@Override
	public void open(Configuration parameters) throws Exception {
		client = callBridge.createClient(userConfig);
		callBridge.verifyClientConnection(client);
		bulkProcessor = buildBulkProcessor(new BulkProcessorListener());
		requestIndexer = callBridge.createBulkProcessorIndexer(bulkProcessor, flushOnCheckpoint, numPendingRequests);
		failureRequestIndexer = new BufferingNoOpRequestIndexer();
	}
Disable flushing on checkpoint. When disabled, the sink will not wait for all
pending action requests to be acknowledged by Elasticsearch on checkpoints.

<p>NOTE: If flushing on checkpoint is disabled, the Flink Elasticsearch Sink does NOT
provide any strong guarantees for at-least-once delivery of action requests.
elasticsearch5-xh/elasticsearch5-xh-sink/src/main/java/org/apache/flink/streaming/connectors/elasticsearch/ElasticsearchSinkBase.java