The MediaWiki core has support for a special X-Wikimedia-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.