Retry, Backoff ve Jitter — Herkes Aynı Anda Tekrar Denerse

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record RetryPolicy(int maxAttempts, Duration base, Duration cap) {
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    static RetryPolicy standard() {
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        return new RetryPolicy(4, Duration.ofMillis(200), Duration.ofSeconds(5));
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    }
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    /** Full jitter: a random wait between zero and the exponential ceiling. */
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    Duration delayBefore(int attempt, RandomGenerator random) {
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        long ceiling = Math.min(cap.toMillis(), base.toMillis() << (attempt - 1));
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        return Duration.ofMillis(random.nextLong(ceiling + 1));
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    }
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}

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final class Retryable {
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    private Retryable() {}
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    static boolean isTransient(Throwable error) {
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        if (error instanceof HttpStatusCodeException http) {
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            int status = http.getStatusCode().value();
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            return status == 429 || status == 502 || status == 503 || status == 504;
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        }
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        return error instanceof ResourceAccessException;   // timeouts, refused connections
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    }
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}

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@Component
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class BalanceClient {
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    private final RestClient rest;
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    private final RetryPolicy policy = RetryPolicy.standard();
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    private final RandomGenerator random = RandomGenerator.getDefault();
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    BalanceClient(RestClient rest) {
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        this.rest = rest;
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    }
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    Balance balanceOf(String accountId) throws InterruptedException {
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        for (int attempt = 1; ; attempt++) {
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            try {
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                return rest.get().uri("/accounts/{id}/balance", accountId).retrieve().body(Balance.class);
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            } catch (RuntimeException error) {
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                if (!Retryable.isTransient(error) || attempt == policy.maxAttempts()) {
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                    throw error;   // permanent, or out of attempts: let the caller decide
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                }
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                Thread.sleep(waitBefore(error, attempt + 1));
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            }
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        }
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    }
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    private Duration waitBefore(RuntimeException error, int attempt) {
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        if (error instanceof HttpStatusCodeException http && http.getResponseHeaders() != null) {
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            String retryAfter = http.getResponseHeaders().getFirst("Retry-After");
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            if (retryAfter != null) {
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                return Duration.ofSeconds(Long.parseLong(retryAfter));   // the server knows best
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            }
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        }
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        return policy.delayBefore(attempt, random);
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    }
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}

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@Component
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class TransferClient {
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    private final RestClient rest;
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    TransferClient(RestClient rest) {
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        this.rest = rest;
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    }
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    /** The same key on every attempt: a retry can never move the money twice. */
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    TransferReceipt send(TransferRequest request, UUID idempotencyKey) {
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        return rest.post().uri("/transfers")
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                .header("Idempotency-Key", idempotencyKey.toString())
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                .body(request)
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                .retrieve()
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                .body(TransferReceipt.class);
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    }
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}