metamodeling vs metaprogramming, a case study on developing client libraries for rest apis

ECMFA 2016: Metamodeling vs Metaprogramming

Metamodeling vs MetaprogrammingA Case Study on Developing Client Libraries for REST APIs

Markus [email protected]

@mscheidgen

Frederik [email protected]

Sven [email protected]

@svenefftinge

1

mailto:[email protected]







RESTful Web ApplicationsMetamodeling vs Metaprogramming

Markus [email protected]

@mscheidgen

Frederik [email protected]

Sven [email protected]

@sefftinge

2








Agenda

■ Model-driven for Web Applications

■ Representational State Transfer (REST)

■ Metamodeling: external DSL

■ Metaprogramming: Active Annotations

■ Conclusions

3


MDD and WEB REST Metamodeling Active Annotations Conclusions

Web Applications4

Endpoints

GWT: Java

Javascript

Cloud Storagee.g. from Google

Hypertext Transfer Protocol (HTTP)

Cliente.g. Web Browser

Servere.g. App Engine



Web Applications – Horizontal “Platforms”5

Endpoints

GWT: Java

Javascript

Graph Databasee.g. Neo4J

Servlet: JavaRuby JavascriptPython PHP

Document Databasee.g. MongoDB

Cloud Stoagee.g. from Google

SQL Databasee.g. MySQL


JavascriptJavae.g. on Android

Objective-Ce.g. on iOS

Dart




Endpoints

GWT: Java

Javascript









Dart



Web Applications – Vertical “Platforms”8

Endpoints

GWT: Java

Javascript









Dart

object.name = “John”

json.put(“name”, “John”);

bean.setName(“John”);

entity.set(“name”,“John”);

{ name : “John” }


Introduction REST Metamodeling Active Annotations Conclusions

Representational State Transfer (REST)

■ Set of principles for distributed architectures by Roy Fielding

■ Most importantly: stateless server

■ Allows robust, scalable, easy to build and easy to maintain server

■ In practice almost always synonymous with web-services based on HTTP and JSON

■ Examples: Facebook, Twitter, Google+, Youtube, ...

9

CLIENT

10

GET api.twitter.com/1.1/

statuses/user_timeline.json?screen_name=”mscheidgen”

GET api.twitter.com/1.1/

statuses/user_timeline.json?screen_name=”mscheidgen”&

max_id=”37..3”

200 OK[ { id : 1233364542, text : “Something els”, favorits : 21, retweets = 3, user : { screen_name : “mscheidgen”, name : “Markus Scheidgen”, follower : 1021, ... }, ... }, ... { id : 36462522 text : “Check out the new ...”, ... }]

SERVER

200 OK [ { id : 1233364542, text : “Going to ECMFA; #STAF”, favorits : 21, retweets = 3, user : { screen_name : “mscheidgen”, name : “Markus Scheidgen”, follower : 1021, ... }, ... }, ... { id : 374256453 text : “Check out the new ...”, ... }]

reprsentational

state transfer



11

REST – How it works

■ Identify resources via URLs: host, path, and parameters

■ CRUD methods, i.e. HTTP’s PUT, GET, POST, DELETE

■ Resource contents is represented platform independently as JSON (or XML)

■ HATEOAS: ideally hyperlinks in resources connect all resources of one service



REST – Metrics and Flaws

+ HTTP and JSON are platform independent, libraries for most systems and programming languages exist

- HTTP and JSON represent the smallest common denominator for most systems and programming languages

- Loads of platform specific boilerplate code to process HTTP and JSON

- No static types for URL and JSON data, no static safety

12



REST – Client Libraries13

val request = HTTP.get("api.twitter.com/1.1/"statuses/user_timeline.json")request.queryString("screen_name", "mscheidgen")

val str = request.asString()val jsonNode = JSON.parse(str)val jsonArray = jsonNode.asArray()

for (i:0..<jsonArray.length) { val jsonItem = jsonArray.get(i); println(jsonItem.get("text"))}

val result = twitter.statuses.userTimeline. userId("mscheidgen").xResult() for (item:result) { println(item.text)}

exists?

right type?

is it an array?

exists/right type?

save



REST – Client Libraries

■ language specific, uses language features, i.e. type safety

■ turns HTTP request/response RPCs into regular method calls

■ comprises

■ a method for each API function

■ a wrapper type for each API resource type

■ Client libraries consist of very canonical, schematic boilerplate code. They are dull, tedious, and error prone to make.

14



Codethe desired platform code, e.g. a REST client library


Code-Generatora tool that processes a metamodel instance to generates code


Platform/Language

Platform/Language

Metamodeling15

Metamodeldefines language constructs,e.g. constructs to define REST request and resource types

Modeluses the given language constructs, e.g. to describe a REST API



Metametamodelthe metamodeling language

Code-Generator Language

Platform/Language



A Metamodel for REST APIs16

method:Methodspath:EString

Request

separator:EString

Parameter

PrimitiveDataType

ComplexDataType

Response

Field

GETPOSTPUTDELETE

«enum»Methods

type 1

response 1{subsets features}

pathPattern:EString

PathParameter

baseURL:EString

REST-API

features 0..*

parameters 0..*{subsets features}

Class

array:EBoolean

Feature

DataType

name:EString

NamedElement

0..* parameters{redefines features} 0..*

dataTypes

0..*requests

QueryParameter

BodyParameter

get request Timeline for “statuses/user_timeline.json”

returns array of Status {query max_id : Stringquery screen_name : String

}

datatype Status {id : Stringtext : Stringfavorits : Integerretweets : Integeruser : User

}

datatype User {screen_name : Stringname : String...

}



xTend17

@request(path=“statuses/user_timeline.json”, response=@Response(type=Status, isArray=true))class Timeline {

String max_idString screen_name

}

@JSON class Status {String idString textint favoritsint retweetsUser user

}

@JSON class User {String screen_nameString name...

}

get request Timeline for “statuses/user_timeline.json”

returns array of Status {query max_id : Stringquery screen_name : String

}

datatype Status {id : Stringtext : Stringfavorits : Integerretweets : Integeruser : User

}

datatype User {screen_name : Stringname : String...

}



Metaprogramming xTend – Active Annotations18

@Request(path="statuses/user_timeline.json", response=@Response(type=Status, isArray=true))class Timeline { String max_id String screen_name}

public class Timeline extends AbstractRequest<List<Status>> { @Override public List<Status> xResult() { JSONArray jsonArray = xResponse().getJSONArray(""); List<Status> result = new ArrayList<Status>(); for (int i = 0; i < jsonArray.length(); i++) { result.add(new Status(jsonArray.getJSONObject(i))); } return Collections.unmodifiableList(result); } ...

xTend compiler + RequestClassProcessor implementsRegisterGlobalsParticipantTransformationParticipantCodeGenerationParticipantValidationParticipant



Active Annotations – Simple Examples19

@AddConstructorclass Customer { private val String id @Accessors(PUBLIC_GETTER, PROTECTED_SETTER) private String name}

public class Customer { private final String id; private String name; public Customer(final String id) { super(); this.id = id; } public String getName() { return this.name; } protected void setName(final String name) { this.name = name; }}



Metamodeling vs Metaprogramming20

xTend Language

xTend Codee.g. uses annotations to describe REST request and resource types

xTend Compiler

Codethe desired Java code, e.g. a REST client library

Active Annotations Class Processors

Codethe desired platform code, e.g.


Code-GeneratorCode-GeneratorMetamodel

Model

Code-Generator




Beyond REST – Active Annotation and Web21

Endpoints

GWT: Java

Javascript

Cloud Storagee.g. from Google

HTTP

@GWTJavaScriptObject@JSONObject@JavaBean@CloudStorageEntityclass User {

String screen_nameString name...

}

object.name = “John”

json.put(“name”, “John”);

bean.setName(“John”);

entity.set(“name”,“John”);

{ name : “John” }



Beyond REST – Active Annotation and Web22

class Statuses { @com.google.endpoints.ApiMethod( name=“statuses/user_timeline.json”) @RequestFromEndpoints @ApiMethodMockup List<StatusBean> getTimeline( String screenName, String maxId) { ... }}

@Request(path=“statuses/user_timeline.json”, response=@Response(type=Status, isArray=true))class Timeline {

String max_idString screen_name

}

REST Client Library Request Code

Server CodeMockup Skeletona bare bone implementa-tion for tests

HTTP



Conclusions

■ Model-driven can be used for Web application development, especially for REST APIs

■ Active annotations can be used like stereotype to extend an existing programming language

■ Active annotation are an internal DSL alternative to custom code-generation

23



Questions24

xTend Language

xTend Codee.g. uses annotations to describe REST request and resource types

xTend Compiler

Codethe desired Java code, e.g. a REST client library

Active Annotations Class Processors



Code-GeneratorCode-GeneratorMetamodel

Model

Code-Generator


metamodeling vs metaprogramming, a case study on developing client libraries for rest apis

Technology