Software Engineering Technical Design Documents

Overview

A software technical design document (TDD) is an extremely important part of the software development effort. While not every development work item / stream necessarily entails a technical design document, engineering leaders should foster a culture where software engineers organically are able to make a judgement call on when to write a technical design document. A technical design document allows for ideas to be fleshed out in a more formal and a transparent manner. It allows for stakeholders to review and respond and to document feedback for future reference. Technical design documents can in some cases also act as feeders for architectural documents, and vice versa.

This post provides a template for software technical design document.

Template

# Title

A short title summarizing the technical design document.

| Metadata                    | Value(s)                                                                                          |
| --------------------------- | ------------------------------------------------------------------------------------------------- |
| Status                      | Draft / Proposed / Under Review / Under Evaluation / Accepted / Amended / Deprecated / Superseded |
| Date Created                | YYYY-MM-DD                                                                                        |
| Date Last Updated           | YYYY-MM-DD                                                                                        |
| Date Approved               | YYYY-MM-DD (when final approval given)                                                            |
| Last Updated By             |                                                                                                   |
| Author(s)                   |                                                                                                   |
| Engineering Team(s)         |                                                                                                   |
| Engineering Lead            |                                                                                                   |
| Primary Product Stakeholder |                                                                                                   |
| Other Stakeholders          |                                                                                                   |
| Reviewer(s)                 |                                                                                                   |
| Commenter(s)                |                                                                                                   |
| Supersedes                  | [Previous design/system name] (if applicable)                                                     |

## Executive Summary

A concise 3-5 sentence overview that captures:

- What is being built/changed
- Why it matters (business/technical value)
- Key architectural approach
- Major trade-offs or risks
- Timeline and effort estimate

This should be readable by executives and allow them to understand the design at a high level.

## Problem Statement

What technical or business problem does this design solve? Be specific about:

- Current pain points or gaps
- Impact of not solving this problem
- Constraints that shape the solution
- Scope boundaries (what's in, what's out)

## Background

Historical context and relevant information:

- Current system state (if replacing/extending existing functionality)
- Previous attempts or related work
- Technical debt or legacy considerations
- Organizational or process context
- Why now? (timing considerations)

## Motivation and Context

Discussion of the technical and/or business requirements / use-cases that was the motivation for this design.

**Business Drivers:**

- Revenue impact
- Customer needs or commitments
- Competitive positioning
- Strategic alignment

**Technical Drivers:**

- Performance requirements
- Scalability needs
- Reliability improvements# Title

### Use Cases / User Stories

This should typically point to a Product document and/or user stories in the Product backlog. But it is OK to provide a brief summary.

**Primary Use Cases:**

1. **Use Case 1:** [Title]
   - Actor: [Who is the user]
   - Goal: [What they want to accomplish]
   - Flow: [Key steps]
   - Success Criteria: [How to measure success]

2. **Use Case 2:** [Title]
   [Repeat structure]

**User Stories:**

- As a [user type], I want to [action] so that [benefit]
- As a [user type], I want to [action] so that [benefit]

## Requirements

### Functional Requirements

**Must Have (P0):**

- [ ] Requirement 1: [Description]
- [ ] Requirement 2: [Description]

**Should Have (P1):**

- [ ] Requirement 3: [Description]
- [ ] Requirement 4: [Description]

**Nice to Have (P2):**

- [ ] Requirement 5: [Description]

**Out of Scope:**

- Item 1: [Why excluded]
- Item 2: [Why excluded]

### Performance Requirements

- **Latency Targets:**
  - P50: [X seconds/ms]
  - P95: [Y seconds/ms]
  - P99: [Z seconds/ms]
- **Throughput Targets:**
  - Peak: [X QPS/requests per minute]
  - Sustained: [Y QPS over 24 hours]
- **Resource Utilization:**
  - CPU: [Target utilization %]
  - Memory: [Target usage]
  - GPU: [If applicable - utilization %, cost]
  - Storage: [Size, IOPS requirements]
  - Network: [Bandwidth requirements]

### Scalability Requirements

- **Initial Scale:**
  - Users: [Number]
  - Data volume: [Size]
  - Traffic: [QPS/requests]
- **Target Scale (12-24 months):**
  - Users: [Number]
  - Data volume: [Size]
  - Traffic: [QPS/requests]
- **Scaling Approach:**
  - Horizontal scaling: [How and where]
  - Vertical scaling: [Limitations]
  - Cost at scale: [Budget implications]
  - Scale testing plan: [How to validate]

### Reliability Requirements

- **Uptime Target:** [e.g., 99.9%, 99.99%]
- **Error Budget:** [Acceptable error rate]
- **Recovery Time Objective (RTO):** [Time to recover from failure]
- **Recovery Point Objective (RPO):** [Acceptable data loss]
- **Failure Modes:**
  - Component X fails: [Mitigation]
  - Database unavailable: [Mitigation]
  - External dependency down: [Mitigation]
- **Data Integrity:**
  - Consistency requirements
  - Backup and recovery strategy
  - Data validation approach

### Security Requirements

- **Authentication & Authorization:**
  - User authentication method
  - Service-to-service auth
  - Permission model
- **Data Protection:**
  - Encryption at rest: [Requirements]
  - Encryption in transit: [Requirements]
  - PII/sensitive data handling
  - Data retention and deletion
- **Compliance:**
  - Federal requirements (if applicable)
  - Air-gapped deployment support
  - Audit logging requirements
  - Regulatory compliance (FedRAMP, SOC2, etc.)

### Observability Requirements

- **Metrics:**
  - Key performance indicators to track
  - Business metrics
  - Technical metrics (latency, error rate, throughput)
- **Logging:**
  - Log levels and content
  - Structured logging format
  - Log retention policy
- **Distributed Tracing:**
  - Critical paths to trace
  - Trace sampling strategy
- **Alerting:**
  - Alert conditions
  - On-call runbook requirements
- **Dashboards:**
  - User-facing dashboards
  - Operational dashboards
  - Executive/business dashboards

## Goals

**Primary Goals:**

1. [Goal 1 - measurable outcome]
2. [Goal 2 - measurable outcome]
3. [Goal 3 - measurable outcome]

**Non-Goals:**

1. [What we're explicitly not trying to achieve]
2. [Scope boundaries]

**Success Metrics:**

- [Metric 1]: [Current state] → [Target state]
- [Metric 2]: [Current state] → [Target state]

## Design Principles Alignment

This design aligns with the following SCM design principles:

**Principles Supported:**

- **[Principle 1 - e.g., Memory Fidelity]**: [How this design supports or exemplifies this principle]
- **[Principle 2 - e.g., Scalability from Start]**: [How this design supports or exemplifies this principle]
- **[Principle 3 - e.g., Async Architecture]**: [How this design supports or exemplifies this principle]

**Principle Trade-offs:**

- **[Principle Name - e.g., Modularity]**: [Where we accept coupling for performance - justification and mitigation plan]

**Principles Not Applicable:**

- **[Principle Name]**: [Why not relevant to this design]

## Proposal

### High-Level Architecture

[Architecture diagram showing major components, data flows, and interactions]

**Overview:**

[2-3 paragraph description of the architecture, key components, and how they work together]

**Key Architectural Decisions:**

- Decision 1: [What and why] - See ADR-XXX
- Decision 2: [What and why] - See ADR-YYY
- Decision 3: [What and why] - See ADR-ZZZ

### Detailed Design

#### Component 1: [Component Name]

**Responsibility:** [Single responsibility - what this component does]

**Technology Stack:**

- Language: [e.g., Go, Python]
- Framework: [e.g., FastAPI, Gin]
- Database: [e.g., PostgreSQL, Cassandra]
- External dependencies: [List]

**Interfaces:**

- **APIs Exposed:**
  - Endpoint 1: `[HTTP method] /path` - [Purpose]
  - Endpoint 2: `[HTTP method] /path` - [Purpose]
- **APIs Consumed:**
  - Service X: [Purpose]
  - External API Y: [Purpose]

**Data Managed:**

- Primary data: [What data this component owns]
- Cache: [What is cached and why]
- Temporary data: [Any temp storage needs]

**Scaling Strategy:**

- Horizontal: [How many instances, auto-scaling triggers]
- Stateless/Stateful: [Design approach]

**Error Handling:**

- Retry logic: [When and how]
- Circuit breakers: [Conditions]
- Graceful degradation: [Fallback behavior]

#### Component 2: [Component Name]

[Repeat structure for each major component]

### Data Models

**Entity 1: [Name]**

```
[Schema definition - SQL, JSON schema, Protobuf, etc.]
```

**Relationships:**

- Relates to Entity X via [relationship type]
- References Entity Y

**Access Patterns:**

- Query 1: [Use case and expected frequency]
- Query 2: [Use case and expected frequency]

**Indexes:**

- Primary key: [Definition]
- Secondary indexes: [List with rationale]

**Entity 2: [Name]**

[Repeat structure]

### API Design

*If this design exposes external or internal APIs*

#### External APIs

**API Endpoint 1:**

```
POST /api/v1/resource
```

**Request:**

```json
{
  "field1": "value",
  "field2": 123
}
```

**Response:**

```json
{
  "id": "uuid",
  "status": "success"
}
```

**Error Responses:**

- 400 Bad Request: [When and why]
- 401 Unauthorized: [When and why]
- 429 Too Many Requests: [Rate limiting]
- 500 Internal Server Error: [When and why]

**Versioning Strategy:**

- How breaking changes are handled
- Deprecation policy
- Version in path vs. header

#### Internal APIs

[Similar structure for service-to-service APIs]

### Sequence Diagrams

**Critical Flow 1: [Name]**

```
[Sequence diagram showing interaction between components]
```

**Description:** [Narrative explanation of the flow]

**Critical Flow 2: [Name]**

[Repeat structure]

### Deployment Architecture

**Infrastructure Components:**

- Load balancers: [Configuration]
- Compute: [VM/container specs, count]
- Storage: [Type, size, performance class]
- Networking: [VPC, subnets, security groups]

**Deployment Environments:**

- Development: [Configuration]
- Staging: [Configuration]
- Production: [Configuration]

**Deployment Process:**

- CI/CD pipeline: [Steps]
- Blue-green / Canary / Rolling deployment: [Strategy]
- Rollback procedure: [Steps]

**Infrastructure as Code:**

- Tool: [Terraform, CloudFormation, etc.]
- Repository: [Location]

### Migration Strategy

*If replacing or significantly changing existing functionality*

**Migration Approach:**

- Big bang vs. incremental: [Choice and rationale]
- Feature flags: [How used]
- Dual-write period: [If applicable]

**Migration Steps:**

1. Step 1: [Description, duration, risk]
2. Step 2: [Description, duration, risk]
3. Step 3: [Description, duration, risk]

**Data Migration:**

- Volume: [Amount of data to migrate]
- Downtime: [Required or zero-downtime]
- Validation: [How to ensure correctness]
- Rollback: [How to undo if needed]

**Backward Compatibility:**

- API compatibility: [Maintained or breaking]
- Data format compatibility: [How handled]
- Client migration: [Required changes]

### Key Takeaways

**For Engineers:**

- [Key technical point 1]
- [Key technical point 2]

**For Product/Business:**

- [Key business impact 1]
- [Key business impact 2]

**For Operations:**

- [Key operational consideration 1]
- [Key operational consideration 2]

## Trade-offs and Risks

### Trade-offs

**Trade-off 1: [e.g., Consistency vs. Performance]**

- **Choice Made:** [What we chose]
- **Rationale:** [Why]
- **Alternative:** [What we gave up]
- **Impact:** [Consequences]
- **Mitigation:** [How we address downsides]

**Trade-off 2: [e.g., Modularity vs. Latency]**

[Repeat structure]

### Risks

**Risk 1: [Description]**

- **Likelihood:** High / Medium / Low
- **Impact:** High / Medium / Low
- **Mitigation:** [How we reduce likelihood or impact]
- **Contingency:** [Plan B if risk materializes]
- **Owner:** [Who is responsible for monitoring]

**Risk 2: [Description]**

[Repeat structure]

### Technical Debt

**Debt Incurred:**

- [What shortcuts or compromises are made]
- [When should this be addressed]
- [Estimated effort to address]

**Debt Reduced:**

- [What existing technical debt this design resolves]

## Security & Compliance Implications

### Security Impact

**Positive Security Improvements:**

- [Security enhancement 1]
- [Security enhancement 2]

**New Security Considerations:**

- [New attack surface or vulnerability]
- [Mitigation approach]

### Compliance Requirements

**Federal/Air-gapped:**

- [Specific requirements for federal deployments]
- [Air-gapped deployment considerations]
- [Data sovereignty requirements]

**Zero Trust Alignment:**

- [How design implements zero trust principles]
- [Authentication/authorization model]

**Audit Trail:**

- [What events are logged for audit]
- [Audit log retention and access]
- [Compliance reporting capabilities]

**Data Protection:**

- [Encryption at rest implementation]
- [Encryption in transit implementation]
- [Key management approach]
- [PII handling and anonymization]

## Observability & Operations

### Monitoring

**Metrics to Track:**

- Business metrics: [e.g., queries processed, accuracy %]
- Performance metrics: [e.g., latency P95, throughput]
- Resource metrics: [e.g., CPU, memory, GPU utilization]
- Error metrics: [e.g., error rate, error types]

**Dashboards:**

- User-facing: [What users can see]
- Operations: [What ops team monitors]
- Executive: [High-level KPIs]

### Logging

**Log Levels:**

- DEBUG: [What gets logged]
- INFO: [What gets logged]
- WARN: [What gets logged]
- ERROR: [What gets logged]

**Structured Logging:**

- Format: [JSON, key-value pairs]
- Required fields: [trace_id, user_id, etc.]
- Sensitive data: [What to redact]

**Log Aggregation:**

- Tool: [e.g., ELK, Splunk, CloudWatch]
- Retention: [How long]
- Search/query capabilities: [What's needed]

### Alerting

**Critical Alerts:**

- Alert 1: [Condition, severity, response]
- Alert 2: [Condition, severity, response]

**Warning Alerts:**

- Alert 3: [Condition, response]

**On-Call Runbook:**

- [Link to runbook or key steps for common issues]

### Operational Maturity Level

**Target Maturity:**

- Q1 2026: Level 1 (MVP) - [Specific capabilities]
- Q2 2026: Level 2 (Medium) - [Specific capabilities]
- Future: Level 3 (High) - [Specific capabilities]

## Alternatives

### Alternative 1: [Alternative Approach Name]

**Description:**

[Brief description of the alternative approach]

**Pros:**

- Benefit 1
- Benefit 2
- Benefit 3

**Cons:**

- Drawback 1
- Drawback 2
- Drawback 3

**Why Not Chosen:**

[Detailed explanation of why this alternative was not selected]

### Alternative 2: [Alternative Approach Name]

[Repeat structure]

### Alternative 3: Do Nothing / Status Quo

**Current State:**

[Description of what happens if we don't implement this design]

**Cost of Inaction:**

- Technical cost: [What problems persist or worsen]
- Business cost: [Revenue/customer impact]
- Opportunity cost: [What we can't do]

**Why This Was Rejected:**

[Rationale for moving forward with new design]

## Success Criteria

### Functional Success

- [ ] All P0 functional requirements implemented and tested
- [ ] All primary use cases supported and validated
- [ ] User acceptance criteria met
- [ ] API contracts stable and documented

### Non-Functional Success

**Performance:**

- [ ] Latency P95 < [X seconds/ms] validated in load testing
- [ ] Throughput of [Y QPS] sustained for 24 hours
- [ ] Resource utilization within targets

**Scalability:**

- [ ] Tested at 2x target scale successfully
- [ ] Horizontal scaling demonstrated
- [ ] Cost model validated

**Reliability:**

- [ ] Uptime target achieved in staging for [X weeks]
- [ ] Failure scenarios tested and mitigated
- [ ] Recovery procedures validated

**Security:**

- [ ] Security review completed and signed off
- [ ] Penetration testing passed
- [ ] Compliance requirements met
- [ ] Audit logging verified

**Observability:**

- [ ] All metrics, logs, traces implemented
- [ ] Dashboards created and reviewed
- [ ] Alerts configured and tested
- [ ] Runbooks created

### Business Success

- [ ] Launched to [X%] of users by [date]
- [ ] Customer adoption: [metric and target]
- [ ] Cost within budget: [actual vs. planned]
- [ ] Timeline: Delivered by [date]

### Validation Plan

**How Success Will Be Measured:**

- Performance testing: [Approach and timeline]
- User acceptance testing: [Approach and timeline]
- Dogfooding: [Internal usage plan]
- Beta program: [External early access plan]

## Implementation Plan

### Phasing

**Phase 1: Foundation (Q1 2026)**

- Scope: [What's included]
- Key deliverables: [List]
- Success criteria: [Metrics]
- Timeline: [Start - End dates]

**Phase 2: Enhancement (Q2 2026)**

- Scope: [What's included]
- Key deliverables: [List]
- Success criteria: [Metrics]
- Timeline: [Start - End dates]

**Phase 3: Optimization (Future)**

- Scope: [What's included]
- Depends on: [Learnings from Phase 1 & 2]

### Work Breakdown

**Backend Development:**

- [ ] Component 1 - Estimate: [X weeks] - Owner: [Name] - Priority: P0
- [ ] Component 2 - Estimate: [Y weeks] - Owner: [Name] - Priority: P0
- [ ] Component 3 - Estimate: [Z weeks] - Owner: [Name] - Priority: P1

**Frontend Development:**

- [ ] UI Component 1 - Estimate: [X weeks] - Owner: [Name]
- [ ] UI Component 2 - Estimate: [Y weeks] - Owner: [Name]

**Infrastructure:**

- [ ] Infrastructure setup - Estimate: [X weeks] - Owner: [Name]
- [ ] CI/CD pipeline - Estimate: [Y weeks] - Owner: [Name]

**Data/ML:**

- [ ] Data pipeline - Estimate: [X weeks] - Owner: [Name]
- [ ] Model integration - Estimate: [Y weeks] - Owner: [Name]

**Testing & Quality:**

- [ ] Test automation - Estimate: [X weeks] - Owner: [Name]
- [ ] Performance testing - Estimate: [Y weeks] - Owner: [Name]
- [ ] Security testing - Estimate: [Z weeks] - Owner: [Name]

**Documentation:**

- [ ] API documentation - Owner: [Name]
- [ ] User documentation - Owner: [Name]
- [ ] Operational runbooks - Owner: [Name]

**Total Estimated Effort:** [X engineer-weeks/months]

### Dependencies

**Technical Dependencies:**

- Dependency 1: [What we need] - Status: [Ready/In Progress/Blocked]
- Dependency 2: [What we need] - Status: [Ready/In Progress/Blocked]

**Team Dependencies:**

- Team X: [What they need to deliver] - Contact: [Name]
- Team Y: [What they need to deliver] - Contact: [Name]

**External Dependencies:**

- Vendor/Tool: [What we need] - Timeline: [When available]
- Third-party API: [What we need] - Timeline: [When available]

**Blocking Dependencies:**

- [Critical dependency that blocks progress] - Mitigation: [Plan]

### Testing Strategy

**Unit Testing:**

- Coverage target: [e.g., 80%]
- Framework: [Tool]
- CI integration: [Automated]

**Integration Testing:**

- Scope: [What integrations to test]
- Environment: [Where tests run]
- Frequency: [When tests run]

**Performance Testing:**

- Load testing: [Approach, tools, scenarios]
- Stress testing: [Breaking point identification]
- Soak testing: [24-hour sustained load validation]
- Benchmarking: [Baseline and targets]

**Security Testing:**

- Static analysis: [Tools]
- Dependency scanning: [Tools]
- Penetration testing: [When and by whom]
- Compliance validation: [Checklist]

**User Acceptance Testing:**

- Internal dogfooding: [Plan]
- Beta testing: [Plan]
- Feedback collection: [Process]

### Rollout Strategy

**Pre-Launch:**

- [ ] All tests passing
- [ ] Security review complete
- [ ] Documentation complete
- [ ] Runbooks validated
- [ ] Alerts configured
- [ ] Rollback plan tested

**Launch Approach:**

- Feature flags: [Which features, how controlled]
- Gradual rollout: [Percentage-based or cohort-based]
- Monitoring plan: [What to watch during rollout]

**Rollout Stages:**

1. Internal (Week 1): [Scope, users, success criteria]
2. Beta (Week 2-3): [Scope, users, success criteria]
3. 10% (Week 4): [Success criteria, go/no-go decision]
4. 50% (Week 5): [Success criteria, go/no-go decision]
5. 100% (Week 6): [Success criteria]

**Rollback Plan:**

- Trigger conditions: [When to rollback]
- Rollback procedure: [Steps]
- Data handling: [What happens to data created during rollout]
- Communication plan: [Who to notify]

### Responsible Parties

- **DRI (Directly Responsible Individual):** [Name]
- **Engineering Lead:** [Name]
- **Implementation Team:** [Team name(s)]
- **Product Owner:** [Name]
- **Technical Reviewers:** [Names]
- **Security Reviewer:** [Name]
- **Operations/SRE Contact:** [Name]

## Potential Future Work

**Post-Launch Enhancements:**

- Enhancement 1: [Description] - Timeline: [When to consider]
- Enhancement 2: [Description] - Timeline: [When to consider]

**Technical Debt to Address:**

- Debt item 1: [What needs improvement] - Effort: [Estimate]
- Debt item 2: [What needs improvement] - Effort: [Estimate]

**Research/Experimentation:**

- Experiment 1: [What to investigate] - Success metric: [How to measure]
- Experiment 2: [What to investigate] - Success metric: [How to measure]

**Scalability Improvements:**

- What optimizations to consider at 10x scale
- When to revisit architecture decisions

## Summary

**What We're Building:**

[1-2 paragraphs summarizing the entire design in simple terms]

**Why It Matters:**

[1 paragraph on business and technical value]

**Key Risks and Mitigations:**

[1 paragraph on biggest risks and how they're addressed]

**Next Steps:**

[1 paragraph on immediate actions and timeline]

## References

### Related ADRs

- ADR-XXX: [Title] - [Relevance to this design]
- ADR-YYY: [Title] - [Relevance to this design]

### Design Principles

- [Design Principle 1]: [How this design aligns]
- [Design Principle 2]: [How this design aligns]

### Supporting Documents

- Architecture and WorkStreams Document: [Link/location]
- Performance Requirements: [Link/location]
- API Specifications: [Link/location]
- Research/Spike Results: [Link/location]

### Discussion References

- Architecture Breakout Session [Date]: Lines [X-Y] - [Topic]
- Working Document: Lines [X-Y] - [Topic]
- Design Review Meeting [Date]: [Notes link]
- Slack Discussion: [Link]

### External Resources

- Industry standard: [Name and link]
- Best practices guide: [Name and link]
- Academic paper: [Citation and link]
- Third-party documentation: [Name and link]
- Competitor analysis: [Document link]

### Product Documentation

- Product Requirements Document: [Link]
- User Stories: [Link to backlog]
- User Research: [Link to findings]

---

## Revision History

| Date       | Version | Changes                     | Author |
| ---------- | ------- | --------------------------- | ------ |
| YYYY-MM-DD | 0.1     | Initial draft created       | [Name] |
| YYYY-MM-DD | 0.5     | Design review feedback      | [Name] |
| YYYY-MM-DD | 1.0     | Approved for implementation | [Name] |
| YYYY-MM-DD | 1.1     | Updated based on Phase 1    | [Name] |

When to use which:

TDD should reference relevant ADRs, and vice versa.