외부 API 연동

@Bean
RestClient pgClient() {
    var factory = new JdkClientHttpRequestFactory(HttpClient.newBuilder()
            .connectTimeout(Duration.ofSeconds(2)).build());
    factory.setReadTimeout(Duration.ofSeconds(3));
    return RestClient.builder().baseUrl("https://pg.example.com")
            .requestFactory(factory).build();
}

@Service
class PgService {
    @Retry(name = "pg")
    @CircuitBreaker(name = "pg", fallbackMethod = "payFallback")
    String pay(String key, String body) {
        return pgClient().post().uri("/pay")
                .header("Idempotency-Key", key).body(body)
                .retrieve().body(String.class);
    }
    String payFallback(String key, String body, Exception e) {
        return "{\"status\":\"PENDING\"}";   // 미확인 상태로 응답, 4.2 절 대사 대상
    }
}

resilience4j:
  retry:
    instances.pg: { max-attempts: 3, wait-duration: 200ms, retry-exceptions: [java.io.IOException] }
  circuitbreaker:
    instances.pg: { sliding-window-size: 10, failure-rate-threshold: 50, wait-duration-in-open-state: 10s }

CREATE TABLE payment (
    id            BIGINT PRIMARY KEY,
    idempotency_key VARCHAR(64) NOT NULL UNIQUE,
    status        VARCHAR(10)  NOT NULL,   -- PENDING, UNKNOWN, PAID, FAILED
    amount        BIGINT       NOT NULL
);

try {
    String res = pgClient.post(url, body, key);
    mapper.updateStatus(key, "PAID");
} catch (ApiClient.ApiException e) {
    mapper.updateStatus(key, "UNKNOWN");   // 처리 여부를 모름, 재시도 금지
}

// @Scheduled(fixedDelay = 60_000), 09 레슨 SafeTask 로 감싼다
for (String key : mapper.findUnknownKeys()) {
    String status = pgClient.get("/pay/status/" + key);   // 상대의 조회 API 로 확정
    mapper.updateStatus(key, status);
}

class TokenBucket {
    private final long capacity = 100, refillMillis = 60_000 / 100;   // 0.6초마다 1개
    private final AtomicLong tokens = new AtomicLong(capacity);
    private final AtomicLong lastRefill = new AtomicLong(System.currentTimeMillis());

    boolean tryAcquire() {
        long now = System.currentTimeMillis();
        long elapsed = now - lastRefill.get();
        long refill = elapsed / refillMillis;
        if (refill > 0 && lastRefill.compareAndSet(lastRefill.get(), now)) {
            tokens.set(Math.min(capacity, tokens.get() + refill));
        }
        return tokens.get() > 0 && tokens.decrementAndGet() >= 0;
    }
}

Map<String, Object> json = MiniJson.parse(body);
Object paymentId = json.get("paymentId");
if (paymentId == null) {
    throw new ApiClient.ApiException(0, "계약 위반: paymentId 누락");
}
if (!(paymentId instanceof String)) {
    throw new ApiClient.ApiException(0, "계약 위반: paymentId 타입 변경 (" + paymentId.getClass() + ")");
}