Elasticsearch has a thread pool and a queue for search per node.
A thread pool will have N number of workers ready to handle the requests. When a request comes and if a worker is free , this is handled by the worker. Now by default the number of workers is equal to the number of cores on that CPU.
When the workers are full and there are more search requests, the request will go to queue. The size of queue is also limited. If by default size is, say, 100 and if there happens more parallel requests than this, then those requests would be rejected as you can see in the error log.
Solutions:
-
The immediate solution for this would be to increase the size of
the search queue. We can also increase the size of threadpool,
but then that might badly affect the performance of individual
queries. So, increasing the queue might be a good idea. But then
remember that this queue is memory residential and increasing the
queue size too much can result in Out Of Memory issues. (more
info) -
Increase number of nodes and replicas – Remember each node has its
own search threadpool/queue. Also, search can happen on primary
shard OR replica.