The MediaWiki core has support for a special [[ https://wikitech.wikimedia.org/wiki/X-Wikimedia-Debug | X-Mediawiki-Debug header ]]. It allows to select which particular backend servers will run the request, allowing to debug and test the code without deploying it everywhere or relying on luck that a certain request hit a certain backend. This is extremely useful and services need a similar thing.
As a bare minimum, the header should support `service_name=server` list that would allow to select a specific node to request whenever requesting #restbase, #parsoid , #mobile-content-service or any other server. The header should be passed through on all the internal communications between the services, so that all request to a particular service ends up on a particular node. Should also support selecting a MediaWiki API node.