Session 02 - Tech Debt Strategy | Knowledge Base

"Tech debt giống nợ tài chính - một chút thì leverage, nhiều quá thì phá sản." - Ward Cunningham

1. Types of Tech Debt

Deliberate vs Accidental

	Deliberate (Cố ý)	Accidental (Vô tình)
Biết là nợ	"Ship trước, refactor sau"	Không nhận ra đang tạo nợ
Ví dụ	Hardcode config để kịp deadline	Copy-paste code vì không biết có utility sẵn
Quản lý	Ghi vào backlog, plan trả nợ	Phát hiện qua code review, static analysis

Reckless vs Prudent

	Reckless (Liều lĩnh)	Prudent (Cân nhắc)
Deliberate	"Không có thời gian design"	"Ship nhanh, biết phải refactor"
Accidental	"Design pattern là gì?"	"Giờ mới biết cách tốt hơn"

2. Tech Debt Quadrant (Martin Fowler)

                    Deliberate
                       │
         ┌─────────────┼─────────────┐
         │             │             │
         │  RECKLESS   │  PRUDENT    │
         │             │             │
         │ "We don't   │ "We must    │
         │  have time  │  ship now   │
         │  for design"│  and deal   │
         │             │  with       │
         │             │  consequences│
Reckless ─┼─────────────┼─────────────┼─ Prudent
         │             │             │
         │ "What's     │ "Now we     │
         │  layered    │  know how   │
         │  architecture│  we should  │
         │  ?"         │  have done  │
         │             │  it"        │
         │  RECKLESS   │  PRUDENT    │
         │             │             │
         └─────────────┼─────────────┘
                       │
                   Accidental

Hành động theo quadrant

Quadrant	Action
Reckless + Deliberate	Stop. Training + code review cần thiết ngay
Reckless + Accidental	Đầu tư vào learning, pair programming, mentoring
Prudent + Deliberate	OK nếu có plan trả nợ rõ ràng trong backlog
Prudent + Accidental	Tự nhiên - refactor khi touch code (Boy Scout Rule)

3. How to Identify and Catalog Tech Debt

Detection Methods

Method	Tool / Approach	What it finds
Static Analysis	SonarQube, NDepend, Roslyn Analyzers	Code smells, complexity, duplication
Code Review	Pull request reviews	Design issues, naming, patterns
Architecture Tests	ArchUnitNET	Dependency violations, layer breaches
Runtime Monitoring	Application Insights, Datadog	Performance debt, memory leaks
Developer Feedback	Survey, retro, tech debt backlog	Pain points chỉ dev biết
Dependency Scan	Dependabot, Snyk	Outdated/vulnerable packages

Tech Debt Catalog Template

markdown
## Tech Debt Item: TD-042

**Title:** OrderService God Class - 2,500 lines, 47 methods
**Category:** Design Debt
**Location:** `src/Services/OrderService.cs`
**Created:** 2024-01-15
**Severity:** High

### Description
OrderService đảm nhiệm quá nhiều responsibilities: order creation,
validation, payment processing, inventory check, notification.
Vi phạm Single Responsibility Principle.

### Impact
- Mỗi lần thay đổi 1 feature → risk regression feature khác
- Unit test coverage chỉ 35% vì quá phức tạp để mock
- Onboarding dev mới mất 2 tuần mới hiểu file này

### Proposed Solution
Tách thành: OrderCreationService, OrderValidationService,
PaymentService, InventoryService, NotificationService.
Apply Mediator pattern cho orchestration.

### Estimated Effort
- 8-13 story points (2-3 Sprint nếu làm incremental)
- Cần regression test toàn bộ order flow

### Business Risk if Not Fixed
- Feature velocity giảm 30% cho order-related stories
- Bug rate tăng mỗi Sprint (3 bugs/Sprint liên quan order)

4. Quantifying Tech Debt

Time Cost Measurement

Metric	Cách đo	Target
Interest Rate	Thời gian extra mỗi Sprint vì debt	Track qua Sprint retro
Fix Time Multiplier	Bug fix mất bao lâu so với expected	< 2x = acceptable
Onboarding Time	Dev mới mất bao lâu productive	< 2 tuần cho senior
Change Failure Rate	% deployments gây incident	< 15%
Lead Time Increase	Story completion time trend	Không tăng qua Sprints

Cost Formula

Annual Cost of Debt = 
  (Extra hours per Sprint due to debt) x 
  (Number of Sprints per year) x 
  (Average developer hourly rate)

Ví dụ:
  5 hours/Sprint x 26 Sprints/year x $50/hour = $6,500/year per item
  20 items in backlog = $130,000/year đang "trả lãi"

Risk Scoring

Debt Priority Score = Impact (1-5) x Frequency (1-5) x Effort_Inverse (5/effort_in_sprints)

Ví dụ:
- God Class OrderService:  Impact=5, Frequency=5, Effort=3 Sprint
  Score = 5 x 5 x (5/3) = 41.7 → HIGH PRIORITY

- Outdated logging library: Impact=2, Frequency=1, Effort=0.5 Sprint  
  Score = 2 x 1 x (5/0.5) = 20 → MEDIUM PRIORITY

5. Prioritization Framework

Tech Debt Decision Matrix

Prioritization Criteria

Tiêu chí	Weight	Low (1)	Medium (3)	High (5)
Business Impact	30%	No user impact	Slowdown	Feature blocked
Developer Productivity	25%	Minor annoyance	Adds 1-2h/week	Adds 1+ day/Sprint
Risk	25%	Low probability	Moderate probability	Ticking time bomb
Effort to Fix	20%	> 2 Sprints	1 Sprint	< 3 days

6. Tech Debt Sprint Allocation - 20% Rule

The 20% Budget

Dành 20% Sprint capacity cho tech debt và improvements:

Sprint 2 tuần, team 5 devs = 50 dev-days
20% = 10 dev-days cho tech debt

Implementation Models

Model	Mô tả	Phù hợp khi
Continuous	Mỗi Sprint dành 20% cho tech debt	Team mature, PO đồng ý
Tech Debt Sprint	Mỗi 4-5 Sprint, 1 Sprint full tech debt	Debt tích tụ nhiều
Boy Scout Rule	Refactor khi touch code (leave it better)	Debt nhỏ, scattered
Dedicated Team	1-2 devs full-time tech debt	Large legacy system
Hack Week	1 tuần/quarter focus innovation + debt	Team morale boost

Track Tech Debt Velocity

Sprint | Feature Points | Tech Debt Points | Ratio
-------|---------------|-----------------|------
S1     | 34            | 5               | 15%
S2     | 30            | 8               | 27%
S3     | 32            | 8               | 25%
S4     | 28            | 5               | 18%
S5     | 35            | 8               | 23%
                                    Average: 21% ✅

7. Communicating Tech Debt to Stakeholders

Nói ngôn ngữ Business, không nói ngôn ngữ Tech

Dev nói (bad)	Business nói (good)
"Code smell trong OrderService"	"Mỗi feature mới liên quan đơn hàng mất gấp đôi thời gian"
"Cần upgrade .NET 6 lên .NET 8"	".NET 6 hết support tháng 11, security patches không còn"
"Cần refactor database layer"	"Query chậm dần, report generation tăng từ 2s lên 8s"
"Technical debt tích tụ nhiều"	"Velocity giảm 20% trong 3 tháng, dự kiến tiếp tục giảm"

Stakeholder Communication Template

markdown
## Tech Debt Quarterly Report - Q1 2025

### Summary
- **Total debt items:** 24 (tăng 6 so với Q4)
- **Estimated annual cost:** $130,000 (developer productivity loss)
- **Velocity trend:** Giảm 15% so với Q4

### Top 3 Critical Items
1. **OrderService refactoring** - blocking 3 upcoming features,
   estimated 2 Sprint delay if not addressed. Fix: 8 points.
2. **.NET 6 → .NET 8 migration** - security support ends Nov 2024.
   Fix: 13 points across 2 Sprints.
3. **Database query optimization** - report generation degrading
   30% per quarter. Fix: 5 points.

### Recommendation
Allocate 1 full Sprint (Sprint 8) for tech debt reduction.
Expected outcome: recover 20% velocity, unblock Q2 features.

### Investment vs Return
- **Investment:** 1 Sprint (50 dev-days = $25,000)
- **Return:** $130,000/year savings + faster Q2 delivery
- **ROI:** 5.2x in first year

Visualization cho Management

Feature Velocity Trend (Story Points/Sprint)
40 |  ██
35 |  ██ ██       ██
30 |  ██ ██ ██    ██ ██
25 |  ██ ██ ██ ██ ██ ██
20 |  ██ ██ ██ ██ ██ ██ ←── "Nếu không trả nợ"
15 |  ██ ██ ██ ██ ██ ██
   |________________________
    Q1  Q2  Q3  Q4  Q1  Q2
              ↑
        Tech debt Sprint here
        → Velocity phục hồi

Tip: Đừng bao giờ nói "cần refactor vì code xấu". Nói "cần refactor để ship feature X nhanh hơn 2 tuần và giảm bug rate 40%."

Session tiếp theo: Session 03 - Engineering Standards sẽ đi sâu vào các tiêu chuẩn kỹ thuật cho dự án .NET.