in flink-connector-elasticsearch-base/src/main/java/org/apache/flink/connector/elasticsearch/sink/ElasticsearchWriter.java [120:126]
public void write(IN element, Context context) throws IOException, InterruptedException {
// do not allow new bulk writes until all actions are flushed
while (checkpointInProgress) {
mailboxExecutor.yield();
}
emitter.emit(element, context, requestIndexer);
}