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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)
	protected by [...] // 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	DATA_TRANSFER_RESOURCE	LINK_LOOKUP_RESOURCE
OPERATIONAL_DATA_HOLDER	MASTER_DATA_HOLDER	REFERENCE_DATA_HOLDER

And the responsibilities (of operations) are:

COMPUTATION_FUNCTION

STATE_CREATION_OPERATION

RETRIEVAL_OPERATION

STATE_TRANSITION_OPERATION

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:

• Parameter Tree (ptree): { {subtree1}, {subtree2}, ap1, ap2, ...}, can appear in pforest
• Atomic Parameter (ap), can appear in ptree and apl: <<pattern>>"name": Role<Type> (see below for explanations)
• Atomic Parameter List (apl): (ap1, ap2, ...), can appear in ptree (note: can and should be modeled as a flat ptree)
• Parameter Forest (pforest): [ {ptree1}; {ptree2}; ... ], can only appear on top level of operation signature

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":D<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, D<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
Data_Element	Metadata_Element	Identifier_Element	Link_ELement
Control_Metadata	Aggregated_Metadata	Provenance_Metadata

Data_Element is the default; L is a shorthand for <<Link_ELement>> D, ID is short for <<Identifier_ELement>> D, and MD is short for <<Metadata_ELement>> D.

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:

D(ata)

MD (Metadata)

ID(entifier)

L(ink)

Data corresponds to the Data Element pattern; the other mappings are straightforward as well.

<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):

• ? means that an element is optional (i.e., none or one instance may appear)
• * represents “any” (i.e., zero or more)
• + indicates that at least one instance of the specified element must appear (i.e., one or more)

The cardinality markers can be applied to Atomic Parameters, Atomic Parameter Lists,¹ and Parameter Trees. Two examples are "listOfIntegers":D<int>* and "optionalInformation": {"part1":D, "part2":D}?. 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:

• | choice

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

Reporting

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

Add the following snippet to the specification of response messages (behind delivering):

reporting 
    error [...]

The placeholder [...] resolves to a data contract (see above). The simplest one is P. Some more advances examples are:

• `error BadRequest “MoreInformation”: {D}

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

• error "soapFault": SOAPFaultElement, with a previous definition:
• data type SOAPFaultElement {"code":D<int>, "string": D<string>, "actor":D<string>, "detail":D<string>}

Security Policy

Finally, a security policy can be specified for each operation:

• protected by policy "UserIdPassword":{"userId":ID<string>, "password":MD<string>}

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

Links

More on service endpoint contract types, data contracts (schemas), bindings and instance-level constructs. Tutorial and tools. Back to MDSL homepage.

Copyright: Olaf Zimmermann, 2018-2020. All rights reserved. See license information.

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