Build a modern replacement service that handles only that functionality
- Route traffic to the new service for that function while the legacy system handles everything else
- Repeat until the legacy system handles nothing and can be switched off
- The legacy system 'strangles' incrementally — no big bang, no risk
Step-by-Step Modernisation Framework
- Step 1: Map the legacy system — every function, integration, data entity, and user workflow
- Step 2: Classify by modernisation strategy: rehost, replatform, refactor, rebuild, or retire
- Step 3: Identify the seams — where the system can be cut and new services inserted
- Step 4: Build the strangler facade — a routing layer that directs traffic to old or new system
- Step 5: Migrate one module at a time, validate, then move to the next
Data Migration: The Hardest Part
Data migration is where most modernisation projects fail. Plan for: schema transformation, data cleansing, dual-write periods during transition, and rollback capability. Never cut over data in a single step without 2–4 weeks of parallel operation first.
Risk Management
- Never go from zero to full cutover — always have a rollback plan
- Run old and new systems in parallel for 4–8 weeks before decommissioning legacy
- Define specific metrics that trigger rollback (error rate, performance degradation)
- Keep a 'break glass' path back to the legacy system for 90 days post-migration
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