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🎨 Design Phase Documentation

The design phase is a critically important stage in software development, transforming approved requirements into a detailed technical implementation plan. This document serves as the official guide for the entire team, providing a single point of reference for architects, developers, testers, and stakeholders.

Key Value of the Design Phase:

  • Creates a "bridge" between business requirements and technical implementation
  • Identifies and resolves potential issues before coding begins
  • Ensures transparency and alignment in decision-making
  • Serves as the foundation for effort estimation and work planning
  • Reduces risks of misunderstandings and rework during implementation

Purpose and Objectives​

Create a technically sound, implementable, and verifiable design that fully aligns with approved requirements and is ready to be broken down into implementation tasks.

Requirements Transformation

  • Translate functional requirements into architectural components
  • Convert non-functional requirements (performance, security) into technical solutions

Conduct Targeted Research

  • Analyze technological options for critical components
  • Study best practices and design patterns
  • Assess risks and constraints of chosen solutions

Define System Structure

  • Identify key modules and their interactions
  • Design interfaces between components
  • Develop data models and their transformations

Plan for Reliability

  • Design error handling and recovery mechanisms
  • Define testing strategies across all levels
  • Ensure compliance with security requirements

Document Decisions

  • Record all architectural decisions with justification
  • Establish traceability between requirements and design elements
  • Prepare materials for handover to developers

Principles of Research Integration​

Defining the Research Scope:

  • Focus on critical decisions. Investigate only those aspects that directly impact architecture or involve high uncertainty. For example, choosing between synchronous and asynchronous payment processing requires in-depth analysis, whereas selecting a UI color scheme can rely on existing guidelines.
  • Time-boxed efforts. Set clear time limits for research activities.
  • Actionable insights. Collect and retain only information that directly influences decision-making.

Documenting Research:

  • Contextual linkage. Each finding must explicitly reference a specific requirement or problem.
  • Source references. Include direct links to documentation, articles, and code examples.
  • Integration into design. Do not store research in isolation. Embed key findings directly into relevant sections of the architectural document.
  • Decision rationale. For every decision made, specify:
    • Alternatives considered
    • Evaluation criteria
    • Reasons for selecting the current option
    • Potential trade-offs

Step-by-Step Process​

Step 1: Requirements Analysis and Research Planning​

Objective: Gain deep understanding of requirements, identify areas needing research, and clearly define scope and boundaries.

This step may be skipped if no new technologies or architectural patterns are planned for adoption.

Process:

  1. Thorough requirements review
  2. Identify research areas
  3. Plan research activities. For each area, define the research objective and success criteria
  4. Establish research boundaries

What to Document:

  • Project context and alignment with business goals
  • List of research topics with justification for prioritization
  • Expected outcomes and their architectural impact
  • Research boundaries and completion criteria

Step 2: Conducting Research and Building Context​

Objective: Gather sufficient information to make informed architectural decisions while avoiding excessive analysis.

This step may be skipped if no new technologies or architectural patterns are planned for adoption.

Process:

  1. Information gathering

    • Review official documentation, technical blogs, and case studies
    • Conduct experimental tests (proof-of-concept) for critical features
    • Consult internal or external experts

  2. Option evaluation

    • For each option, assess:
      • Technical characteristics
      • Required effort
      • Potential risks
      • Alignment with non-functional requirements

  3. Document research outcomes

    • Create a concise summary focused on decisions
    • Provide source references for verification
    • Note uncertainties and need for further research

  4. Make preliminary decisions

    • Formulate recommendations based on research
    • Specify rationale and potential trade-offs

What to Document:

  • Key findings linked to specific requirements
  • Comparison of alternatives with criteria-based evaluation
  • Justification for selected technologies and patterns
  • Source references and verification materials
  • Uncertainties and recommendations for resolution

Step 3: Creating System Architecture​

Objective: Define a high-level solution structure that fully satisfies requirements and is ready for detailed elaboration.

Architecture Components:

  1. System Overview

    • Component diagram (C4 model recommended)
    • Brief description of primary data flows
    • Integration points with existing infrastructure

  2. Component Architecture

    • List of core modules with purpose descriptions
    • Responsibility boundaries for each component
    • Component interactions (synchronous/asynchronous)

  3. Data Flow

    • Description of key entity lifecycles
    • Data storage locations at each stage
    • Data transformations between components

  4. Integration Points

    • External systems and APIs with version specifications
    • Communication protocols and data formats
    • Strategies for handling external system unavailability

  5. Technology Stack

    • Explicit justification for each technology choice
    • Versions of tools used
    • Migration plan for legacy components

What to Document:

  • Architecture diagram with explanations
  • Justification for chosen architectural style (microservices, monolith, etc.)
  • How the architecture satisfies functional and non-functional requirements
  • Risks of architectural decisions and mitigation strategies

Important: Describe only components necessary to fulfill current requirements. Avoid designing "for the future" without explicit requirements.

Step 4: Defining Components and Interfaces​

Objective: Detail the internal system structure and component interaction mechanisms to ensure readiness for implementation.

Component Design Elements:

  1. Component responsibilities. Clear description of what each component does
  2. Interface definitions
  3. Dependency relationships
  4. Configuration and setup

What to Document:

  • Separate subsection for each component with complete description
  • Example requests and responses for all interfaces
  • Sequence diagrams for key scenarios
  • Component-level error handling rules

Step 5: Data Model Design​

Objective: Define data structures and processing rules that ensure integrity and compliance with business rules.

Data Model Elements:

  1. Entity definitions: data and responsibilities
  2. Entity relationships
  3. Validation rules and business logic for entities
  4. Storage strategies

What to Document:

  • ERD diagram with explanations
  • Complete field descriptions for each entity
  • Example data for key scenarios
  • Data migration strategies when schema changes occur

Step 6: Planning Error Handling and Edge Cases​

Objective: Ensure system reliability during failures by defining clear strategies for handling all possible scenarios.

Error Handling Design:

  1. Categorize errors. System errors, data validation errors, etc.
  2. Error response strategies
  3. User experience in error scenarios
  4. Recovery mechanisms
  5. Monitoring mechanisms

What to Document:

  • Error matrix for each key operation with handling strategies
  • Error handling examples for critical scenarios
  • Log and metric formats for error tracking
  • Incident response procedures for critical failures

Step 7: Defining Testing Strategy​

Objective: Ensure implementation quality through a well-thought-out testing strategy covering all system aspects.

Testing Strategy Elements:

  1. Define testing levels
  2. Test coverage: coverage criteria and priorities
  3. Test scenarios
  4. Testing tools
  5. Quality checkpoints

What to Document:

  • Required test level and type for each component
  • Quality metrics and target values
  • Example test scenarios for key features
  • Integration of testing strategy with development process

Step 8: Final Design Quality Review​

Objective: Ensure the design is complete, understandable, implementation-ready, and meets all quality criteria.

Quality Checklist:

CategoryVerification CriterionVerification Method
CompletenessAll requirements addressed in designRequirements-to-design mapping
Core system components definedDiagram and description review
Data models cover all required entitiesERD and description analysis
Error handling covers expected failure modesError matrix verification
ClarityDesign decisions clearly explainedDocument review by new developer
Component responsibilities well-definedComponent description review
Component interfaces specifiedAPI contract analysis
Technical choices include justificationResearch section verification
FeasibilityDesign technically achievable with chosen technologiesExpert consultation
Performance requirements can be metOptimization strategy analysis
Security requirements addressedSecurity measure verification
Implementation complexity aligns with project estimatesDeveloper assessment
TraceabilityDesign elements linked to specific requirementsTraceability matrix
All requirements covered by design componentsCompleteness verification
Design decisions support requirement fulfillmentCompliance analysis
Testing strategy validates requirement satisfactionTest scenario verification

Common Design Mistakes​

Mistake 1: Over-Engineering​

Problem: Designing for requirements that don't exist or adding functionality "for the future."

Symptoms:

  • Design includes components with no direct link to current requirements
  • Complex abstractions for scenarios that may never materialize
  • Extended design timeline without clear benefit

Solution:

  • Focus strictly on current requirements
  • Apply the YAGNI principle (You Aren't Gonna Need It)
  • Design should be extensible but not implement unused features
  • Regularly ask: "Which specific requirement justifies this component?"

Mistake 2: Poorly Specified Interfaces​

Problem: Vague component boundaries and interactions leading to implementation misunderstandings.

Symptoms:

  • Unclear component responsibilities
  • Missing clear API contracts
  • Numerous clarification questions during implementation

Solution:

  • Clearly define inputs, outputs, and errors for each interface
  • Use formal specifications (OpenAPI, Protobuf)
  • Include example requests and responses
  • Conduct interface reviews with developers before implementation

Mistake 3: Ignoring Non-Functional Requirements​

Problem: Focusing only on functional behavior while neglecting performance, security, and other non-functional aspects.

Symptoms:

  • No mention of response time, load capacity, or security
  • Missing scaling or failover strategies
  • Undefined quality metrics

Solution:

  • Explicitly document all non-functional requirements in a dedicated section
  • Specify measurable metrics for each (e.g., "Response time < 500ms at 1000 RPS")
  • Include design elements that ensure NFR compliance
  • Verify NFR compliance during final review

Mistake 4: Technology-Driven Design​

Problem: Selecting technologies before fully understanding requirements, leading to suboptimal solutions.

Symptoms:

  • Technologies mentioned before requirements are defined
  • Technology comparisons without specific task alignment
  • Unnecessary complexity from using "trendy" technologies

Solution:

  • Let requirements drive technology choices, not vice versa
  • For each technology, specify the exact requirement it satisfies
  • Consider simple solutions before complex ones
  • Use a technology evaluation matrix with requirement-based criteria

Mistake 5: Inadequate Error Handling Design​

Problem: Designing only for the "happy path" while ignoring failure scenarios.

Symptoms:

  • Missing error handling section
  • No recovery strategies for failures
  • Undefined user error messages

Solution:

  • Explicitly design error handling alongside main workflows
  • Define possible failure scenarios for each operation
  • Include retry mechanisms, fallback strategies, and monitoring in design
  • Ensure user experience is considered for all scenarios

Resolving Design Issues​

Issue: Design Becomes Overly Complex​

Symptoms:

  • Design document exceeds 2500 lines without clear necessity
  • Too many components and interactions
  • Difficulty explaining architecture to new team members

Solution:

  • Return to requirements and remove elements without direct linkage
  • Consider phased implementation (MVP + subsequent iterations)
  • Refactor architecture by consolidating related components

Issue: Requirements Don't Map to Design​

Symptoms:

  • Difficulty tracing requirements to design elements
  • Some requirements missing from design
  • No clear connection between business goals and technical decisions

Solution:

  • Create a requirements-to-design traceability matrix
  • Conduct step-by-step verification of each requirement
  • Add requirement references to relevant design sections

Issue: Technology Choices Are Unclear​

Symptoms:

  • Multiple viable options without clear selection criteria
  • Missing justification for chosen technologies

Solution:

  • Define decision criteria based on requirements and constraints
  • Create a comparison matrix with key criteria evaluation
  • Conduct proof-of-concept for critical decisions
  • Document selection rationale including trade-offs

Issue: Design Lacks Implementation Details​

Symptoms:

  • Many questions during implementation
  • Ambiguity in interfaces and contracts

Solution:

  • Add specific example requests and responses
  • Clarify data formats and error codes
  • Include sequence diagrams for key scenarios

Conclusion​

A design document is not merely a formal artifact but a living tool that ensures successful project implementation. High-quality design:

  • Reduces errors and rework
  • Accelerates development through clear guidance
  • Simplifies knowledge transfer among team members
  • Serves as the foundation for implementation quality assessment

Key Principles of Effective Design:

  1. Minimal sufficiency β€” design exactly what's needed for implementation
  2. Practical orientation β€” focus on solutions ready for implementation
  3. Decision transparency β€” every decision must have clear justification
  4. Living document β€” regularly update design as new information emerges

Document Template​

# Design "[Brief Feature Name]"

[Brief description of business objectives, alignment with corporate strategy, key stakeholders]

[Clear system boundaries definition: what's included/excluded in the solution]

---

## System Architecture

[General description of feature workflow, component listing, their relationships, and data flows between them]

---

## Components and Interfaces

**For each key component, create a subsection:**

### [Component Name]

[What the component does]

[Relationships with other components]

[Link to source requirements]

**Non-Functional Requirements**

[Non-functional requirements for the component]

- Performance: [Metrics + strategies]  
- Security: [Protection mechanisms]  
- Reliability: [Failover strategies]

**Error Handling Strategy**

[Error handling strategy]

**Testing Strategy**

* [Test case]
   * Test case description
* [Another test case]
   * Test case description

---

## System Entities

### [Entity Name]

[Entity description]

[Link to source requirements]

**Entity Methods**

* [Entity method]
   * Method description and behavior
* [Another entity method]
   * Method description and behavior

**Entity Data**

* [Entity field]
   * Field description and behavior
* [Another entity field]
   * Field description and behavior

**Testing Strategy**

* [Test case]
   * Test case description
* [Another test case]
   * Test case description

---

## Requirements Quality Checklist

*(Completed after document finalization)*

| Criterion                                                             | Completed | Comment |
| -------------------------------------------------------------------- | --------- | ------- |
| All requirements have unambiguous representation in design            | ☐        |         |
| Non-functional requirements translated into measurable technical solutions | ☐        |         |
| Error handling designed for all key scenarios                         | ☐        |         |
| Data model covers all business entities and rules                     | ☐        |         |
| Testing strategy defined with levels and quality metrics              | ☐        |         |
| Design follows minimal sufficiency principle (YAGNI)                  | ☐        |         |
| Traceability system exists from requirements β†’ design elements        | ☐        |         |