v1.0.2 - Post MVP Improvements

Post MVP Improvements

Feature enhancements, reliability improvements, and documentation updates added after the initial MVP implementation.

Features

• [PAYGATE-20] Idempotency key support — Optional Idempotency-Key header (UUID) on POST prevents duplicate bank charges on client retries. Cached responses are returned
  without calling the bank again.
• [PAYGATE-21] Luhn algorithm validation — Card numbers are validated against the Luhn checksum before contacting the bank, catching typos and clearly fake numbers early.
• [PAYGATE-22] HTTP 201 Created — POST /v1/payment now returns 201 Created with a Location header pointing to the created payment resource, following REST
  conventions.
• [PAYGATE-23] Retry with exponential backoff — Bank calls are retried up to 3 times (500ms base, 2x multiplier) on transient failures before falling through to the
  circuit breaker.
• [PAYGATE-24] Correlation ID in error responses — Non-validation errors (404, 429, 502, 500) now include correlationId and timestamp in the JSON body for
  traceability.
• [PAYGATE-25] Bank failure logging — Failed bank authorization attempts are logged with payment context (paymentId, cardLast4, currency, amount) for incident
  investigation.

Tests

• [PAYGATE-26] Rejected payment verification — Added tests confirming that rejected payments (expired card, Luhn-invalid) are not stored and return 404 on GET.

Documentation

• [PAYGATE-27] Error reference — Added comprehensive doc/ERROR_REFERENCE.md covering all error types, HTTP status codes, validation rules, and troubleshooting guide.
• [PAYGATE-28] README and manual testing guide — Updated README and doc/MANUAL_TESTING_GUIDE.md with all new features: idempotency, Luhn validation, 201 status, retry
  config, correlation ID in errors.

Fixes

• [PAYGATE-29] Dockerfile JAR version — Replaced hardcoded JAR version in Dockerfile with wildcard glob to prevent build failures on version bumps.
• [PAYGATE-30] Luhn-valid test cards — Replaced test card numbers that fail Luhn validation with valid alternatives (declined: 2222405343248828, bank error:
  2222405343248810).

v0.2.0 — MVP

v0.2.0 — MVP

Highlights

MVP payment gateway with full observability, resilience patterns, build-time quality enforcement, and API versioning.

What's Changed

Observability

• Structured logging — JSON-formatted logs with correlation IDs and PCI-compliant masking ([PAYGATE-13])
• Business metrics — payment_processed_total counter by status, bank_authorization_duration_seconds histogram ([PAYGATE-13])
• Docker support — Dockerfile and docker-compose.yml with 6 services ([PAYGATE-13])
• Monitoring stack — Prometheus, Grafana, and Tempo with pre-provisioned dashboards (Application, Business, Infrastructure, Alerts) ([PAYGATE-14])
• Alerting — 5 Prometheus alert rules (HighBankErrorRate, HighRejectionRate, HighLatency, HighErrorRate, HighHeapUsage) routed through Alertmanager ([PAYGATE-15])

Resilience

• Circuit breaker — Resilience4j bankClient instance; opens on configurable failure rate, returns 502 with structured error ([PAYGATE-16])
• Rate limiting — Bucket4j per-IP token-bucket filter with separate GET/POST limits; returns 429 with Retry-After header ([PAYGATE-16])

Configuration & Quality

• Externalized config — All settings driven by .env via spring-dotenv (23 variables across 8 categories) ([PAYGATE-17])
• Quality gates — ./gradlew check enforces JaCoCo >= 80% line coverage, Google Java Style (Checkstyle), and SpotBugs + FindSecBugs ([PAYGATE-18])

Bug Fixes

• API path versioning — Controller and rate limit filter now serve /v1/payment matching the OpenAPI spec ([PAYGATE-19])

Documentation

• README rewrite — Services table, full configuration reference, resilience, monitoring, dashboards, and alerting sections ([PAYGATE-19])

Changelog

Task	Type	Description
PAYGATE-13	feat	Add structured logging, metrics, and Docker support
PAYGATE-14	feat	Add Prometheus, Tempo, and Grafana observability stack
PAYGATE-15	feat	Add Prometheus alerting rules and Alertmanager
PAYGATE-16	feat	Add circuit breaker and per-IP rate limiting
PAYGATE-17	chore	Externalize config with spring-dotenv and .env
PAYGATE-18	feat	Add build-time quality gates
PAYGATE-19	docs	Update README and fix API path versioning

Services

Service	Image	Port(s)
payment-gateway	(local build)	8090
bank_simulator	bbyars/mountebank:2.8.1	8080, 2525
prometheus	prom/prometheus:v2.48.0	9090
grafana	grafana/grafana:10.2.2	3000
tempo	grafana/tempo:2.3.1	3200, 4317, 4318
alertmanager	prom/alertmanager:v0.26.0	9093

API Endpoints

Method	Path	Success	Error
POST	/v1/payment	200	400, 429, 502
GET	/v1/payment/{id}	200	404, 429

v0.1.0 — Functional MVP

Functional MVP — Payment Gateway

Functional MVP implementation of the payment gateway service.

Features

• PAYGATE-1 — Implementation plan, OpenAPI specs, and C4 diagrams
• PAYGATE-2 — Characterization tests for GET /payment/{id}
• PAYGATE-3 — OpenAPI code generation for server interface and bank client
• PAYGATE-4 — Replace hand-written DTOs with generated models
• PAYGATE-5 — Domain model, entity, MapStruct mappers, and CQRS use cases
• PAYGATE-6 — Bank simulator integration with POST /payment
• PAYGATE-7 — PCI DSS log masking for PAN and CVV
• PAYGATE-8 — Externalize test fixtures to JSON files
• PAYGATE-9 — Externalize bank config with @ConfigurationProperties
• PAYGATE-10 — Validation edge-case tests for payment processing
• PAYGATE-11 — Catch-all exception handler to prevent stack trace leaks
• PAYGATE-12 — README rewrite and centralized dependency versions

Highlights

• POST /payment and GET /payment/{id} endpoints
• OpenAPI-generated server interfaces and bank client
• Request validation with descriptive error responses
• In-memory payment storage
• Configurable bank simulator connection (URL, timeouts)
• PCI-compliant log masking
• Swagger UI at /swagger-ui/index.html