Towards a Graphical Language Server Protocol for Modeling Tools
The use of graphical concrete syntax is one of the most common approaches to express and visualize the concepts of a modeling language. With the help of specialized modeling tools developers are able to create and edit models based on their graphical representation. Traditional graphical modeling editors are powerful, heavy weight desktop applications that are strongly tailored towards a particular language and built on top of existing modeling frameworks like the Eclipse Modeling Framework (EMF). This causes major limitations in terms of re-usability and extensibility. It’s often not possible to develop a graphical language that is supported by multiple modeling tools without major modi cations and the anguage has to be integrated into each platform/tool separately. Another aspect that has to be considered is the rise of web technologies, and in particular cloud based concepts, that can be observed over the last couple of years. This trend causes a shift in Model-Driven Engineering resulting in an increased interest to migrate existing model editing tools to the web. The motivations driving this shift are twofold: increased usability through the usage of modern UI technology, such as TML5 and CSS, and simpli ed deploy ability via a browser. In addition, this enables new opportunities like cloud-based collaborative modeling tools but at the same time opens new challenges and potential problems. Web-based solutions require a decoupled architecture composed of dedicated client and server artifacts. However due to their monolithic nature current state of the art modeling tools are not suitable for distributed deployment. The development of full- edged graphical modeling tools is a challenging and complex task  so re-implementing them on a web-based technology stack is often not feasible. Instead a proper approach to develop and maintain modern web modeling tools whilst reusing well established modeling platforms has to be found.
These problems are not only limited to the context of graphical languages but are also applicable to other language development areas. Looking at textual programming languages similar issues have been concluded and led to the creation of
the Language Server Protocol (LSP). This eort eectively decouples textual language IDE development into a client-server architecture. The client is generic and language-agnostic which means an LSP client can (potentially) connect to
any number of language-speci c servers via standardized protocol. Following a similar path for graphical languages seems logical but it’s currently not clear how to properly achieve this. Technically LSP provides a powerful extension mechanism which would allow the implementation of a graphical language server protocol on top of LSP. In practice this approach has some major downsides and might even be insucient to cover all common use cases. In order
to use LSP each graphical language would have to conform to a textual language. Since not all potential graphical languages are or even can be derived from textual artifacts another solution might be preferable.
It also has to be considered that the concept of completely language-agnostic clients might not be applicable in this context. Graphical language clients need a lot additional information to properly identify model elements, render their
graphical representation and to be able to trigger model modi cations.