bookforge-domain-logic-pattern-selector（bookforge 域逻辑模式选择器）

Domain Logic Pattern Selector

Choose between Transaction Script, Domain Model, and Table Module for a subsystem, then decide how thick to make the Service Layer. Based on Fowler's Patterns of Enterprise Application Architecture (Ch 2 + Ch 9).

When to Use

Use this skill when starting or refactoring a subsystem and the central question is: _how should business logic be organized?_ This is distinct from _where_ that logic persists (data-source patterns) or _how_ it is presented.

Typical triggers: greenfield module design, Transaction Script code growing unmanageable, a team debate about "should we use DDD?", evaluating whether to add a service layer, diagnosing anemic domain model or script sprawl in legacy code.

Prerequisites: none. Works from a verbal description of the subsystem; codebase access improves diagnosis.

Context & Input Gathering

Ask the user (or read from codebase) in this order:

Required:

1. What does the subsystem do? One paragraph — the nouns (entities), the operations (use cases), and the outputs.
2. How complex is the business logic? Pick the best match:

- Simple: mostly CRUD, a few validations, no conditional algorithms - Moderate: validations + multi-step calculations + some conditional rules - Complex: many rules that interact, vary by product/customer/time, change frequently

1. What is the team's OO experience? Low (procedural background), moderate (OO-familiar but not DDD-trained), high (experienced with rich domain models, strategy/composite patterns).
2. What is the language and stack? Java/JVM, C#/.NET, Python, TypeScript, Ruby, etc. If .NET, is ADO.NET / RecordSet usage widespread?

Observable from codebase (if provided):

• Service classes with many long methods containing if/else chains → Transaction Script
• Classes with fields but no methods (getters/setters only) → anemic domain model signal
• Behavior scattered across utility/helper classes → Transaction Script sprawl
• SQL or stored-procedure calls embedded directly in business-logic code → stored-procedure leakage
• Dataset/DataTable types flowing through business logic (.NET) → Table Module context

Defaults if not specified:

• Team OO experience: moderate
• Stack: JVM (Java/Kotlin) unless .NET signals are present

Sufficiency test: Proceed once complexity level and team experience are known. Stack is tie-breaker for moderate complexity.

Process

Step 1 — Classify Domain Complexity

WHY: Fowler's core insight is that the cost curves for the three patterns cross at different complexity points. Choosing the wrong pattern for the complexity level locks the team into a wall of effort as logic grows.

Map the subsystem to one of three complexity bands:

If the codebase is available: grep for service methods longer than ~50 lines with nested conditionals. That is a high-complexity signal even if the requirement description sounds simple.

Step 2 — Apply the Routing Matrix

WHY: Complexity alone does not determine the best pattern. Team OO experience shifts the cost curve for Domain Model (familiar teams pay the ramp-up cost once; unfamiliar teams pay it every sprint). Platform shifts the appeal of Table Module (RecordSet tooling makes it a natural fit in .NET; without that tooling it is pointless friction).

ROUTING MATRIX
─────────────────────────────────────────────────────────────────────
Complexity   Team OO Exp    Platform         → Recommended Pattern
─────────────────────────────────────────────────────────────────────
Low          any            any              → Transaction Script
Moderate     low            any              → Transaction Script (with extraction discipline)
Moderate     moderate/high  .NET + RecordSet → Table Module
Moderate     moderate/high  Java/Python/TS   → Domain Model (simple variant)
High         low            any              → Domain Model + coaching (or Table Module in .NET as interim)
High         moderate/high  any              → Domain Model
─────────────────────────────────────────────────────────────────────

Platform defaults (Fowler's explicit guidance):

• .NET with ADO.NET / DataSet: Table Module is the natural fit. Fowler says "I don't see a reason to use Transaction Script in a .NET environment" once RecordSet tooling is present.
• Java / JVM: Domain Model is the natural target for moderate-to-high complexity. POJOs (plain old Java objects) with Data Mapper is the recommended implementation — avoid entity beans for rich domain models.
• Python / Ruby / TypeScript: Domain Model with Active Record is viable for simple-to-moderate domains; Data Mapper for rich ones.

Refactoring direction rule: If starting fresh or refactoring up — Transaction Script → Domain Model is a well-worn path. Going the other direction (Domain Model → Transaction Script) is rarely worthwhile unless you can also simplify the data-source layer. Start simple; refactor up when complexity demands.

Mixed patterns are allowed. Transaction Script for some use cases, Domain Model for the core rules — Fowler explicitly calls this common.

Step 3 — Decide Service Layer

WHY: A Service Layer defines the application boundary — where transaction control and security live, and what coarse-grained API client layers (UI, APIs, messaging consumers) call into. Adding it unnecessarily layers overhead; omitting it when multiple client types exist causes cross-cutting logic to scatter.

Add a Service Layer when ANY of these are true:

• More than one kind of client (UI + REST API, UI + message consumer, UI + batch job)
• A use case response must be transacted atomically across multiple resources (database + message queue + email)
• Need a distinct, stable API boundary (versioning, remote access, access control)

Skip the Service Layer when:

• Single client type (a UI), simple use cases, single transactional resource
• Page Controllers can control transactions directly by delegating to the domain or data-source layer

Thickness — three variants:

Avoid stored procedures for domain logic. Reserve stored procedures for batch jobs and extreme performance requirements only. Placing business rules in stored procedures creates logic you cannot test, version, or refactor as a domain model.

Step 4 — Check Anti-Pattern Risk

WHY: Pattern choices create pressure toward specific failure modes. Flagging the risk up front lets the team establish guards before the anti-pattern takes hold.

Anemic Domain Model is Fowler's most prominent warning: a system that uses Domain Model classes but has stripped behavior out into service classes reduces the pattern to an expensive object-relational mapping exercise with none of the encapsulation benefit. If your "domain model" has no methods beyond getters and setters, it is a data transfer layer, not a domain model.

Step 5 — Produce the Decision Record

WHY: A written record anchors the decision, gives future maintainers the context to maintain pattern consistency, and becomes a checklist for code review.

Produce a Domain-Logic Pattern Decision Record containing:

## Domain-Logic Pattern Decision Record: [Subsystem Name]

### Classification
- Complexity band: [low / moderate / high]
- Team OO experience: [low / moderate / high]
- Stack / platform: [Java / .NET / Python / ...]

### Primary Pattern: [Transaction Script | Domain Model | Table Module]
**Rationale:** [1-2 sentences applying the routing matrix]

### Service Layer: [None | Domain Facade | Operation Script | Controller-Entity]
**Rationale:** [Why needed / not needed; thickness reasoning]

### Anti-Pattern Watch
- Risk: [name]  Warning sign: [observable symptom]

### Migration Path (if refactoring)
[Starting point → recommended direction → end state]

### Implementation Sketch
[5-10 lines of pseudocode or language-idiomatic code showing the structural skeleton]

### Pair With Data-Source Pattern
[Brief note on which data-source pattern pairs with the chosen domain pattern]

Inputs

Outputs

• Domain-Logic Pattern Decision Record (markdown) — primary output
• Routing rationale — explicit complexity × team × platform reasoning
• Anti-pattern risk flags — with early warning signs and guards
• Implementation skeleton — language-idiomatic structural sketch
• Service Layer decision — thickness recommendation with rationale
• Data-source pairing note — which data-source pattern to combine with the chosen domain pattern

Key Principles

1. Complexity determines the crossover point, not personal preference. Fowler's complexity-vs-effort graph shows that Domain Model costs more upfront but grows sub-linearly while Transaction Script cost grows super-linearly. The crossover is real but cannot be measured precisely — use experienced judgment on domain complexity signals.

1. Team experience shifts the curve but does not override complexity. A team unfamiliar with OO domain models lowers the break-even point (they pay ramp-up cost longer), but high-complexity domains still demand Domain Model eventually. The alternative is unbounded Transaction Script sprawl.

1. Platform preference is Fowler's explicit tie-breaker. .NET RecordSet tooling makes Table Module the natural mid-complexity choice. Without that tooling, Table Module adds no value over a simple Domain Model with Active Record.

1. Refactor up, rarely down. If you start with Transaction Script and complexity grows: refactor toward Domain Model. If you start with Domain Model and want simplicity: going back to Transaction Script is rarely worth it unless you simultaneously simplify the data-source layer.

1. The anemic domain model is a pattern failure, not a design choice. Stripping behavior from domain classes into services defeats the purpose of Domain Model. If your domain classes have no methods, you are paying ORM complexity without the encapsulation benefit.

1. Service Layer thickness should match application-logic responsibility. Thin facade when the Domain Model carries all logic. Operation script when use cases require coordinating notifications, messaging, and multi-resource transactions. Avoid controller-entity as an architectural style — it tends to replicate Transaction Script duplication inside the service layer.

1. Mixed patterns are legitimate. Simple subsystems within a Domain Model application can use Transaction Script. The patterns are not mutually exclusive — the key is conscious, documented choice.

Examples

Scenario A — Revenue Recognition Engine (Complex Rules → Domain Model)

Trigger: A team building a SaaS billing system. Revenue recognition rules vary by product type: word processors recognize 100% immediately; spreadsheets spread recognition over three dates; databases over two; and the CFO adds new product categories quarterly.

Process:

• Classify: High complexity — rules vary by product, change frequently, involve calculations and date logic.
• Route: Domain Model (complexity high → Domain Model regardless of team experience).
• Service Layer: Operation Script — use case requires updating the database and notifying contract administrators atomically.
• Anti-pattern check: Risk of anemic domain model if the recognition strategy is placed in the service layer. Guard: RevenueRecognition objects should carry the isRecognizableBy(date) method; strategy subclasses should carry calculateRevenueRecognitions(). The service layer only coordinates the transaction and sends notifications.

Output: Domain Model with Contract, Product, RevenueRecognition, and a strategy hierarchy (CompleteRecognitionStrategy, ThreeWayRecognitionStrategy). RecognitionService is an Operation Script service layer that delegates domain logic to domain objects and handles email/integration notifications.

Scenario B — Simple Order Management (.NET CRUD → Table Module)

Trigger: A .NET team building internal order management. Rules are mostly: validate fields, calculate totals, apply standard discounts. All UI is data-grid based. ADO.NET DataSets flow through every layer.

Process:

• Classify: Low-to-moderate complexity. No complex invariants, no algorithmic variation.
• Route: Table Module. .NET RecordSet tooling is present; the calculation logic fits naturally in table-oriented classes.
• Service Layer: None needed — single client (a Windows Forms UI), single transactional resource.
• Anti-pattern check: Watch for stored-procedure logic drift. Keep discount calculation in the Order Table Module, not in a SQL stored procedure.

Output: Contract Table Module, Product Table Module, RevenueRecognition Table Module operating on a shared DataSet. No domain object identity — every operation takes a primary key parameter.

Scenario C — Legacy Transaction Scripts Hitting Complexity Wall → Refactor to Domain Model

Trigger: A Java team's billing service has grown to a 2,000-line BillingService class. Discount logic is copy-pasted across five methods. Adding a new rule requires touching seven places.

Process:

• Classify: Transaction Script sprawl — the complexity has exceeded the pattern's capacity.
• Route: Refactor toward Domain Model. Direction: up, not down.
• Service Layer: Keep BillingService as an Operation Script service layer; move business rules onto Contract, LineItem, and Discount domain objects.
• Migration path: Extract duplicate validation into domain object methods first; introduce Data Mapper to decouple persistence; migrate rule-specific conditional branches into strategy classes.
• Anti-pattern check: Do not introduce an anemic domain model by moving methods from BillingService to new classes that only hold data. Behavior must move too.

Output: Decision record documenting the refactor direction, a domain object skeleton (Contract, LineItem, Discount), BillingService reduced to coordinating transactions and notifications, and a Data Mapper recommendation for persistence.

References

• references/complexity-vs-effort-graph.md — Fowler's unquantified-but-instructive chart described textually with decision thresholds
• references/revenue-recognition-four-ways.md — the three-table schema (products / contracts / revenueRecognitions) implemented as Transaction Script (Java), Domain Model (Java), Table Module (C#), and Service Layer (Java)
• references/service-layer-thickness-guide.md — detailed comparison of domain facade, operation script, and controller-entity with code sketches

If unclear which data-source pattern to pair with the chosen domain pattern → invoke data-source-pattern-selector. If unclear whether a distribution boundary (Remote Facade, DTO) is needed → invoke distribution-boundary-designer. If unsure which overall layering approach fits the application → invoke enterprise-architecture-pattern-stack-selector.

License

This skill is licensed under CC-BY-SA-4.0. Source: BookForge — Patterns of Enterprise Application Architecture by Martin Fowler et al. (Addison-Wesley, 2002).

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