  In this paper, we propose a general architecture for designing language
servers for hybrid modeling languages, that is, modeling languages that contain
both textual and graphical representations. The architecture consists of a
textual language server, a graphical language server, and a client that
communicates with the two servers. The servers are implemented using the
Language Server Protocol (LSP) and the Graphical Language Server Protocol
(GLSP) and are based on a shared abstract syntax of the hybrid language. This
means that only static resources need to be common between the graphical and
textual language servers. The servers' separation allows each to be developed
and maintained independently, while also enabling forward-compatibility with
their respective dependencies.
  We describe a prototype implementation of our architecture in the form of a
hybrid editor for the UML-RT language. The evaluation of the architecture via
this prototype gives us useful insight into further generalization of the
architecture and the way it is used. We then sketch a suitable extension of the
architecture to enable support for multiple diagram types and, thus, multiple
graphical views.
