This is a protection mechanism to prevent a single search request from
hitting a large number of shards in the cluster concurrently. If a search is
executed against all indices in the cluster this can easily overload the cluster
causing rejections etc. which is not necessarily desirable. Instead this PR adds
a per request limit of `max_concurrent_shard_requests` that throttles the number of
concurrent initial phase requests to `256` by default. This limit can be increased per request
and protects single search requests from overloading the cluster. Subsequent PRs can introduces
addiontional improvemetns ie. limiting this on a `_msearch` level, making defaults a factor of
the number of nodes or sort shards iters such that we gain the best concurrency across nodes.
Now that we have incremental reduce functions for topN and aggregations
we can set the default for `action.search.shard_count.limit` to unlimited.
This still allows users to restrict these settings while by default we executed
across all shards matching the search requests index pattern.
This commit adds the new `action.search.shard_count.limit` setting which
configures the maximum number of shards that can be queried in a single search
request. It has a default value of 1000.
Simon Willnauer
Nik Everett
jasonquick
Adrien Grand
javanna
Joel Taddei
Clinton Gormley