| name | eventmodeling-integrating-legacy-systems |
| description | Apply Event Modeling to legacy systems using side-car pattern. Freeze old system, extract events, build new features without rewriting. Use when modernizing legacy applications. Do not use for: greenfield systems without existing legacy constraints (use eventmodeling-orchestrating-event-modeling) or translating inbound events from external APIs (use eventmodeling-translating-external-events). |
| allowed-tools | ["AskUserQuestion","Write"] |
Integrating Legacy Systems
Interview Phase (Critical - Not Optional)
When to Interview: This step is high-risk and often initiated prematurely. Always interview unless the user has explicit organizational buy-in for a freeze agreement AND has already assessed legacy system state and extraction feasibility.
Interview Strategy: Assess organizational readiness, understand legacy system constraints, and validate event extraction feasibility before designing the side-car. Poor planning here leads to costly integration failures.
Critical Questions
Always conduct this interview unless all context is provided:
-
Legacy System State & Documentation (Impact: Determines how much reverse-engineering is needed; affects timeline dramatically)
- Question: "Tell me about the legacy system: (A) Technology stack, (B) Age/last major update, (C) Database size/complexity, (D) Current users/traffic, (E) Known documentation or audit trails?"
- Why it matters: Undocumented systems require exploration; modern systems may have better audit logs; scaling affects extraction approach
- Follow-up triggers: If documentation is incomplete โ ask for at least data schema; if no audit trail โ ask about update_at timestamps; if very large โ ask about partitioning strategy
-
Organizational Freeze Agreement (Impact: Most criticalโdetermines if side-car is even feasible)
- Question: "Has business leadership agreed to FREEZE the legacy system? Specifically: (A) No new features in legacy, (B) Only bug fixes permitted, (C) No schema changes?"
- Why it matters: Without explicit freeze, teams keep modifying legacy โ events become stale โ side-car becomes incorrect โ project fails
- Follow-up triggers: If not frozen โ ask "What would get stakeholder buy-in for freeze?"; if partially frozen โ clarify exact boundaries
-
Event Extraction Feasibility (Impact: Determines if extraction is reverse-engineer-able or if it requires external data)
- Question: "For event extraction: (A) Can you query the legacy database directly, (B) Is there an audit log/change tracking, (C) Will you use CDC (Change Data Capture), (D) Can you modify the legacy system for hooks?"
- Why it matters: Direct query extraction is fastest but may be imperfect; CDC is cleaner but requires infrastructure; modified legacy defeats freeze
- Follow-up triggers: If (A) โ ask about query access and schema understanding; if (B) โ ask format of audit log; if (C) โ discuss CDC tool selection
-
Integration Timeline & Staffing (Impact: Determines realistic phase durations; affects approach choices)
- Question: "What's your timeline? (A) Need new features within 3 months (aggressive), (B) 6-12 months (standard), (C) 18+ months (gradual). And team capacity: (A) Full team on side-car, (B) Part of team, (C) Skeleton crew?"
- Why it matters: Aggressive timeline might skip historical extraction, reducing risk surface; team size affects whether extraction and side-car can happen in parallel
- Follow-up triggers: If aggressive โ ask "What's the MVP scope?"; if skeleton crew โ ask "Can you backfill?"
-
Risk Tolerance & Failure Recovery (Impact: Determines safety margins and validation rigor)
- Question: "How critical is the system? (A) Revenue-critical/zero downtime tolerance, (B) Important but can tolerate brief outages, (C) Low-risk migration path acceptable?"
- Why it matters: Affects how much validation you need before user cutover; determines rollback strategy importance
- Follow-up triggers: If (A) โ propose comprehensive testing and gradual rollout; if (C) โ can be more aggressive
Interview Flow
No Conditional Skip: This interview is critical. Even if some context is provided, confirm all five dimensions.
Phase 1: System Understanding (Question 1)
- Document legacy system constraints
- Assess documentation gaps
- Plan information gathering
Phase 2: Organizational Alignment (Question 2) CRITICAL
- Confirm freeze agreement (required)
- Document boundaries
- Identify stakeholders
Phase 3: Technical Feasibility (Question 3)
- Validate extraction approach
- Identify data challenges
- Plan extraction strategy
Phase 4: Timeline & Staffing (Questions 4-5)
- Set realistic phases
- Identify team structure
- Plan rollout phases
Capturing Interview Findings
REQUIRED: Document findings in detail before proceeding:
## Interview Findings: Legacy System Integration
**System Overview**:
- Name: [System name]
- Age: [X years]
- Tech: [Stack]
- Scale: [Users/Data size]
- Documentation: [State]
**Freeze Agreement** CRITICAL
- Status: [Agreed / Pending / Blocked]
- Boundaries: [What's frozen, what's allowed]
- Stakeholders signed off: [Yes/No - names if yes]
**Event Extraction Approach**:
- Method: [Direct query / CDC / Audit log / Hooks]
- Feasibility: [High / Medium / Low]
- Challenges: [List specific data challenges]
**Timeline & Staffing**:
- Target completion: [Date]
- Phase 1 duration: [Months]
- Team capacity: [% allocated to project]
**Risk Assessment**:
- Criticality: [High / Medium / Low]
- Acceptable downtime: [None / Minutes / Hours]
- Rollback strategy: [How to recover if side-car fails]
**Blockers or Concerns**:
- [Any showstoppers identified]
- [Questions for stakeholders]
**Green Light Status**:
- [ ] Freeze agreement obtained (REQUIRED)
- [ ] Extraction approach validated (REQUIRED)
- [ ] Team staffing confirmed (REQUIRED)
- [ ] Risk mitigation plan accepted (REQUIRED)
RECOMMENDATION: [Proceed with side-car / Resolve blockers first / Not recommended at this time - explain why]
CRITICAL: Write findings to the project's event modeling file:
File: .trogonai/interviews/[project-name]/EVENTMODELING.md
Append this section (place it in "Additional Steps" or create new section if legacy integration is primary):
## Legacy System Integration (eventmodeling-integrating-legacy-systems)
### System Overview
[From Q1]
- Name: [System name]
- Age: [X years]
- Tech: [Stack]
- Scale: [Users/Data size]
### Freeze Agreement CRITICAL
[From Q2]
- Status: [Agreed / Pending / Blocked]
- Boundaries: [What's frozen]
- Stakeholders: [Signed off: Yes/No]
### Event Extraction Approach
[From Q3]
- Method: [Direct query / CDC / Audit log / Hooks]
- Feasibility: [High / Medium / Low]
- Challenges: [Data challenges]
### Timeline & Staffing
[From Q4 & Q5]
- Target: [Date]
- Criticality: [High / Medium / Low]
- Rollback Strategy: [How to recover]
### Green Light Checklist
- [ ] Freeze agreement obtained (REQUIRED)
- [ ] Extraction approach validated (REQUIRED)
- [ ] Team staffing confirmed (REQUIRED)
- [ ] Risk mitigation accepted (REQUIRED)
**RECOMMENDATION**: [Proceed / Resolve blockers / Not recommended - why]
Update Interview Trail with integration-specific findings.
This section is the risk management document for high-risk integrations.
The Side-Car Pattern
Instead of rewriting the legacy system, build new features alongside it:
Traditional Approach (Risky):
Legacy System โ Rewrite everything from scratch โ New System
Problem: Risk of losing functionality, expensive, long timeline
Side-Car Approach (Safe):
Legacy System (Frozen - no new changes)
- Still handles existing features
- Still processes existing users
- No modifications, no new bugs
> Database (Event Source)
Query existing data
Extract domain events
> Event Store (New)
Captured events
New event source of truth
Side-Car System (New Event-Modeled Features)
- New functionality using events
- New UI/APIs
- Runs in parallel with legacy
Contains:
Event Store (primary source of truth)
Commands/Handlers
Read Models
User-facing UIs/APIs
Result: Legacy system frozen, new features built safely alongside.
Workflow
1. Analyze the Legacy System
Document what the legacy system does:
Legacy System: Order Management (10-year-old monolith)
Current capabilities:
Create orders
Confirm orders
Track shipments
Process refunds
Generate invoices
Known issues:
No audit trail
Hard to modify order status
Performance degrades with large datasets
No clear separation of concerns
Tightly coupled to specific customer
Technology:
- Database: MySQL (20+ GB)
- Code: Monolithic Rails application
- API: XML-based SOAP
- Users: 500+ directly using legacy UI
Cost of rewrite:
- Effort: 6-12 months
- Risk: High (functionality gaps)
- Cost: $500k+
2. Define the Freeze
Establish what won't change in the legacy system:
Freeze Agreement with Business
We will NOT change:
Legacy UI (users continue using it)
Legacy database schema
Legacy business logic
Legacy APIs
We WILL do:
Maintain legacy system (bug fixes, support)
Extract events from legacy data
Build new features in side-car
Gradually migrate users to new features
Benefits:
Zero risk to existing operations
Can start immediately (no design cycle)
Old users continue with familiar UI
New users get modern features
Can integrate both systems gradually
Timeline:
Year 1: Side-car handles new functionality
Year 2-3: Gradually migrate users
Year 3-4: Phase out legacy system
3. Extract Domain Events from Legacy Data
The legacy database is your event source:
Legacy Database Schema:
Orders table:
id, customer_id, status, created_at, updated_at, items_json, total, ...
Payments table:
id, order_id, amount, status, gateway_ref, created_at, ...
Shipments table:
id, order_id, carrier, tracking_num, delivered_at, created_at, ...
Event Extraction Strategy:
For Orders table:
When status = 'draft' and record exists
โ Extract: OrderCreated event (created_at, items_json, customer_id, ...)
When status = 'confirmed' and previous was 'draft'
โ Extract: OrderConfirmed event
When status = 'shipped' and previous was 'confirmed'
โ Extract: OrderShipped event
When status = 'cancelled'
โ Extract: OrderCancelled event
For Payments table:
When status = 'authorized'
โ Extract: PaymentAuthorized event (amount, gateway_ref, ...)
When status = 'failed'
โ Extract: PaymentFailed event
For Shipments table:
When delivered_at is populated
โ Extract: DeliveryConfirmed event
Key insight: Legacy tables contain the data that represents events that happened.
We reverse-engineer: State changes โ Events.
4. Build Event Capture Pipeline
Create a process to extract events:
Option A: One-time Historical Extraction (Catch-up)
Script:
1. Query legacy Orders: SELECT * WHERE id > last_extracted_id
2. For each record, reverse-engineer what events happened
3. Create events in new Event Store
4. Continue polling for changes
Process:
order_id=123, status=confirmed, updated_at=2024-01-15
โ Determine: OrderCreated happened at created_at
โ Determine: OrderConfirmed happened at updated_at
โ Persist: { OrderCreated, OrderConfirmed } to Event Store
--- Option B: Real-time Sync (Ongoing)
Trigger on legacy writes:
1. When legacy system creates/updates record
2. Database trigger OR Change Data Capture (CDC)
3. Event generated and sent to new system
4. Both systems stay in sync
Benefits:
- Zero delay between legacy action and event capture
- Can serve new features in real-time
- Cleaner integration
--- Option C: Hybrid (Start with historical, add real-time)
Year 1:
- Historical extract existing 10 years of orders
- New events captured in real-time
Advantage: Smooth onboarding, future-proof
5. Build the Side-Car System
Create new Event-Modeled system alongside legacy:
New Side-Car System Architecture:
Event Store (Source of Truth for new features)
- OrderCreated
- OrderConfirmed
- PaymentAuthorized
- OrderShipped
- DeliveryConfirmed
- CustomerCreated (new feature)
- ReturnRequested (new feature)
> Handlers (Process events)
PaymentProcessor
InventoryProcessor
NotificationProcessor
> Read Models (Projections)
OrderStatusView
CustomerDashboard (new!)
ReturnStatusView (new!)
> APIs & UIs (New user-facing)
REST API
GraphQL endpoint
New web UI
Mobile app
6. Handle Y-Valve User Traffic
Gradually redirect users from legacy to new:
Phase 1: New features only in side-car
User action โ Legacy system handles
Phase 2: Read model shown alongside legacy
User views โ Legacy UI + New dashboards
Phase 3: New features accessible, legacy still available
User can: Use legacy OR new features
Gradual migration as users opt-in
Phase 4: Deprecation period
New features required
Legacy features deprecated
Legacy system in read-only mode
--- Y-Valve Pattern (Traffic Routing):
User Request
Is this a NEW feature? โ Route to Side-Car system
Is user opted-in to new UI? โ Route to Side-Car system
Default โ Route to Legacy system
Example code:
if (isNewFeature(request)) {
return sideCarSystem.handle(request);
} else if (user.preferNewUI) {
return sideCarSystem.handle(request);
} else {
return legacySystem.handle(request);
}
7. Develop New Features in Side-Car
Use Event Modeling for new functionality:
New Feature: Order Returns & Refunds
Legacy system has: Nothing (returns handled via email/phone)
New side-car feature: Self-service return management
Event Model for Returns:
Commands:
- RequestReturn (from customer)
- ApproveReturn (from support agent)
- ProcessRefund (from payment system)
Events:
- ReturnRequested
- ReturnApproved
- RefundInitiated
- RefundCompleted
Views:
- ReturnStatusView (by order)
- ReturnQueueView (pending approvals)
- RefundHistoryView (completed refunds)
Benefits:
Built with modern Event Modeling patterns
Clear contracts
Easy to test
Scalable architecture
No need to modify legacy code
Output Format
Present as:
# Legacy System Integration: [Organization Name]
## Current Legacy System
**System Name**: [Name]
**Age**: [Years]
**Technology**: [Tech stack]
**Users**: [Count]
**Database Size**: [Size]
**Current Capabilities**:
- [Feature 1]
- [Feature 2]
**Known Issues**:
- [Issue 1]
- [Issue 2]
**Rewrite Impact**:
- Effort: [Months/Years]
- Risk: [Low/Medium/High]
- Cost: [Estimate]
---
## The Freeze Agreement
**What we won't change**:
- [Legacy UI]
- [Legacy database]
- [Legacy APIs]
**What we will do**:
- [Maintain legacy]
- [Extract events]
- [Build new features]
**Timeline**:
- [Phase 1]
- [Phase 2]
- [Phase 3]
---
## Event Extraction Strategy
### Source: [Legacy Table/System]
**Data available**:
- [Field 1]
- [Field 2]
**Events extracted**:
- [Event 1] when [condition]
- [Event 2] when [condition]
**Extraction logic**:
[Reverse-engineering approach]
---
## Side-Car System Architecture
### Events Captured from Legacy
- OrderCreated
- OrderConfirmed
- [Other events...]
### Events Generated by Side-Car
- CustomerCreated (new feature)
- ReviewSubmitted (new feature)
- [Other new events...]
### New Features Built
- [Feature 1]: Commands โ Handlers โ Events โ Views
- [Feature 2]: Commands โ Handlers โ Events โ Views
---
## User Migration Plan
### Phase 1: Read-Only Views
**Timeline**: [Duration]
**Users**: [Gradual rollout]
**Change**: New dashboards available alongside legacy
### Phase 2: New Features
**Timeline**: [Duration]
**Users**: [% of user base]
**Change**: New functionality accessible, legacy still primary
### Phase 3: Preference Switch
**Timeline**: [Duration]
**Users**: [Opt-in]
**Change**: Users choose new UI
### Phase 4: Deprecation
**Timeline**: [Duration]
**Change**: Legacy system read-only
---
## Risks & Mitigations
| Risk | Mitigation |
|------|-----------|
| Data inconsistency | Event capture validation |
| User confusion | Gradual rollout, clear messaging |
| Operational overhead | Automated monitoring, alerts |
| Performance | Side-car scales independently |
Quality Checklist
Common Integration Patterns
Pattern 1: Read Models from Legacy
Legacy Database โ Query โ Extract state โ Create Read Model for side-car
Benefit: New UI shows unified data (legacy + new)
Pattern 2: Event Stream from Audit Logs
Legacy audit log (if available) โ Parse โ Extract events โ Event Store
Benefit: Complete history, less guesswork
Pattern 3: Scheduled Sync
Every N minutes โ Query legacy changes โ Generate events โ Event Store
Benefit: Simple to implement, eventual consistency
Pattern 4: Mirror-Write via Integration Boundary (Use with Caution)
Default: Avoid dual-write. Keep side-car-only and sync via legacy events.
Exception: Dual-write is allowed only with an approved freeze exception.
Required controls when dual-write is approved:
- Route all writes through a controlled integration boundary (not direct DB access)
- Make every write idempotent (safe to replay on retry or failure)
- Add a reconciliation check and a documented rollback plan
- Document the owner, the sunset date, and the freeze exception approval
Without these controls: keep side-car-only.
Key Principles
- Freeze First: Establish clear freeze agreement before starting
- No Rewrites: Side-car builds NEW features, doesn't duplicate legacy
- Gradual Migration: Don't force everyone at once
- Dual Operation: Both systems run in parallel indefinitely (your timeline)
- Clear Ownership: Legacy team maintains legacy, side-car team owns new
- Event-Driven: Use events as integration point
- User Choice: Where possible, let users choose when to migrate
When Side-Car Is Right
Use side-car when:
- Legacy system works but is hard to modify
- Business needs new features quickly
- Rewrite would take 6+ months
- High risk of rewrite failure
- Users are comfortable with both systems
- Clear separation of old vs. new features
Don't use side-car when:
- Legacy system is broken and needs fixes
- Complete integration required (tight coupling)
- Users only want one unified system
- Legacy holds critical IP that can't be replicated
Anti-Patterns to Avoid
Hybrid approach: Trying to modify legacy AND build side-car (confusion)
Forced unification: Requiring all users to switch at once (disruption)
Incomplete event extraction: Missing important legacy data in events
Tight coupling: Side-car depends on legacy database directly (defeats purpose)
No clear separation: Users don't know which system they're using (confusion)
