# Patterns of Enterprise Application Architecture vs Working Effectively with Legacy Code

Status: reviewed
Research basis: mini-only

Verdict: ✅ Complementary

Conflict: 12%
Overlap: 38%
Complementarity: 78%

## Loading Decision

Use together when changing existing code: one rule set controls safe change sequencing while the other defines the target design, construction, architecture, data, or production quality.

## Book A Pressure

- Patterns of Enterprise Application Architecture should drive tasks where enterprise pattern forces across workflow, persistence, transactions, integration, session state, or remoting dominate.
- Evidence: `patterns-of-enterprise-application-architecture/patterns-of-enterprise-application-architecture.mini.md` lines 3-5: applies to enterprise code crossing presentation, workflow, domain, persistence, transactions, concurrency, integration, session state, or remote boundaries.

## Book B Pressure

- Working Effectively with Legacy Code should drive tasks where unclear or weakly tested code requires characterization, seams, dependency breaking, and small safe changes.
- Evidence: `working-effectively-with-legacy-code/working-effectively-with-legacy-code.mini.md` lines 3-5: applies when code is expensive to change safely because behavior is unclear, tests are weak, dependencies hidden, or runtime/framework setup blocks feedback.

## Complementary Forces

- Claim: Patterns of Enterprise Application Architecture contributes enterprise-pattern pressure across presentation, workflow, domain logic, persistence, transactions, integration, state, and remoting; Working Effectively with Legacy Code contributes characterization, seam, dependency-breaking, small-change, and local-refactoring pressure. Together they are useful only where both scopes are active.
- Evidence:
  - `patterns-of-enterprise-application-architecture/patterns-of-enterprise-application-architecture.mini.md` lines 13-30: requires explicit responsibility ownership, earned layering, force-based business logic patterns, Service Layer boundaries, Remote Facade/DTOs at remote boundaries, deliberate persistence patterns, visible identity/write/loading behavior, explicit ORM mapping forces, workflow-owned transactions/concurrency, focused presentation, translated external systems, deliberate session state, concrete-pressure base patterns, no default distribution, ordered code generation, and responsibility-level tests.
  - `working-effectively-with-legacy-code/working-effectively-with-legacy-code.mini.md` lines 31-39: fires on uncertain behavior, excessive test setup, hard runtime boundaries, large methods/classes, database/UI/framework/API-boundary code, magical seams, repeated edits, and rewrite temptation.

## Overlap

- Claim: They overlap where both affect safe existing-code change, tests, behavior preservation, ownership, and stopping before speculative cleanup; the overlap score reflects how often an agent would receive similar pressure from both.
- Evidence:
  - `patterns-of-enterprise-application-architecture/patterns-of-enterprise-application-architecture.mini.md` lines 47-54: checks intentional separation of enterprise responsibilities, business logic pattern fit, persistence pattern fit, explicit transactions/concurrency/loading, remote/integration boundaries, session-state ownership, and responsibility-aligned tests.
  - `working-effectively-with-legacy-code/working-effectively-with-legacy-code.mini.md` lines 42-50: checks legacy risk, behavior delta/preservation, characterization, close fast tests, smallest seam, reduced blocking dependency, separated behavior/refactor/cleanup, cleanup path for temporary seams, and improved understandability/testability.

## Conflicts

- Claim: The tension is scope creep: design or architecture improvements must not override behavior preservation, characterization, or the current-smell stop condition.
- Evidence:
  - `patterns-of-enterprise-application-architecture/patterns-of-enterprise-application-architecture.mini.md` lines 7-9: corrects inventing architecture for every feature and letting frameworks, ORMs, schemas, or transports choose the design.
  - `working-effectively-with-legacy-code/working-effectively-with-legacy-code.mini.md` lines 7-9: corrects improving design before gaining control by requiring behavior understanding, preservation, smallest useful seam, dependency breaking, requested change, and local testability improvement.

## Use Together When

- Use together when changing weakly tested code toward Patterns of Enterprise Application Architecture goals: first characterize behavior and create the smallest seam, then apply the other rule set inside the controlled change area.

## Prefer One When

- Prefer Working Effectively with Legacy Code when tests are weak or behavior is unclear; prefer the other book only after control, characterization, or seams make the change safe.

## Source Basis

- `patterns-of-enterprise-application-architecture/patterns-of-enterprise-application-architecture.mini.md` lines 3-5: applies to enterprise code crossing presentation, workflow, domain, persistence, transactions, concurrency, integration, session state, or remote boundaries.
- `patterns-of-enterprise-application-architecture/patterns-of-enterprise-application-architecture.mini.md` lines 7-9: corrects inventing architecture for every feature and letting frameworks, ORMs, schemas, or transports choose the design.
- `patterns-of-enterprise-application-architecture/patterns-of-enterprise-application-architecture.mini.md` lines 13-30: requires explicit responsibility ownership, earned layering, force-based business logic patterns, Service Layer boundaries, Remote Facade/DTOs at remote boundaries, deliberate persistence patterns, visible identity/write/loading behavior, explicit ORM mapping forces, workflow-owned transactions/concurrency, focused presentation, translated external systems, deliberate session state, concrete-pressure base patterns, no default distribution, ordered code generation, and responsibility-level tests.
- `patterns-of-enterprise-application-architecture/patterns-of-enterprise-application-architecture.mini.md` lines 47-54: checks intentional separation of enterprise responsibilities, business logic pattern fit, persistence pattern fit, explicit transactions/concurrency/loading, remote/integration boundaries, session-state ownership, and responsibility-aligned tests.
- `working-effectively-with-legacy-code/working-effectively-with-legacy-code.mini.md` lines 3-5: applies when code is expensive to change safely because behavior is unclear, tests are weak, dependencies hidden, or runtime/framework setup blocks feedback.
- `working-effectively-with-legacy-code/working-effectively-with-legacy-code.mini.md` lines 7-9: corrects improving design before gaining control by requiring behavior understanding, preservation, smallest useful seam, dependency breaking, requested change, and local testability improvement.
- `working-effectively-with-legacy-code/working-effectively-with-legacy-code.mini.md` lines 31-39: fires on uncertain behavior, excessive test setup, hard runtime boundaries, large methods/classes, database/UI/framework/API-boundary code, magical seams, repeated edits, and rewrite temptation.
- `working-effectively-with-legacy-code/working-effectively-with-legacy-code.mini.md` lines 42-50: checks legacy risk, behavior delta/preservation, characterization, close fast tests, smallest seam, reduced blocking dependency, separated behavior/refactor/cleanup, cleanup path for temporary seams, and improved understandability/testability.

## Review Notes

- External context was not used as decisive evidence for Patterns of Enterprise Application Architecture vs Working Effectively with Legacy Code; the verdict is based on the cited local `mini` line ranges.
