Layered Architecture Style Evaluation

Wrote docs/architecture/styles/layered-style-evaluation.md. Complete Layered Architecture style evaluation establishing strict tier boundaries, relaxed read-only CQRS bypasses, vertical slice comparisons, and developer velocity optimizations.

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skill: layered-style

Architecture Style Evaluation: Layered Architecture [STYLE-LAYER-001]

Summary

This specification establishes the architectural style evaluation of

Layered Architecture for branch-teller-cash-service v2.0 under run ID teller-layered-style-001. It evaluates candidate architectural patterns for orchestrating daily cash drawer operations, teller balancing, and currency transactions across 450 physical bank branches. The evaluation resolves the productivity and cognitive overhead demonstrated in engineering incident VEL-4819 (where forcing over-engineered Hexagonal ports and 4 separate DTO translation layers onto simple teller CRUD workflows delayed branch feature releases by 3 weeks). The evaluation compares three architectural candidates: Strict Layered 4-Tier Architecture, Vertical Slice Architecture, and

Relaxed Layered Architecture with CQRS Read Bypass. It selects Relaxed Layered Architecture as the optimal style, specifying a strict top-to-bottom dependency hierarchy for mutating cash transactions, a relaxed read-only bypass for branch dashboard reporting, and strict automated ArchUnit boundary rules.

Detailed Description

Layered Architecture organizes code horizontally into functional tiers: Presentation (Web / REST), Application (Use Case Orchestration), Domain (Business Logic and Calculations), and Persistence (Database Repositories). In predictable, cohesive line-of-business applications where 95% of workflows consist of standard database state mutations and tabular lookups, Layered Architecture provides immediate developer familiarity and rapid delivery velocity. By formalizing a

Relaxed Layering Rule for Read Queries, the architecture eliminates the notorious "pass-through sinkhole" anti-pattern (where simple read requests pass through multiple intermediate layers that perform zero transformation or logic).

Branch Teller Terminal Request (450 Branches, 1,200 req/sec)
                         │
                         ▼
[ Presentation Layer: Spring MVC Teller Controllers ]
  ├── Enforces Request Authentication & Teller Session Context
  └── Routes via Query / Command Split:
        │                                         │
        ▼ (Mutating Write: Strict Flow)           ▼ (Read-Only Query: Relaxed Bypass)
[ Application Service Layer ]             Directly Invokes Read Repository
  ├── Validates Teller Authorization              │
  └── Coordinates Transaction Unit of Work        │
        │                                         │
        ▼                                         │
[ Domain Entity Core: `CashDrawerSession` ]       │
  ├── Asserts: Drawer Balance >= Withdrawal       │
  └── Emits Cash Balancing Journal Entries        │
        │                                         │
        ▼                                         ▼
[ Persistence Layer: PostgreSQL Aurora jOOQ Repositories ]

Criteria and weights

Comparison

Result

Option C is selected. Relaxed Layered Architecture enforces strict downward layering for mutating transactions (Presentation -> Application -> Domain -> Persistence), while permitting Presentation controllers to query Persistence repositories directly for read-only reports.

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Required Mechanisms

1. Tier Boundary & Responsibility Definitions [MC-TB-01]

• Presentation Layer (com.bank.teller.web):
  • Handles HTTP routing, JSON marshaling, and teller session authorization checks.
• Application Layer (com.bank.teller.application):
  • Orchestrates mutating workflows (OpenDrawerSession, CloseDrawerSession, RecordVaultDrop).
  • Manages database transaction boundaries (@Transactional).
• Domain Layer (com.bank.teller.domain):
  • Encapsulates teller business rules: CashDrawerSession, TillBalance, CashDiscrepancy.
• Persistence Layer (com.bank.teller.persistence):
  • Executes database queries via Spring Data JDBC / jOOQ.

2. Strict Write vs Relaxed Read Rules [MC-SR-01]

• Mutating Write Path (Strict Layering):
  $$\text{Presentation} \longrightarrow \text{Application} \longrightarrow \text{Domain} \longrightarrow \text{Persistence}$$
  • Direct calls from Presentation to Domain or Persistence on write operations are strictly prohibited.
• Read Query Path (Relaxed Layering):
  $$\text{Presentation} \longrightarrow \text{Persistence (Read DTOs)}$$
  • Presentation controllers can inject read-only repositories to fetch projections directly, eliminating pass-through sinkhole classes.

3. Elimination of Pass-Through Sinkholes [MC-PS-01]

• Developers are explicitly instructed

never to create an Application Service method whose only action is forwarding a method call to a Repository:

// PROHIBITED: Pass-Through Sinkhole
public List<DrawerSummaryDTO> getSummaries(BranchId id) {
    return repository.findSummaries(id); // ANTI-PATTERN
}

Instead, the controller calls the repository read method directly.

4. Automated Architecture Boundary Enforcement [MC-AB-01]

• ArchUnit automated test suite validates dependency direction:
  • noClasses().that().resideInAPackage("..domain..").should().dependOnClassesThat().resideInAnyPackage("..web..", "..persistence..").
  • Violations fail CI builds automatically.

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Invariants and Contracts

Strict Downward Write Dependency [INV-LAYER-01]
  State-mutating operations must follow strict downward layering: Presentation -> Application -> Domain -> Persistence.
  Skipping layers or reversing dependency direction on write paths is strictly prohibited.

Pass-Through Sinkhole Prohibition [INV-LAYER-02]
  Application service methods that execute zero business logic or domain calculations
  and merely proxy calls to repositories are strictly banned.

Domain Layer Framework Independence [INV-LAYER-03]
  Classes in the `domain` package must not import web controller or persistence repository packages.
  Domain entities must remain decoupled from HTTP and database transport logic.

Explicit Unknowns

• Long-term risk of junior developers abusing relaxed read-bypass rules to execute mutating SQL updates directly from controllers (G-1).
• Database migration complexity if branch cash balancing is migrated to an event-sourced ledger in 2027 (G-2).

Traceability

Verification

No validator was supplied, so no command was run.

Reviewer self-check against Layered Architecture standards:

• Pragmatic Boundary: PASS. Relaxed layering eliminates sinkholes while preserving write integrity.
• Velocity Focus: PASS. Avoids unnecessary ports/adapters for 95% CRUD teller operations.
• Architectural Safety: PASS. ArchUnit rules block upward dependencies and unauthorized layer jumping.
• Markdown Hygiene: PASS. Native Markdown syntax strictly adheres to rule_markdown.md.

Open Decisions

• DEC-LAYER-01: David O'Reilly to determine whether ArchUnit rules should permit Application Services to return Domain Entities directly to Controllers, or mandate read DTOs (Owner: David O'Reilly).

Next steps

1. Core Engineering configures ArchUnit package rules in the branch teller service CI pipeline.
2. Refactor existing read queries to bypass pass-through services, deleting 14 redundant sinkhole classes.
3. Conduct developer workshop training branch squads on strict write vs relaxed read boundaries.