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Designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases. Use when modernizing leg
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Designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases. Use when modernizing legacy systems, implementing strangler fig pattern or branch by abstraction, decomposing monoliths, upgrading frameworks or languages, or reducing technical debt without disrupting business operations.
Designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases. Use when modernizing legacy systems, implementing strangler fig pattern or branch by abstraction, decomposing monoliths, upgrading frameworks or languages, or reducing technical debt without disrupting business operations.
Assess system — Analyze codebase, dependencies, risks, and business constraints. Produce a dependency map and risk register before proceeding.
Plan migration — Design an incremental roadmap with explicit rollback strategies per phase. Reference references/system-assessment.md for code analysis templates.
Build safety net — Create characterization tests and monitoring before touching production code. Target 80%+ coverage of existing behavior.
Migrate incrementally — Apply strangler fig pattern with feature flags. Route traffic via a facade; shift load gradually.
Validate & iterate — Run full test suite, review monitoring dashboards, and confirm business behavior is preserved before retiring legacy code.
Load detailed guidance based on context:
| Topic | Reference | Load When |
|---|---|---|
| Strangler Fig | references/strangler-fig-pattern.md | Incremental replacement, facade layer, routing |
| Refactoring | references/refactoring-patterns.md | Extract service, branch by abstraction, adapters |
| Migration | references/migration-strategies.md | Database, UI, API, framework migrations |
| Testing | references/legacy-testing.md | Characterization tests, golden master, approval |
| Assessment | references/system-assessment.md | Code analysis, dependency mapping, risk evaluation |
# facade.py — routes requests to legacy or new service based on a feature flag
import os
from legacy_service import LegacyOrderService
from new_service import NewOrderService
class OrderServiceFacade:
def __init__(self):
self._legacy = LegacyOrderService()
self._new = NewOrderService()
def get_order(self, order_id: str):
if os.getenv("USE_NEW_ORDER_SERVICE", "false").lower() == "true":
return self._new.fetch(order_id)
return self._legacy.get(order_id)
# feature_flags.py — thin wrapper around an environment or config-based flag store
import os
def flag_enabled(flag_name: str, default: bool = False) -> bool:
"""Check whether a migration feature flag is active."""
return os.getenv(flag_name, str(default)).lower() == "true"
# Usage
if flag_enabled("USE_NEW_PAYMENT_GATEWAY"):
result = new_gateway.charge(order)
else:
result = legacy_gateway.charge(order)
# test_characterization_orders.py
# Captures existing legacy behavior as a golden-master safety net.
import pytest
from legacy_service import LegacyOrderService
service = LegacyOrderService()
@pytest.mark.parametrize("order_id,expected_status", [
("ORD-001", "SHIPPED"),
("ORD-002", "PENDING"),
("ORD-003", "CANCELLED"),
])
def test_order_status_golden_master(order_id, expected_status):
"""Fail loudly if legacy behavior changes unexpectedly."""
result = service.get(order_id)
assert result["status"] == expected_status, (
f"Characterization broken for {order_id}: "
f"expected {expected_status}, got {result['status']}"
)
When implementing modernization, provide:
Strangler fig pattern, branch by abstraction, characterization testing, incremental migration, feature flags, canary deployments, API versioning, database refactoring, microservices extraction, technical debt reduction, zero-downtime deployment
Maintained by @jeffallan, Principal Consultant at Synergetic Solutions