Technical Requirements Document (TRD)

Overview

A Technical Requirements Document (TRD) defines how a system or feature will be built. It translates the product requirements from the PRD into concrete technical specifications -- architecture, APIs, data models, security controls, performance targets, and deployment strategy. The TRD is the engineering team's blueprint for implementation.

The TRD sits downstream of the PRD and upstream of implementation:

PRD ("What are we building?") --> TRD ("How do we build it?") --> Implementation + ADRs

Standard TRD Structure

1. Title and Metadata

Document identification, ownership, and status tracking.

2. System Overview

High-level description of the system, its purpose, and how it fits into the broader architecture.

3. Architecture

System architecture including components, interactions, technology stack, and diagrams.

4. API Specifications

Endpoints, request/response schemas, authentication, error handling, and versioning.

5. Data Models

Database schema, entity relationships, data types, constraints, and migration strategy.

6. Security Requirements

Authentication, authorization, encryption, compliance, and threat model.

7. Performance Requirements

Latency, throughput, availability, and scalability targets.

8. Dependencies

External services, libraries, infrastructure, and team dependencies.

9. Technical Constraints

Technology mandates, backward compatibility, regulatory, and organizational constraints.

10. Deployment Plan

Environments, CI/CD pipeline, rollout strategy, and rollback procedures.

Complete TRD Template

# TRD: [System/Feature Name]

| Field            | Value                          |
|------------------|--------------------------------|
| Author           | [Name]                         |
| Status           | Draft / In Review / Approved   |
| Created          | YYYY-MM-DD                     |
| Last Updated     | YYYY-MM-DD                     |
| PRD Reference    | [Link to PRD]                  |
| Reviewers        | [Names]                        |
| Approvers        | [Names]                        |

## 1. System Overview

### 1.1 Purpose
[Brief description of what this system does and why it exists.
Reference the PRD for product context.]

### 1.2 Scope
[What is covered by this TRD. What is explicitly excluded.]

### 1.3 Context
[How this system fits into the broader architecture.
Include a C4 Context diagram or equivalent.]

```mermaid
graph LR
    User[End User] --> WebApp[Web Application]
    WebApp --> API[API Gateway]
    API --> AuthService[Auth Service]
    API --> OrderService[Order Service]
    OrderService --> Database[(PostgreSQL)]
    OrderService --> MessageQueue[RabbitMQ]
    MessageQueue --> NotificationService[Notification Service]

2. Architecture

2.1 Architecture Style

[Monolith / Microservices / Serverless / Event-driven / Hybrid] [Justification for the chosen approach. Link to ADR if applicable.]

2.2 Component Overview

2.3 Technology Stack

2.4 Communication Patterns

3. API Specifications

3.1 API Design Principles

[REST / GraphQL / gRPC. Versioning strategy. Naming conventions.]

3.2 Authentication and Authorization

[OAuth 2.0, JWT, API keys. Token format and validation. Scope/role model.]

3.3 Endpoints

POST /api/v1/orders

Description: Create a new order Authentication: Bearer token (JWT) Authorization: Role: customer

Request:

{
  "items": [
    {
      "productId": "WM001",
      "quantity": 2
    }
  ],
  "shippingAddress": {
    "street": "123 Main St",
    "city": "Springfield",
    "state": "IL",
    "zip": "62701"
  },
  "paymentMethodId": "pm_abc123"
}

Response (201 Created):

{
  "orderId": "ord_789xyz",
  "status": "pending",
  "total": 59.98,
  "currency": "USD",
  "estimatedDelivery": "2025-02-15",
  "createdAt": "2025-02-08T14:30:00Z"
}

Error Responses:

3.4 Rate Limiting

[Requests per minute per user/IP. Rate limit headers. Retry-After behavior.]

3.5 Pagination

[Cursor-based / offset-based. Page size limits. Response envelope format.]

4. Data Models

4.1 Entity-Relationship Diagram

erDiagram
    User ||--o{ Order : places
    Order ||--|{ OrderItem : contains
    OrderItem }o--|| Product : references
    Order ||--o| Payment : "paid by"
    User ||--o{ Address : has

4.2 Schema Definitions

Users Table

Orders Table

4.3 Data Migration Strategy

[How schema changes are applied. Tool (e.g., Flyway, EF Migrations). Backward compatibility during rolling deployments.]

4.4 Data Retention and Archival

[Retention policies. PII handling. Archival strategy for historical data.]

5. Security Requirements

5.1 Authentication

[Mechanism: OAuth 2.0 + PKCE, JWT tokens, session management. Token lifetime and refresh strategy.]

5.2 Authorization

[RBAC / ABAC model. Role definitions. Permission matrix.]

5.3 Data Protection

[Encryption at rest (AES-256). Encryption in transit (TLS 1.2+). PII handling. GDPR/CCPA compliance measures.]

5.4 Input Validation

[Validation strategy. Sanitization. SQL injection prevention. XSS prevention. CSRF protection.]

5.5 Secrets Management

[Vault / Key Vault / SSM. Secret rotation policy. No secrets in code or environment files.]

5.6 Audit Logging

[What is logged. Log retention. Tamper-proof storage.]

6. Performance Requirements

6.1 Latency Targets

6.2 Throughput

[Expected RPS. Peak load projections. Burst capacity.]

6.3 Availability

[SLA target (e.g., 99.9%). Planned maintenance windows. Failover strategy. RTO and RPO.]

6.4 Scalability

[Horizontal vs vertical scaling strategy. Auto-scaling triggers and limits. Database read replicas / sharding plan.]

6.5 Caching Strategy

[Cache layers (CDN, application cache, database cache). Cache invalidation strategy. TTL policies.]

7. Dependencies

7.1 External Services

7.2 Internal Dependencies

[Other team services, shared libraries, platform services.]

7.3 Third-Party Libraries

[Key libraries with versions. License compliance. Update strategy.]

8. Technical Constraints

• •[Constraint 1: e.g., Must run on .NET 8 LTS]
• •[Constraint 2: e.g., Must use PostgreSQL (organizational standard)]
• •[Constraint 3: e.g., Must support IE 11 (legacy customer requirement)]
• •[Constraint 4: e.g., Must integrate with existing SSO via SAML 2.0]
• •[Constraint 5: e.g., Maximum deployment artifact size 500MB]

9. Deployment Plan

9.1 Environments

9.2 CI/CD Pipeline

[Build → Test → Security Scan → Deploy → Smoke Test → Rollout] [Tool: GitHub Actions / Azure DevOps / Jenkins]

9.3 Rollout Strategy

[Blue-green / Canary / Rolling update. Canary percentage and promotion criteria. Feature flags for gradual rollout.]

9.4 Rollback Plan

[Automated rollback triggers (error rate, latency). Manual rollback procedure. Database rollback strategy (backward-compatible migrations).]

9.5 Monitoring and Alerting

[Metrics: latency, error rate, CPU, memory, queue depth. Tools: Prometheus, Grafana, Datadog, Azure Monitor. Alert thresholds and escalation policy.]

10. Open Technical Questions

• • [Question 1 -- owner, target date]
• • [Question 2 -- owner, target date]
• • [Question 3 -- owner, target date]

Appendix

A. Glossary

[Domain-specific technical terms and acronyms.]

B. Related Documents

• •PRD: [link]
• •ADRs: [links to relevant architecture decisions]
• •API Schema: [link to OpenAPI/Swagger spec]
• •Runbook: [link to operational runbook]

## TRD Example: Order Service

```markdown
# TRD: Order Service

| Field            | Value                     |
|------------------|---------------------------|
| Author           | Jane Engineer             |
| Status           | In Review                 |
| Created          | 2025-01-15                |
| PRD Reference    | PRD-042: Checkout Redesign|

## 1. System Overview

The Order Service manages the complete order lifecycle from creation
through fulfillment. It replaces the order module in the monolith
as part of the microservices migration (see ADR-007).

## 2. Architecture

Architecture style: Event-driven microservice (see ADR-012).

The service exposes a REST API for synchronous order operations
and publishes domain events to RabbitMQ for downstream consumers
(notification, analytics, fulfillment).

## 3. Key API Endpoints

- POST /api/v1/orders -- Create order
- GET /api/v1/orders/{id} -- Get order by ID
- GET /api/v1/orders?userId={id} -- List user orders
- PATCH /api/v1/orders/{id}/cancel -- Cancel order
- POST /api/v1/orders/{id}/refund -- Initiate refund

## 4. Data Model

Orders table with OrderItems join table. Uses PostgreSQL with
EF Core migrations. See schema definitions in Section 4.2 above.

## 5. Events Published

| Event              | Trigger               | Consumers                  |
|--------------------|-----------------------|----------------------------|
| OrderCreated       | New order placed      | Notification, Analytics    |
| OrderCancelled     | Order cancelled       | Notification, Fulfillment  |
| OrderFulfilled     | Order shipped         | Notification, Analytics    |
| PaymentProcessed   | Payment confirmed     | Order (self), Analytics    |

Relationship to Other Documents

BRD (Business Requirements)
 |  Business case and justification
 v
PRD (Product Requirements)
 |  What to build, user stories, success metrics
 v
TRD (Technical Requirements)  <-- YOU ARE HERE
 |  How to build it -- architecture, APIs, data, deployment
 |
 +---> ADR (Architecture Decision Records)
 |     Individual decisions made during TRD creation
 |     (e.g., "Why PostgreSQL over MongoDB?")
 |
 +---> RFC (Request for Comments)
 |     Proposals for significant technical changes
 |     that need team-wide input before finalizing
 |
 v
Implementation
 |  Code guided by the TRD
 v
Gherkin / Gauge (Executable Specs)
    Verification that implementation meets requirements

Best Practices

• •Reference the PRD -- every technical decision should trace back to a product requirement. If a TRD section cannot be justified by the PRD, question whether it belongs.
• •Include diagrams -- architecture diagrams (C4, sequence, ERD) communicate structure far better than prose alone. Use Mermaid for Markdown-native rendering.
• •Specify API contracts precisely -- include request/response schemas, error codes, authentication, pagination, and rate limiting. Consider providing an OpenAPI spec as an appendix.
• •Define performance targets with percentiles -- P50, P95, and P99 targets are more useful than averages, which hide tail latency.
• •Document the security model explicitly -- authentication, authorization, encryption, input validation, and secrets management deserve dedicated sections, not afterthoughts.
• •Plan for failure -- include rollback procedures, circuit breaker strategies, fallback behaviors, and disaster recovery in the deployment plan.
• •Record decisions as ADRs -- when the TRD makes a significant technical choice (database, framework, architecture style), create a linked ADR to capture the context and trade-offs.
• •Keep it current -- update the TRD as the system evolves. An outdated TRD is a liability, not an asset.
• •Review with the full engineering team -- the TRD should be reviewed by backend, frontend, infrastructure, security, and QA engineers before implementation begins.
• •Use version control -- store the TRD in the repository alongside the code it describes so it evolves with the system.