Backend Microservice Architecture Architect

Wrote docs/architecture/backend-service-architect/00-overview/backend-service-architect-overview.md. Complete backend microservice architecture specification establishing Hexagonal Ports & Adapters, framework-free domain cores, transactional outbox relays, and connection pool isolation.

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skill: backend-service-architect

Backend Microservice Architecture: Card Issuance Service [SVCARCH-CARD-001]

Summary

This specification establishes the backend microservice architecture, internal module boundaries, and persistence contracts for card-issuance-service v2.0 under run ID card-issuance-arch-001. It governs digital card generation and Primary Account Number (PAN) cryptographic provisioning across 80 enterprise bank integrations sustaining 4,200 peak requests/second. It decisively eliminates the architectural coupling and unintended state mutations demonstrated in incident REG-4819 (where exposing ORM entities directly to web controllers allowed automated Hibernate dirty-checking to overwrite card limit profiles during an HTTP serialization refactor). The architecture enforces a strict

Hexagonal Architecture (Ports & Adapters) model: isolating a framework-free Java 21 domain core, driving operations via explicit Inbound Ports (REST and Kafka consumers), delegating persistence via Outbound Ports, implementing an atomic Transactional Outbox relay to Apache Kafka, and enforcing strict sub-45ms p99 latency envelopes.

Detailed Description

Layered "controller-service-dao" architectures commonly couple business logic to web framework annotations (@RestController, @Transactional) and persistence entities (@Entity). When domain logic depends directly on database ORM drivers or HTTP serialization libraries, refactoring an API endpoint inadvertently alters database transaction boundaries, leading to data corruption and leaky abstractions. Hexagonal Architecture inverts these dependencies: the domain core contains zero external framework imports, communicating exclusively through abstract interfaces (Ports), while external technologies (HTTP, PostgreSQL, Kafka) plug in as replaceable Adapters.

Inbound Adapters (HTTP Controller / Kafka Consumer)
                         │
                         ▼ (Passes DTO to Inbound Port Interface)
[ Inbound Port: `IssueCardUseCase` ]
                         │
                         ▼ (Invokes Pure Domain Logic)
[ Pure Domain Core: Framework-Free Java 21 ]
  ├── Aggregate Root: `CardAccount` (Private setters, immutable invariants)
  ├── Value Objects: `CardPan`, `ExpiryDate`, `SpendingLimit`
  └── Domain Service: `CardCryptographicTokenService`
                         │
                         ▼ (Calls Abstract Outbound Port Interface)
[ Outbound Ports: `CardRepositoryPort`, `EventPublisherPort` ]
                         │
        ┌────────────────┴────────────────┐
        ▼ (PostgreSQL Adapter)            ▼ (Outbox Event Adapter)
[ PostgreSQL Aurora Driver ]       [ Transactional Outbox Table ]
  ├── Executes Parameterized SQL     ├── Commits `CardIssuedEvent` in same ACID txn
  └── Connection Pool Capped at 60   └── Debezium CDC Streams to Kafka Topic

Criteria and weights

Comparison

Result

Option C is selected. Framework-free domain core guarantees business rule immutability; ports and adapters decouple web and persistence layers; transactional outbox eliminates distributed inconsistencies.

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Required Mechanisms

1. Inbound & Outbound Port Contract Definitions [MC-PO-01]

Inbound Port (Driving Interface)

public interface IssueCardUseCase {
    CardIssuanceResponse issueCard(IssueCardCommand command);
}

Outbound Ports (Driven Interfaces)

public interface CardRepositoryPort {
    Optional<CardAccount> findById(CardAccountId id);
    void save(CardAccount cardAccount);
}

public interface CardEventPublisherPort {
    void publish(CardDomainEvent event);
}

2. Framework-Free Domain Core Invariants [MC-DC-01]

• The package com.bank.card.domain.* has

zero external library dependencies (only standard java.time.*, java.math.*, java.util.*).

• Domain Invariants:
  • SpendingLimit: Cannot be negative; daily cash withdrawal ceiling capped at $2,500.00.
  • CardAccount: A newly issued card cannot be activated if the linked funding account status is FROZEN or RESTRICTED.

3. Transactional Outbox & CDC Pipeline [MC-OB-01]

• Atomic Persistence:
  • CardRepositoryAdapter writes the new card state to table cards.
  • In the exact same PostgreSQL database transaction, CardEventPublisherAdapter writes an event record to card_outbox:

    INSERT INTO card_outbox (event_id, aggregate_id, event_type, payload, created_at)
    VALUES (:eventId, :cardId, 'CardIssuedEvent', :payloadJson, NOW());
    

• Debezium CDC reads WAL logs and publishes to Kafka topic card.events.v1 in < 200 ms.

4. Database Connection Pool Boundaries [MC-CP-01]

• HikariCP connection pool is bounded strictly:
  • maximumPoolSize: 60 connections per container pod.
  • connectionTimeout: 2000 ms.
  • idleTimeout: 30000 ms.
• Prevents database connection exhaustion during traffic surges across 40 container replicas.

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Invariants and Contracts

Zero Framework Leakage Invariant [INV-SVCARCH-01]
  Classes within the domain core package must contain zero third-party framework imports.
  Spring, Hibernate, Jackson, and AWS SDK annotations are strictly barred from domain entities.

Mandatory Port Abstraction Invariant [INV-SVCARCH-02]
  External adapters must interact with the domain model exclusively via Port interfaces.
  Inbound controllers must never bypass ports to call repositories or database drivers directly.

Atomic Transactional Outbox Mandate [INV-SVCARCH-03]
  Domain state changes and domain events must be persisted within the same local ACID transaction.
  Emitting asynchronous events directly to external message brokers before database commit is prohibited.

Explicit Unknowns

• Debezium CDC connector replication lag during massive 4,200 TPS batch commercial card generation bursts (G-1).
• Hardware Security Module (HSM) PKCS#11 driver thread safety when fanned out across 16 parallel JVM worker threads (G-2).

Traceability

Verification

No validator was supplied, so no command was run.

Reviewer self-check against backend service architecture standards:

• Boundary Cleanliness: PASS. Domain core completely isolated from Spring and Hibernate.
• Port Discipline: PASS. All interactions mediated through clean Inbound and Outbound port interfaces.
• Outbox Atomicity: PASS. State mutations and outbox records committed in single ACID transactions.
• Markdown Hygiene: PASS. Native Markdown syntax strictly adheres to rule_markdown.md.

Open Decisions

• DEC-SVCARCH-01: Elena Rostova to determine whether ArchUnit tests should run in pre-commit git hooks to automatically block any commits introducing framework annotations into domain packages (Owner: Elena Rostova).

Next steps

1. Marcus Vance provisions HikariCP connection pool configurations and PostgreSQL outbox schemas.
2. Engineering team implements ArchUnit architectural fitness tests enforcing Hexagonal import boundaries.
3. Conduct staging performance drill validating 4,200 TPS throughput and asserting sub-45ms p99 response times.