Domain-Driven Design Aggregate Contract Design

Wrote docs/architecture/tasks/banking-aggregate-001/aggregate-design/aggregate-contract-spec.md. Complete Domain-Driven Design aggregate specification establishing LoanDisbursement aggregate root boundaries, Value Object encapsulation, reference-by-identity rules, and transactional event outbox contracts.

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skill: aggregate-design

DDD Aggregate Contract Specification: Commercial Loan Disbursement [AGG-LOAN-001]

Summary

This specification establishes the Domain-Driven Design (DDD) aggregate boundaries, invariant enforcement rules, entity lifecycle contracts, and domain event outbox specifications for loan-disbursement-aggregate v2.0 under run ID banking-aggregate-001. It governs commercial credit facilities up to $50M executing across multi-tranche disbursement schedules. It decisively eliminates the database lock contention and transaction failures demonstrated in incident DB-4910 (where a monolithic mega-aggregate spanning borrower, collateral, and loan entities induced massive database deadlocks, freezing commercial credit operations for 45 minutes). The architecture establishes LoanDisbursement as an isolated

Aggregate Root, models immutable concepts as

Value Objects (Money, InterestRateMargin, DisbursementSchedule), enforces strict

reference-by-identity for external entities (BorrowerId, CollateralId), restricts database ACID transactions to a single aggregate boundary, and emits transactional outbox domain events (TrancheDisbursedEvent).

Detailed Description

In complex financial domains, developers frequently design "mega-aggregates" that cluster disparate business concepts into a single entity hierarchy. When an aggregate boundary grows too large, multiple concurrent business operations (such as collateral revaluations and tranche disbursements) contend for the exact same database row lock, inducing thread starvation and deadlocks. A formal DDD aggregate boundary defines an atomic consistency island: internal entities are encapsulated behind the Aggregate Root, external aggregates are referenced exclusively by immutable identity keys, and inter-aggregate synchronization is achieved asynchronously via domain events.

Commercial Credit Operation (Disburse Tranche $2.5M)
                           │
                           ▼
[ LoanDisbursement Aggregate Root (Boundary Boundary) ]
  ├── 1. Internal Encapsulated Entities:
  │      └── `DisbursementTranche` (Entity: ID, Amount, Status)
  ├── 2. Immutable Value Objects:
  │      ├── `Money` (Currency, Decimal Value, Rounding Mode)
  │      ├── `InterestRateMargin` (Spread Floor: >= 2.50%)
  │      └── `DisbursementState` (Enum: PENDING, DISBURSED, CANCELLED)
  ├── 3. External References by Identity (Zero Direct Object Graphs):
  │      ├── `BorrowerId`: "borr_corp_8812"
  │      └── `CollateralPledgeId`: "collat_pledge_9941"
  └── 4. Invariant Assertion Gate:
         Asserts: Sum(Tranches) <= CreditFacilityCap ($50M)
                           │
       ┌───────────────────┴───────────────────┐
       ▼ (Atomic Commit: Single Aggregate)     ▼ (Invariant Breach: Over-Allocation)
[ Aurora PostgreSQL 16 (Optimistic Lock) ] [ Business Rule Exception ]
  ├── 1. Updates `LoanDisbursement` Row      └── Throws `CreditLimitExceededException`
  └── 2. Writes `TrancheDisbursedEvent`          Rolls back local transaction in < 2ms
         to Transactional Outbox Table

Criteria and weights

Comparison

Result

Option C is selected. LoanDisbursement encapsulates tranche lifecycle logic; reference-by-identity eliminates cross-aggregate deadlocks; optimistic versioning guarantees race-free commits.

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Required Mechanisms

1. Aggregate Root Boundary & Encapsulation [MC-AR-01]

• Aggregate Root: LoanDisbursement (Root entity owning lifecycle).
• Internal Entities: DisbursementTranche:
  • Tranches have local entity IDs scoped within the aggregate root.
  • External callers cannot query or mutate DisbursementTranche directly; all additions or state transitions must invoke methods on LoanDisbursement:

    loanDisbursement.requestTrancheDisbursement(trancheId, amount, margin);
    

2. Value Object Classification & Immutability [MC-VO-01]

Money: Contains BigDecimal amount and Currency currency. Enforces Banker's Rounding (half-even); prohibits arithmetic across mismatched currencies.

InterestRateMargin: Enforces business invariant: spread must be >=

2.50% (250 basis points). Instantiation with $< 2.50%$ throws IllegalArgumentException.

DisbursementSchedule: Encapsulates tranche due dates and maturity windows as an immutable value object collection.

3. Transactional Invariant Enforcement [MC-IE-01]

• Invariant 1 (INV-FACILITY-CAP):
  $$\sum_{i=1}^{N} \text{Tranche}_i.\text{amount} \le \text{FacilityCeiling} \quad ($50,000,000.00)$$
  If a new tranche causes the aggregate total to exceed $50M, the operation throws FacilityCeilingExceededException and aborts.

Invariant 2 (INV-TRANCHE-SEQ): Tranche $N$ cannot be disbursed unless Tranche $N-1$ has achieved status SETTLED or CANCELLED.

4. Reference-by-Identity & Domain Event Outbox [MC-EV-01]

• Identity References:
  • BorrowerId: String identifier (borr_corp_8812).
  • CollateralId: String identifier (collat_pledge_9941).
• Domain Event Contract:
  • Upon successful tranche disbursement, the root appends TrancheDisbursedEvent to its internal domain events collection:

    {
      "event_id": "evt_01J8N6B5H2QZ3R8V8",
      "aggregate_id": "disb_facility_881204",
      "event_type": "loan.tranche.disbursed",
      "occurred_at": "2026-09-15T14:22:00.123Z",
      "borrower_id": "borr_corp_8812",
      "tranche_id": "trn_04",
      "amount_cents": 250000000,
      "currency": "USD"
    }
    

  • Saved atomically to the local outbox table in the same database transaction.

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Invariants and Contracts

Single Aggregate Transaction Boundary [INV-AGG-01]
  A single database transaction must mutate exactly one aggregate instance.
  Mutating multiple aggregate roots within the same database commit is strictly prohibited.

Reference-by-Identity Invariant [INV-AGG-02]
  Aggregates must not hold direct memory references or object pointers to external aggregate roots.
  External entities must be referenced exclusively via immutable identity value objects.

Optimistic Concurrency Control Floor [INV-AGG-03]
  The aggregate persistence entity must maintain a numerical version column (`@Version`).
  Concurrent race conditions must trigger `OptimisticLockingFailureException` and roll back cleanly.

Explicit Unknowns

• Eventual consistency latency when downstream accounting ledgers consume TrancheDisbursedEvent during peak Kafka broker rebalances (G-1).
• Maximum cardinality of historical tranche entities per loan facility before aggregate rehydration latency exceeds 10 ms (G-2).

Traceability

Verification

No validator was supplied, so no command was run.

Reviewer self-check against aggregate design standards:

• Boundary Cleanliness: PASS. LoanDisbursement is the sole root; internal tranches are strictly encapsulated.
• Reference Hygiene: PASS. External borrower and collateral entities referenced exclusively by identity strings.
• Invariant Rigor: PASS. Mathematical facility cap ($50M) and tranche sequencing enforced within the root.
• Markdown Hygiene: PASS. Native Markdown syntax strictly adheres to rule_markdown.md.

Open Decisions

• DEC-AGG-01: David O'Reilly to determine whether completed loans older than 3 years should be pruned from the active aggregate table and archived to cold Parquet lakehouses (Owner: David O'Reilly).

Next steps

1. Core Engineering implements the LoanDisbursement aggregate root and Money Value Object in Java 21.
2. Platform team sets up the transactional outbox table schema and Debezium Kafka publisher.
3. Conduct concurrency stress drill simulating 35 parallel tranche disbursement requests on the same facility to verify race-free optimistic locking.