Problem: So far all rank eval requests are being executed in parallel. If there
are more than the search thread pool can handle, or if there are other search
requests executed in parallel rank eval can fail.
Solution: Make number of max_concurrent_searches configurable.
Name of configuration parameter is analogous to msearch. Default
max_concurrent_searches set to 10: Rank_eval isn't particularly time critical so
trying to avoid being more clever than probably needed here. Can set this value
through the API to a higher value anytime.
Fixes#21403
Problem: We introduced the ability to shorten the rank eval request by using a
template in #20231. When playing with the API it turned out that there might be
use cases where - e.g. due to various heuristics - folks might want to translate
the original user query into more than just one type of Elasticsearch query.
Solution: Give each template an id that can later be referenced in the
actual requests.
Closes#21257
* Reference documentation for rank evaluation API
This adds a first page of reference documentation to the current state of the
rank evaluation API.
Closes to #21402
* Add default values for precision metric.
Add information on default relevant_rating_threshold and ignore_unlabeled
settings.
Relates to #21304
* Move under search request docs, fix formatting
Also removes some detail where it seemed unneeded for reference docs
Christoph Büscher
Isabel Drost-Fromm
Isabel Drost-Fromm