With the upcoming cross-cluster search alternate execution mode, the CCS
node will be able to split a CCS request into multiple search requests,
one per remote cluster involved. In order to do that, the CCS node has
to be able to signal to each remote cluster that such sub-requests are
part of a CCS request. Each cluster does not know about the other
clusters involved, and does not know either what alias it is given in
the CCS node, hence the CCS coordinating node needs to be able to provide
the alias as part of the search request so that it is used as index prefix
in the returned search hits.
The cluster alias is a notion that's already supported in the search shards
iterator and search shard target, but it is currently used in CCS as both
index prefix and connection lookup key when fanning out to all the shards.
With CCS alternate execution mode the provided cluster alias needs to be
used only as index prefix, as shards are local to each cluster hence no
cluster alias should be used for connection lookups.
The local cluster alias can be set to the SearchRequest at the transport layer
only, and its constructor/getter methods are package private.
Relates to #32125
51fe20e0c3