Installing the Content Service in a migration scenario
For a RESTful, service-based setup of Content Delivery to work, you must install the Content Service in a Content Delivery environment if both of the following are true: one, you never installed the Content Delivery Web service Server Role, and two, you did not install the Session-enabled Content Service Server Role. So if you are migrating to the new architecture, either directly from an earlier version or from a setup you already upgraded to SDL Web 8.5, installing either Content Service or Session-enabled Content Service is mandatory. You can run the Content Service as a standalone microservice. It is not yet necessary to install the Content Service if you are doing a minimal upgrade only; that is, if you are continuing to use the in-process APIs for the time being.