private void acquireSlot() {
        try {
            while (inflight.get() >= throttler.targetInflight())
                Thread.sleep(1);

            inflight.incrementAndGet();
        }
        catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }
Retries throttled requests (429), adjusting the target inflight count, and server unavailable (503).
vespa-feed-client/src/main/java/ai/vespa/feed/client/impl/HttpRequestStrategy.java