@VisibleForTesting
	long getNumPendingRequests() {
		if (flushOnCheckpoint) {
			return numPendingRequests.get();
		} else {
			throw new UnsupportedOperationException(
					"The number of pending requests is not maintained when flushing on checkpoint is disabled.");
		}
	}
The interval time which check client whether available.
elasticsearch5-xh/elasticsearch5-xh-sink/src/main/java/org/apache/flink/streaming/connectors/elasticsearch/ElasticsearchSinkBase.java