The shards preference on a search request enables specifying a list of
shards to hit, and then a secondary preference (e.g., "_primary") can be
added. Today, the separator between the shards list and the secondary
preference is ';'. Unfortunately, this is also a valid separtor for URL
query parameters. This means that a preference like "_shards:0;_primary"
will be parsed into two URL parameters: "_shards:0" and "_primary". With
the recent change to strict URL parsing, the second parameter will be
rejected, "_primary" is not a valid URL parameter on a search
request. This means that this feature has never worked (unless the ';'
is escaped, but no one does that because our docs do not that, and there
was no indication from Elasticsearch that this did not work). This
commit changes the separator to '|'.
Relates #20786
This commit removes the search preference _only_node as the same
functionality can be obtained by using the search preference
_only_nodes. This commit also adds a test that ensures that _only_nodes
will continue to support specifying node IDs.
Relates #18875
The search preference _prefer_node allows specifying a single node to
prefer when routing a request. This functionality can be enhanced by
permitting multiple nodes to be preferred. This commit replaces the
search preference _prefer_node with the search preference _prefer_nodes
which supplants the former by specifying a single node and otherwise
adds functionality.
Relates #18872
Just like specifying `?preference=_primary`, this adds the ability to
specify `?preference=_replica` or `?preference=_replica_first` on
requests that support it.
Resolves#12222
Jason Tedor
Isabel Drost-Fromm
Lee Hinman
Nirmal Chidambaram
Clinton Gormley