Trạng thái và mã lỗi

queued → sending → sent → delivered
                     ↓
                   failed

function sendZnsSafely(array $payload, int $maxAttempts = 3): array {
    $trackingId = $payload['tracking_id'];

    for ($attempt = 1; $attempt <= $maxAttempts; $attempt++) {
        try {
            return sendZns($payload);
        } catch (NetworkOrServerError $e) {
            $existing = findByTrackingId($trackingId);
            if ($existing !== null) return $existing;

            if ($attempt === $maxAttempts) throw $e;
            usleep((int)(pow(2, $attempt) * 500_000));
        }
    }
}

function findByTrackingId(string $trackingId): ?array {
    $r = getJson("/api/v1/messages?tracking_id=" . urlencode($trackingId));
    return $r['messages'][0] ?? null;
}