TransportClient: Timeout waiting for task

ElasticSearch version: 1.5.2

Hi all,
I am using the java transport client to index documents. Recently I sometimes met this exception:

However, it seems that my Elasticsearch node is still in healthy state. Noting error logs in the Elasticsearch nodes can be found.
What' worse, if I retry index the documets, the error will happen again.

Here are part of the smaple codes that I am using to index documents.

BulkRequestBuilder bulkRequest = client.prepareBulk(); bulkRequest.add(client.prepareIndex(indexName, indexType, docId) .setSource(dataMaps)); BulkResponse bulkResponse = bulkRequest.execute().actionGet(15000);

My question:
What's the reason for the exception?
Is there any proposal for problem?

Thank you

Today I also met this problems.