When using a bulk processor in test, you might write something like:
```java
BulkProcessor bulkProcessor = BulkProcessor.builder(client, new BulkProcessor.Listener() {
@Override public void beforeBulk(long executionId, BulkRequest request) {}
@Override public void afterBulk(long executionId, BulkRequest request, BulkResponse response) {}
@Override public void afterBulk(long executionId, BulkRequest request, Throwable failure) {}
})
.setBulkActions(10000)
.setFlushInterval(TimeValue.timeValueSeconds(10))
.build();
for (int i = 0; i < 10000; i++) {
bulkProcessor.add(new IndexRequest("foo", "bar", "doc_" + i)
.source(jsonBuilder().startObject().field("foo", "bar").endObject()
));
}
bulkProcessor.flush();
client.admin().indices().prepareRefresh("foo").get();
SearchResponse response = client.prepareSearch("foo").get();
// response does not contain any hit
```
The problem is that by default bulkProcessor defines the number of concurrent requests to 1 which is using behind the scene an Async BulkRequestHandler.
When you call `flush()` in a test, you expect it to flush all the content of the bulk so you can search for your docs.
But because of the async handling, there is a great chance that none of the documents has been indexed yet when you call the `refresh` method.
We should advice in our Java guide to explicitly set concurrent requests to `0` so users will use behind the scene the Sync BulkRequestHandler.
```java
BulkProcessor bulkProcessor = BulkProcessor.builder(client, new BulkProcessor.Listener() {
@Override public void beforeBulk(long executionId, BulkRequest request) {}
@Override public void afterBulk(long executionId, BulkRequest request, BulkResponse response) {}
@Override public void afterBulk(long executionId, BulkRequest request, Throwable failure) {}
})
.setBulkActions(5000)
.setFlushInterval(TimeValue.timeValueSeconds(10))
.setConcurrentRequests(0)
.build();
```
Closes#22158.
e32c7f1d72