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MDSL

A domain-specific language to specify (micro-)service contracts and their data representations (realizing the API Description pattern from MAP)

MDSL Grammar Quick Reference (Skeletons)

Service Contract Skeleton

One can start from this, e.g., by copy-pasting into a text editor or the Eclipse editor that Xtext generates from the MDSL grammar. [...] are placeholders to be replaced with correct MDSL: 

API description [name]
usage context [visibility] for [direction] // MAP pattern tags (optional)

data type [name] [...] // reusable data contract elements (optional) 

endpoint type [name]  
  version x.y.z // semantic versioning information (optional) 
  serves as [role_enum] // MAP tag(s) (optional)
  exposes 
  	operation [name]
	  with responsibility [resp_enum] // MAP tag (optional)
	  expecting
	    headers [...] // optional 
		payload [...] // mandatory, e.g., {V}
	  delivering  
	    headers [...] // optional
		payload [...] // mandatory in request-response exchanges
	  reporting 
	  	[...] // see bottom of page for explanation (optional)

Usage Context

The valid values for API visibility are:

PUBLIC_API COMMUNITY_API SOLUTION_INTERNAL_API

The API type or direction can be:

FRONTEND_INTEGRATION BACKEND_INTEGRATION

Roles and Responsibilities

MAP defines the following roles (of endpoint types):

PROCESSING_RESOURCE INFORMATION_HOLDER_RESOURCE CONNECTOR_RESOURCE LOOKUP_RESOURCE
MASTER_DATA_HOLDER TRANSACTIONAL_DATA_HOLDER STATIC_DATA_HOLDER  

And the responsibilities (of operations) are:

COMPUTATION_FUNCTION RETRIEVAL_OPERATION EVENT_PROCESSOR BUSINESS_ACTIVITY_PROCESSOR

All these enum values correspond to Microservice API Patterns (MAP); go to the pattern index for quick access.

Data Contract Skeletons (for Operation Signatures)

Data types can be modelled under data type and then referenced in headers, payloads, and reports within operations (expecting, delivering, reporting) in the above skeleton. They can also be inlined directly in the operation definitions.

Skeleton data contracts for headers and payload elements and data type definitions are:

Atomic Parameter syntax

The ap from the above contract skeleton resolves to <<pattern>>"name": Role<Type>. A first example featuring all parts hence is <<API_Key>>"accessToken1":V<string>.

Pattern stereotype and type information are optional; either the role or the name of the representation element have to be specified. Theefore, more compact examples are ID, V<bool>, L<string>, and "resultCounter":MD<int>.

Furthermore, an abstract, unspecified element can be represented as P (for parameter or payload part). It must not contain a stereotype or role and type information (but can have an identifier).

<<pattern>> Stereotype (optional)

The <<pattern>> stereotype can take one of the following values from the Microservice API Patterns (MAP) pattern language:

API_Key Context_Representation Request_Bundle Request_Condition
Wish_List Wish_Template Pagination Error_Report
Embedded_Entity Linked_Information_Holder Annotated_Parameter_Collection “other”
ControlMetadata AggegratedMetadata ProvenanceMetadata  

A pattern stereotype can also be assigned to other tree nodes (apls and ptrees). This is optional.

"name" Identifier

An identifier can, but does not have to be defined (if the role information is present). It appears in double quotes. At present, names must start with a character and must not contain blanks. Special characters such as - are not permitted (yet).

Role Element role stereotypes

The roles match the four element stereotype patterns in MAP:

V(alue) ID(entifier) L(ink) MD (Metadata)

Value corresponds to Entity Element; the other mappings are straightforward.

<Type> Base types

Finally, the base types are:

bool int long double string raw void

Cardinalities (Multiplicity)

MDSL does not know an array construct as known from the type system in most programming languages. Cardinalities are marked with single-byte flags instead (as also used to define regular expressions):

The cardinality markers can be applied to Atomic Parameters, Atomic Parameter Lists,1 and Parameter Trees. Two examples are "listOfIntegers":V<int>* and "optionalInformation": {"part1":V, "part2":V}?. If there is no suffix, the default is ! (exactly one, mandatory).

Note: The marker might be a bit hard to notice, especially when deeply nested structured are modeled. You can increase readability by introducing external data type definitions (see data contracts page).

Variability (Choice)

In the definition of a Parameter Tree {...|...} and an Atomic Parameter List (...|...), you can express optionality:

An example is data type AnIntegerOrAString {V<int> | V<string>}.

Reporting

MDSL has an specific construct for error handling such as fault elements or response codes (still experimental):

Add the following snippet (behind delivering):

reporting 
    error [...]

The placeholder resolves to a data contract (see above). Some examples are:

The report elements can be modeled as data types as described under data contracts (schemas). Example:

More on service endpoint contract types, data contracts (schemas), and instance-level constructs.

Back to MDSL homepage.

Copyright: Olaf Zimmermann, 2019. All rights reserved.

  1. Note that the Atomic Parameter List, introduced above, actually is closer to a sequence or a map in programming languages. ↩