Legacy System Modernization Planner

You are a modernization consultant who specializes in migrating legacy systems without big-bang rewrites. Help me modernize my legacy system.

Legacy system: [LEGACY_SYSTEM]
Age of system: [AGE]
Technology: [LEGACY_TECH]
Business criticality: [CRITICALITY]
Team knowledge of legacy code: [TEAM_KNOWLEDGE]
Target technology: [TARGET_TECH]
Business constraints: [CONSTRAINTS]
Timeline: [TIMELINE]

Create a modernization plan:

**1. Assessment**
- Business value map: Which parts of the legacy system generate the most value?
- Risk map: Which parts are most fragile or most likely to fail?
- Complexity map: Which parts are most tangled or hardest to understand?
- Prioritization: Where the three maps overlap = start here

**2. Strategy Selection**
- **Retain**: Keep as-is (some parts aren't worth migrating)
- **Rehost**: Lift and shift to cloud (quick wins)
- **Replatform**: Move to managed services without rewriting
- **Refactor**: Restructure for cloud-native
- **Rewrite**: Build new (last resort — highest risk)
- **Retire**: Turn off (the best migration is no migration)

Recommendation for each system component.

**3. Strangler Fig Implementation**
- Identify the seam: Where can you intercept traffic between old and new?
- Proxy/facade pattern: Route requests to old or new based on readiness
- Event interception: Capture events from legacy to feed new system
- Incremental migration sequence (ordered by risk and value)

**4. Data Migration**
- Schema mapping (old → new)
- Data transformation and cleaning
- Dual-write period (write to both during migration)
- Verification and reconciliation
- Rollback plan for data

**5. Risk Mitigation**
- Feature parity tracking (what the old system does vs. new)
- Shadow mode: Run both systems, compare outputs
- Canary deployment: Route small percentage to new system
- Instant rollback capability at every phase

**6. Team Strategy**
- Knowledge extraction from legacy experts (before they leave!)
- Training plan for new technology
- Parallel team structure vs. single team

**7. Phased Roadmap**
- Phase 1: Understand and contain (wrap the legacy, add observability)
- Phase 2: Extract and modernize (start with highest-value, lowest-risk components)
- Phase 3: Migrate data and traffic
- Phase 4: Decommission legacy components
- Each phase with go/no-go criteria

**8. Anti-patterns to Avoid**
- Second system syndrome, big-bang rewrite, stop-the-world migration, ignoring data migration complexity, underestimating legacy business logic