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Domain Model - Audit Trail Platform (ATP)

Domain-Driven Design at the core — ATP's domain model captures business logic, enforces invariants, and provides the foundation for tamper-evident, compliant audit trail management.

📋 Documentation Generation Plan

This document will be generated in 4 cycles. Current progress:

Cycle Topics Estimated Lines Status
Cycle 1 Domain Layer Overview & DDD Introduction (1-2) ~2,500 ⏳ Not Started
Cycle 2 Domain Model Structure & Navigation (3-4) ~2,500 ⏳ Not Started
Cycle 3 Reader Guides & Integration Patterns (5-6) ~2,000 ⏳ Not Started
Cycle 4 Domain Evolution & Best Practices (7-8) ~2,000 ⏳ Not Started

Total Estimated Lines: ~9,000

Purpose & Scope

This document serves as the navigation hub and conceptual overview for the Audit Trail Platform (ATP) domain model, introducing Domain-Driven Design (DDD) principles, explaining the domain layer structure, and providing guided paths to detailed domain documentation based on reader role and learning objectives.

What this document covers

  • Introduce ATP's domain-driven approach: Strategic and tactical DDD patterns, bounded contexts, ubiquitous language
  • Explain the domain layer architecture: Aggregates, entities, value objects, domain events, domain services
  • Provide navigation guides: Links to all domain documentation with reading paths per role
  • Define bounded contexts: Seven contexts in ATP with responsibilities and integration points
  • Describe domain model principles: Immutability, event sourcing, multi-tenancy, compliance by design
  • Outline integration contracts: REST APIs, message schemas, webhooks, and shared contracts
  • Document domain evolution: How the model changes over time, versioning, backward compatibility
  • Specify best practices: Domain modeling, testing, documentation, and code organization

Out of scope (covered in linked documents)

Readers & ownership

  • All Team Members (readers): Entry point to understand ATP domain model
  • Domain Experts/Product (owners): Domain model design and evolution
  • Architects: Bounded context design, integration patterns
  • Backend Developers: Domain implementation guidance
  • New Team Members: Onboarding to ATP domain concepts

Artifacts produced

  • Domain Model Overview: High-level introduction to ATP's domain
  • Navigation Guide: Reader-specific paths through domain documentation
  • Bounded Context Map: Visual and textual context relationships
  • Integration Matrix: Contracts between contexts
  • Domain Glossary Index: Quick reference to all domain terms
  • Reading Paths: Recommended documentation sequences per role
  • Evolution Guide: How domain model changes over time
  • Best Practices Catalog: Domain modeling guidelines

Acceptance (done when)

  • Overview section introduces ATP domain model with DDD principles
  • Navigation guides provide clear paths for all reader types
  • Bounded context map visualizes all contexts and relationships
  • Links to detailed docs are comprehensive and well-organized
  • Quick reference sections enable fast lookups of terms and concepts
  • Reader feedback indicates index is helpful for onboarding and navigation
  • Documentation complete with diagrams, examples, and cross-references

Detailed Cycle Plan

CYCLE 1: Domain Layer Overview & DDD Introduction (~2,500 lines)

Topic 1: ATP Domain Layer Introduction

What will be covered:

  • What is the Domain Layer?
  • Core business logic and rules
  • Independent of infrastructure concerns
  • Heart of the application (DDD layered architecture)

  • Technology-agnostic (no database, framework dependencies)

  • Why Domain-Driven Design for ATP?

  • Complex business domain (audit, compliance, classification, retention)
  • Multiple stakeholders with different mental models
  • Need for precise terminology (legal/regulatory)
  • Evolving requirements (new regulations, new use cases)

  • DDD provides structure for managing complexity

  • ATP Domain Characteristics

  • Immutable by design: Audit events never change after creation
  • Event-driven core: State changes captured as domain events
  • Multi-tenant from the start: Tenant context in every operation
  • Compliance-aware: GDPR, HIPAA, SOC 2 encoded in domain rules
  • Tamper-evident: Hash chains and signatures in domain model

  • Append-only semantics: No updates or deletes, only new events

  • Domain Layer Architecture

  • Aggregates: Consistency boundaries (AuditEvent, EventStream, Tenant)
  • Entities: Objects with identity (Classification, Retention)
  • Value Objects: Immutable values (EventId, Hash, Timestamp)
  • Domain Events: Business state changes (EventReceived, StreamSealed)
  • Domain Services: Cross-aggregate operations (Integrity Service)
  • Repositories: Persistence abstraction
  • Factories: Complex object creation

  • Specifications: Business rule queries

  • Layered Architecture in ATP

  • Presentation Layer: APIs, gRPC services, webhooks
  • Application Layer: Use cases, command handlers, query handlers
  • Domain Layer: Business logic, aggregates, domain events (THIS LAYER)

  • Infrastructure Layer: Database, messaging, caching, external services

  • Domain Independence

  • Domain layer has no dependencies on other layers
  • Domain models don't reference EF Core, Service Bus, etc.
  • Pure C# POCOs (Plain Old CLR Objects)
  • Testable without infrastructure

Code Examples: - Domain layer namespace structure - Aggregate base class (no infrastructure dependencies) - Domain event base class - Value object base class - Repository interface (in domain, implementation in infrastructure)

Diagrams: - DDD layered architecture - ATP domain layer components - Dependency direction (all point inward to domain) - Domain model purity

Deliverables: - Domain layer introduction - DDD rationale for ATP - Architecture overview - Layer dependency rules

Topic 2: Domain-Driven Design Primer

What will be covered:

  • Strategic DDD
  • Bounded Contexts: Explicit boundaries around models
  • Context Mapping: Relationships between contexts
  • Ubiquitous Language: Shared vocabulary
  • Subdomains: Core, supporting, generic

  • Anti-Corruption Layers: Protecting domain model

  • Tactical DDD

  • Aggregates: Consistency boundaries, transactional scope
  • Entities: Identity and lifecycle
  • Value Objects: Immutability and equality
  • Domain Events: State changes as events
  • Repositories: Aggregate persistence
  • Domain Services: Cross-aggregate logic
  • Factories: Complex creation logic

  • Specifications: Encapsulated business rules

  • DDD Patterns in ATP

  • Event Sourcing: Events as source of truth
  • CQRS: Separate write and read models
  • Saga Pattern: Distributed transactions
  • Outbox/Inbox: Reliable messaging

  • Aggregate Design: Consistency and performance

  • ATP's DDD Implementation

  • Strategic: 7 bounded contexts (Ingestion, Storage, Query, etc.)
  • Tactical: 8 core aggregates, 20+ value objects, 50+ domain events
  • Event sourcing for AuditEvent aggregate
  • CQRS for query optimization
  • Multi-tenancy as first-class concept

Code Examples: - DDD pattern examples from ATP - Strategic vs tactical code examples - Event sourcing implementation - CQRS command and query handlers

Diagrams: - Strategic DDD concepts - Tactical DDD building blocks - ATP's DDD implementation - Bounded context map

Deliverables: - DDD primer for ATP - Pattern selection guide - Implementation examples - Best practices

CYCLE 2: Domain Model Structure & Navigation (~2,500 lines)

Topic 3: ATP Bounded Contexts Overview

What will be covered:

  • What are Bounded Contexts?
  • Explicit boundaries around models
  • Each context has its own ubiquitous language
  • Context isolation and autonomy

  • Integration contracts between contexts

  • ATP's Seven Bounded Contexts

1. Ingestion Context - Responsibility: Receive and validate audit events from producers - Core Concepts: Ingestion, Validation, Batch Processing, Rate Limiting - Aggregates: BatchIngestion - Integration: REST/gRPC APIs, SDK clients - Owned By: Ingestion Service

2. Storage Context - Responsibility: Append-only event store with hash chains - Core Concepts: Event Store, Event Stream, Immutability, Seal - Aggregates: EventStream, IntegrityProof - Integration: Domain events from Ingestion - Owned By: Storage Service

3. Classification Context - Responsibility: Auto-classify events, detect PII, apply redaction - Core Concepts: Classification, Sensitivity, PII Detection, Redaction - Aggregates: ClassificationRule, PIIPattern - Integration: Events from Storage, policies from Policy context - Owned By: Classification Service

4. Query Context - Responsibility: Queryable read models, search, filtering - Core Concepts: Read Model, Projection, Index, Search - Aggregates: QueryView, SearchIndex - Integration: Projections from all events - Owned By: Query Service

5. Policy Context - Responsibility: Tenant policies for classification, retention, access - Core Concepts: Policy, Rule, Compliance Configuration - Aggregates: Policy, RetentionPolicy, ClassificationPolicy - Integration: Policies applied by all contexts - Owned By: Policy Service

6. Export Context - Responsibility: eDiscovery, legal hold, SIEM integration - Core Concepts: Export, eDiscovery, Legal Hold, SIEM - Aggregates: ExportRequest, LegalHold - Integration: Queries from Query context, storage from Storage - Owned By: Export Service

7. Integrity Context - Responsibility: Cryptographic verification, tamper detection - Core Concepts: Hash Chain, Signature, Verification, Merkle Tree - Aggregates: IntegrityProof, VerificationJob - Integration: Hashes from Storage, verification requests - Owned By: Integrity Service

  • Context Relationships
  • Upstream/downstream dependencies
  • Shared kernel (common value objects)
  • Published language (integration events)
  • Anti-corruption layers at boundaries

Code Examples: - Bounded context namespace structure - Context integration patterns - Shared kernel value objects - Anti-corruption layer example

Diagrams: - Bounded context map with all 7 contexts - Context relationships (upstream/downstream) - Integration patterns between contexts - Shared kernel visualization

Deliverables: - Bounded context specifications - Context map - Integration matrix - Shared kernel documentation

Topic 4: Domain Documentation Navigation

What will be covered:

  • Domain Documentation Structure 

    domain/
├── index.md (THIS FILE)
├── ubiquitous-language.md
├── aggregates-entities.md
├── events-catalog.md
└── contracts/
    ├── index.md
    ├── rest-apis.md
    ├── message-schemas.md
    ├── webhooks.md
    └── shared/
        └── idempotency.md

  • Document Relationships

  • index.md: Overview and navigation hub
  • ubiquitous-language.md: Shared vocabulary (read first!)
  • aggregates-entities.md: Domain model details
  • events-catalog.md: All events with schemas

  • contracts/: Integration contracts (APIs, messages, webhooks)

  • Reading Paths by Role

New Developers: 1. Start here (domain/index.md) 2. Read ubiquitous-language.md (learn the vocabulary) 3. Read aggregates-entities.md (understand core model) 4. Read events-catalog.md (understand event flows) 5. Read contracts/ (understand integration)

Domain Experts: 1. Start with ubiquitous-language.md (validate terminology) 2. Review aggregates-entities.md (validate business rules) 3. Review events-catalog.md (validate event semantics)

Architects: 1. Read index.md (bounded contexts overview) 2. Read aggregates-entities.md (consistency boundaries) 3. Read contracts/ (integration patterns) 4. Review ../architecture/ (system design)

Integration Engineers: 1. Start with contracts/index.md 2. Read rest-apis.md (HTTP endpoints) 3. Read message-schemas.md (async messaging) 4. Read webhooks.md (callbacks) 5. Reference ubiquitous-language.md (terminology)

QA/Test Engineers: 1. Read ubiquitous-language.md (test terminology) 2. Read aggregates-entities.md (invariants to test) 3. Read events-catalog.md (event contracts to validate)

Document Status Lines Topics Last Updated
ubiquitous-language.md 📝 Planned ~15,500 12 TBD
aggregates-entities.md 📝 Planned ~23,000 16 TBD
events-catalog.md 📝 Planned ~28,000 20 TBD
contracts/index.md 📝 Planned ~8,000 8 TBD
contracts/rest-apis.md 📝 Planned ~12,000 10 TBD
contracts/message-schemas.md 📝 Planned ~10,000 8 TBD
contracts/webhooks.md 📝 Planned ~6,000 6 TBD

Code Examples: - Namespace organization - Import structure across domain docs - Cross-reference examples

Diagrams: - Domain documentation structure - Reading paths by role - Document relationships

Deliverables: - Navigation guide - Reading paths per role - Quick reference index - Completeness tracking

CYCLE 3: Reader Guides & Integration Patterns (~2,000 lines)

Topic 5: Domain Model Learning Paths

What will be covered:

  • Learning Path 1: Understanding ATP Domain (Beginners)
  • Week 1: Ubiquitous language + Core concepts
  • Week 2: Aggregates and entities
  • Week 3: Events and event flows
  • Week 4: Integration contracts
  • Hands-on exercises

  • Validation checkpoints

  • Learning Path 2: Implementing Aggregates (Developers)

  • Aggregate design patterns
  • Entity implementation
  • Domain event publishing
  • Repository pattern
  • Unit testing aggregates

  • Code examples and exercises

  • Learning Path 3: Event-Driven Architecture (Architects)

  • Event sourcing patterns
  • CQRS implementation
  • Bounded context integration
  • Eventual consistency

  • Saga patterns

  • Learning Path 4: Integration (Integration Engineers)

  • REST API contracts
  • Message schemas
  • Webhooks configuration
  • SDK usage

  • Contract testing

  • Learning Path 5: Compliance (Compliance Officers)

  • GDPR compliance in domain model
  • HIPAA requirements
  • SOC 2 controls
  • Retention and deletion
  • Audit trail generation

Code Examples: - Sample exercises per learning path - Hands-on tutorials - Test-driven examples

Diagrams: - Learning path flowcharts - Knowledge progression - Skill-building roadmap

Deliverables: - Learning path guides - Exercises and tutorials - Assessment checkpoints - Certificate/badge criteria

Topic 6: Domain Integration Patterns

What will be covered:

  • Integration Between Bounded Contexts
  • Context map patterns (shared kernel, customer/supplier, etc.)
  • Integration via domain events (choreography)
  • Integration via commands (orchestration)

  • Anti-corruption layers

  • REST API Integration

  • Command APIs (POST /audit-events)
  • Query APIs (GET /audit-events)
  • API versioning strategy

  • Error handling and validation

  • Message-Based Integration

  • Integration events (TenantOnboarded, ExportCompleted)
  • Message routing
  • Schema evolution

  • Idempotency and deduplication

  • Webhook Integration

  • Outbound webhooks (event notifications)
  • Webhook security (signing, verification)

  • Retry and failure handling

  • SDK Integration

  • C# SDK for producers
  • JavaScript SDK for frontend

  • SDK versioning and compatibility

  • Shared Contracts

  • Idempotency contracts
  • Correlation ID propagation
  • Tenant context propagation
  • Error response formats

Code Examples: - Integration patterns code - Anti-corruption layer - SDK usage examples - Contract validation

Diagrams: - Integration architecture - Contract relationships - SDK integration flow - Anti-corruption layer

Deliverables: - Integration patterns guide - Contract specifications - SDK documentation - Anti-corruption layer examples

CYCLE 4: Domain Evolution & Best Practices (~2,000 lines)

Topic 7: Domain Model Evolution

What will be covered:

  • Why Domain Models Evolve
  • New business requirements
  • Regulatory changes (new GDPR requirements)
  • Performance optimizations
  • Bug fixes and clarifications

  • Refactoring for clarity

  • Evolution Strategies

  • Additive Changes: New aggregates, events, properties (safe)
  • Modifications: Changing existing behavior (careful)
  • Deprecations: Phasing out old patterns (planned)

  • Breaking Changes: Incompatible changes (avoided when possible)

  • Versioning Domain Model

  • Aggregate version numbers
  • Event schema versioning
  • API versioning strategy

  • Backward compatibility rules

  • Refactoring Domain Model

  • Identifying refactoring needs
  • Planning refactoring safely
  • Migration strategies

  • Testing during refactoring

  • Managing Breaking Changes

  • Impact analysis
  • Communication plan
  • Migration guide for consumers
  • Dual-running old and new versions

  • Sunset old version

  • Domain Model Governance

  • Who can change the domain model?
  • Approval process for changes
  • Architecture Decision Records (ADRs)

  • Documenting rationale

  • Evolution Documentation

  • Changelog maintenance
  • Migration guides
  • Deprecation notices
  • Version compatibility matrix

Code Examples: - Versioned aggregates - Migration scripts - Backward compatibility patterns - Deprecation annotations

Diagrams: - Evolution timeline - Breaking change process - Migration workflow - Version compatibility

Deliverables: - Evolution guide - Change management process - Migration templates - Compatibility matrix

Topic 8: Domain Best Practices

What will be covered:

  • Aggregate Design Best Practices
  • Keep aggregates small (single responsibility)
  • One aggregate per transaction
  • Reference other aggregates by ID only
  • Enforce invariants in aggregate

  • Use value objects for complex concepts

  • Entity and Value Object Best Practices

  • Entities for objects with identity and lifecycle
  • Value objects for attributes and measurements
  • Value objects are immutable

  • Equality by identity (entities) vs value (value objects)

  • Domain Event Best Practices

  • Events are past-tense (what happened)
  • Events are immutable
  • Events contain all necessary data
  • Events are published after persistence

  • Event versioning from the start

  • Repository Best Practices

  • One repository per aggregate
  • Repository returns full aggregate
  • No lazy loading across aggregates

  • Repository interface in domain layer

  • Domain Service Best Practices

  • Use when operation doesn't belong to an aggregate
  • Stateless services

  • Depend on repositories, not infrastructure

  • Testing Best Practices

  • Unit test aggregates in isolation
  • Test invariants thoroughly
  • Test domain events published correctly
  • Use test data builders

  • Property-based testing for value objects

  • Code Organization Best Practices

  • Namespace per bounded context
  • Folder structure: Aggregates/, Events/, ValueObjects/, Services/
  • File per aggregate

  • Keep domain layer pure (no infrastructure references)

  • Documentation Best Practices

  • Document business rules in code (comments)
  • Document invariants explicitly
  • Keep ubiquitous language updated
  • Example code in documentation

  • Architecture Decision Records (ADRs)

  • Common Anti-Patterns to Avoid

  • Anemic domain model (no behavior)
  • Transaction spanning multiple aggregates
  • Domain logic in application layer
  • Infrastructure dependencies in domain
  • Mutable value objects
  • Lazy loading across aggregates
  • Technical terms in ubiquitous language

Code Examples: - Good vs bad aggregate design - Good vs bad entity/value object usage - Good vs bad domain service design - Anti-pattern examples with fixes

Diagrams: - Code organization structure - Testing strategy - Good vs bad patterns comparison

Deliverables: - Best practices catalog - Anti-patterns guide - Code organization templates - Testing framework

Domain Model Quick Reference

Core Aggregates (8): - AuditEvent, EventStream, Tenant, Policy, ClassificationRule, RetentionPolicy, ExportRequest, IntegrityProof

Bounded Contexts (7): - Ingestion, Storage, Classification, Query, Policy, Export, Integrity

Domain Events (50+): - Ingestion: AuditEventReceived, BatchIngested - Storage: EventStreamCreated, EventStreamSealed - Classification: EventClassified, PIIDetected - Retention: RetentionApplied, EventTombstoned - Policy: PolicyCreated, PolicyUpdated - Tenant: TenantOnboarded, TenantSuspended - Export: ExportRequested, ExportCompleted - Integrity: HashChainValidated, SignatureVerified

Value Objects (20+): - Identity: TenantId, EventId, StreamId - Time: Timestamp, TimeRange, RetentionPeriod - Classification: Classification, Sensitivity - Security: Hash, Signature - Actor/Resource: Actor, Resource, Action

Integration Contracts: - REST APIs: Ingestion, Query, Policy, Export endpoints - Message Schemas: Domain events, integration events - Webhooks: Event notifications, status updates

Understanding the Domain

→ Ubiquitous Language - Start here to learn ATP terminology
→ Aggregates & Entities - Core domain model details
→ Events Catalog - All events with schemas

Integration Contracts

→ Contracts Overview - Integration hub
→ REST APIs - HTTP endpoints
→ Message Schemas - Async messaging
→ Webhooks - Event callbacks
→ Idempotency - Duplicate handling

→ Architecture Overview - System design
→ Components - Microservices
→ Data Model - Persistence
→ Sequence Flows - Behavior

Implementation Guidance

→ Messaging - Event bus
→ Persistence - Data access
→ Outbox/Inbox - Reliable messaging

Summary & Implementation Plan

Implementation Phases

Phase 1: Introduction (Cycle 1) - 1 week - Domain layer overview and DDD introduction

Phase 2: Structure (Cycle 2) - 1 week
- Bounded contexts and navigation

Phase 3: Learning (Cycle 3) - 1 week - Reader guides and integration patterns

Phase 4: Evolution (Cycle 4) - 1 week - Domain evolution and best practices

Success Metrics

  • Navigation Clarity: Users can find relevant docs in < 30 seconds
  • Onboarding Speed: New developers understand domain in < 1 week
  • Cross-References: 100% of terms link to definitions
  • Reader Satisfaction: 90%+ find index helpful
  • Documentation Coverage: All domain topics have plans

Ownership & Maintenance

  • Domain Experts: Topic 1, 2, 7
  • Architects: Topic 2, 3, 6
  • Technical Writers: Topic 4, 5, 8
  • All Team Members: Feedback and continuous improvement

Document Status: ✅ Plan Approved - Ready for Content Generation

Target Start Date: Q2 2025

Expected Completion: Q2 2025 (1 month)

Owner: Domain Modeling Team

Last Updated: 2024-10-30