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Microservice DSL (MDSL)

A Domain-Specific Language (DSL) to specify (micro-)service contracts, their data representations and API endpoints.

PrimerEndpoint TypesAPI Provider and ClientTutorialToolsTransformations


Microservice Domain-Specific Language (MDSL) abstracts from technology-specific interface description languages such as OpenAPI/Swagger, WSDL, and Protocol Buffers.

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A First Example

An API with a single endpoint HelloWorldEndpoint that exposes a single operation called sayHello can be modelled as this:

API description HelloMDSLWorld

data type SampleDTO {ID, D<string>} 

endpoint type HelloWorldEndpoint
  operation sayHello 
    expecting payload "in": D<int>  
    delivering payload SampleDTO

API provider HelloWorldAPIProvider
  offers HelloWorldEndpoint 
  at endpoint location "http://localhost:8000" 
  via protocol HTTP 
    binding resource HomeResource at "/"
      operation sayHello to POST 
API client HelloWorldAPIClient
  consumes HelloWorldEndpoint 
  from HelloWorldAPIProvider
  via protocol HTTP 

sayHello accepts a single scalar string value D<string> as input. This operation returns a Data Transfer Object (DTO) called SampleDTO as output, which is modeled explicitly so that its specification can be reused. This SampleDTO is specified incompletely as an identifier-data pair {ID, D}. Its two elements are an identifier ID and some data D. The names of these two “parameters” have not been specified yet. The data element D is a string; the type of the ID parameter is not unspecified. This is different from "in": D<int>, the data send in the request message of sayHello, which gives its single parameter a name, a role, and a type.

In addition to the endpoint type (a.k.a. service contract) HelloWorldEndpoint, an API client and an API provider working with this contract are defined (and bound to HTTP, a single home resource in this simple case).

Take a look at Hello World in Swagger/OpenAPI Specification in comparison. You can find such contract specification example here (note: this OpenAPI specification contains a few more details about the HTTP binding of the abstract contract).

An advanced example showing more contract feature appears in the MDSL Primer.


Language Specification Elements

  1. Service endpoint contract types follow this template:
    • endpoint type ... exposes operation ... expecting ... delivering ...
  2. Data contracts (schemas)
    • data type SampleDTO {ID, V} in the above example
  3. Optional instance-level concepts:
    • API Provider with protocol bindings and, optionally, Service Level Agreements and evolution strategies such as Two in Production
    • API Client instances consuming the contracts exposed by providers
    • API Gateways (or intermediaries) acting both in client and in provider role; note that gateways are not featured in the simple example above
  4. Integration scenarios including user/job/test stories
  5. Orchestration flows.

Next Steps

MDSL Tools and Their Application

The following tools already support MDSL (see MDSL Tools page for more information):

Context Mapper can generate MDSL from strategic and tactic domain-driven design models.

Two blog posts, “Story-Driven Service Design: From Feature Story to Minimum Viable API Product” and “Event-Driven Service Design: Five Steps from Whiteboard to OpenAPI and Camel Flow” feature these MDSL tools in action.

MDSL Documentation (this website)

Copyright: Olaf Zimmermann, 2018-2022. All rights reserved. See license information.