Courier Fetch: shards failed

I got this error when my query was missing a closing quote:

field:"value

In my ElasticSearch logs I see these exceptions:

Caused by: org.elasticsearch.index.query.QueryShardException:
    Failed to parse query [field:"value]
...
Caused by: org.apache.lucene.queryparser.classic.ParseException: 
    Cannot parse 'field:"value': Lexical error at line 1, column 13.  
    Encountered: <EOF> after : "\"value"

Using Elasticsearch 5.4 thread_pool has an underscore it it.

thread_pool.search.queue_size: 10000

See documentation at Elasticsearch Thread Pool module documentation

For me tuning the threadpool search queue_size solved the issue. I tried a number of other things and this is the one that solved it.

I added this to my elasticsearch.yml

threadpool.search.queue_size: 10000

and then restarted elasticsearch.

Reasoning... (from the docs)

A node holds several thread pools in order to improve how threads memory consumption are managed within a node. Many of these pools also have queues associated with them, which allow pending requests to be held instead of discarded.

and for search in particular...

For count/search operations. Defaults to fixed with a size of int((# of available_processors * 3) / 2) + 1, queue_size of 1000.

For more information you can refer to the elasticsearch docs here...

I had trouble finding this information so I hope this helps others!