A unified view approach to software development automation

These properties are defined based on a conceptual layeredsoftware model that includes the domain model at the core, an intermediate module layersurrounding this core and an outer softwa

Vietnam National University, Hanoi

VNU University of Engineering and Technology

ĐẠI HỌC QUỐC GIA HÀ NỘI

TRƯỜNG ĐẠI HỌC CÔNG NGHỆ

LÊ MINH ĐỨC

PHƯƠNG PHÁP TIẾP CẬN KHUNG NHÌN HỢP NHẤT CHO

TỰ ĐỘNG HÓA PHÁT TRIỂN PHẦN MỀM

LUẬN ÁN TIẾN SĨ NGÀNH CÔNG NGHỆ THÔNG TIN

Vietnam National University, Hanoi

VNU University of Engineering and Technology

LE MINH DUC

A Unified View Approach to

Software Development Automation

Specialisation: Software Engineering

Code: 9480103.01

Doctor of Philosophy Dissertation

in Information Technology

Supervisors:

1 Assoc Prof., Dr Nguyen Viet Ha

2 Dr Dang Duc Hanh

ĐẠI HỌC QUỐC GIA HÀ NỘI

TRƯỜNG ĐẠI HỌC CÔNG NGHỆ

LÊ MINH ĐỨC

PHƯƠNG PHÁP TIẾP CẬN KHUNG NHÌN HỢP NHẤT CHO

TỰ ĐỘNG HÓA PHÁT TRIỂN PHẦN MỀM

Chuyên ngành: Kỹ thuật Phần mềm

Mã số: 9480103.01

LUẬN ÁN TIẾN SĨ NGÀNH CÔNG NGHỆ THÔNG TIN

NGƯỜI HƯỚNG DẪN KHOA HỌC:

1 PGS TS Nguyễn Việt Hà

2 TS Đặng Đức Hạnh

I hereby declare that the materials presented in this dissertation are my own work, conductedunder the supervision of Assoc Prof., Dr Nguyen Viet Ha and Dr Dang Duc Hanh, atthe Faculty of Information Technology, University of Engineering and Technology, VietnamNational University, Hanoi All the research data and results presented in this dissertationare authentic and (to the best of my knowledge) have not previously been published in anyacademic publications by other authors

Le Minh Duc

An important software engineering methodology that has emerged over the past twentyyears is model-based software development At the heart of this methodology lies twocomplementary methods: model-driven software engineering (MDSE) and domain-drivendesign (DDD) While the aim of MDSE is ambitiously broad, DDD’s goal is more modestand direct but not less important – to apply model-based engineering techniques to tackle thecomplexity inherent in the domain requirements The state-of-the-art DDD method includes

a set of principles for constructing a domain model that is feasible for implementation in atarget programming language However, this method lacks the solutions needed to addressthe following important design questions facing a technical team when applying DDD in

object oriented programming language (OOPL) platforms: (i) what constitues an essentially expressive domain model and (ii) how to effectively construct a software from this model.

The dissertation aims to address these limitations by using annotation-based domain-specificlanguage (aDSL), which is internal to OOPL, to not only express an essential and unifieddomain model but generatively construct modular software from this model

First, we propose an aDSL, named domain class specification language (DCSL), whichconsists in a set of annotations that express the essential structural constraints and the essentialbehaviour of a domain class We carefully select the design features from a number ofauthoritative software and system engineering resources and reason that they form a minimumdesign space of the domain class

Second, we propose a unified domain (UD) modelling approach, which uses DCSL toexpress both the structural and behavioural modelling elements We choose UML activitydiagram language for behavioural modelling and discuss how the domain-specific constructs

of this language are expressed in DCSL To demonstrate the applicability of the approach wedefine the UD modelling patterns for tackling the design problems posed by five core UMLactivity flows

Third, we propose a 4-property characterisation for the software that are constructeddirectly from the domain model These properties are defined based on a conceptual layeredsoftware model that includes the domain model at the core, an intermediate module layersurrounding this core and an outer software layer.

Fourth, we propose a second aDSL, named module configuration class language (MCCL),that is used for designing module configuration classes (MCCs) in a module-based softwarearchitecture An MCC provides an explicit class-based definition of a set of module con-figurations of a given class of software modules The MCCs can easily be reused to createdifferent variants of the same module class, without having to change the module class design.Fifth, we develop a set of software tools for DCSL, MCCL and the generators associatedwith these aDSLs We implement these tools as components in a software framework, namedjDomainApp, which we have developed in our research

To evaluate the contributions, we first demonstrate the practicality of our method byapplying it to a relatively complex, real-world software construction case study, concerningorganisational process management We then evaluate DCSL as a design specification lan-guage and evaluate the effectiveness of using MCCL in module-based software construction

We focus the latter evaluation on module generativity

We contend that our contributions help make the DDD method more concrete and morecomplete for software development On the one hand, the method becomes more concretewith solutions that help effectively apply the method in OOPL platforms On the other hand,the method is more complete with solutions for the design aspects that were not originallyincluded

Tóm tắt

Trong vòng hai thập kỷ gần đây, phương pháp luận phát triển phần mềm dựa trên mô hình nổi lên là một phương pháp luận quan trọng trong kỹ nghệ phần mềm Ở trung tâm của phương pháp luận này có hai phương pháp có tính bổ trợ nhau là: kỹ nghệ phần mềm hướng mô hình (model-driven software engineering (MDSE)) và thiết kế hướng miền (domain-driven design (DDD)) Trong khi MDSE mang một mục tiêu rộng và khá tham vọng thì mục tiêu của DDD lại khiêm tốn và thực tế hơn, đó là tập trung vào cách áp dụng các kỹ thuật của kỹ nghệ dựa trên mô hình để giải quyết

sự phức tạp vốn có trong yêu cầu miền Phương pháp DDD hiện tại bao gồm một tập các nguyên lý để xây dựng một mô hình miền ở dạng khả thi cho triển khai viết

mã trên một ngôn ngữ lập trình đích Tuy nhiên phương pháp này còn thiếu các giải pháp cần thiết giúp giải đáp hai câu hỏi quan trọng mà người phát triển phần mềm thường gặp phải khi áp dụng DDD vào các nền tảng ngôn ngữ lập trình hướng đối tượng (object oriented programming language (OOPL)): (i) những thành phần nào cấu tạo nên một mô hình miền có mức độ diễn đạt thiết yếu? và (ii) xây dựng một cách hiệu quả phần mềm từ mô hình miền như thế nào? Luận án này đặt mục đích khắc phục hạn chế trên của DDD bằng cách sử dụng ngôn ngữ chuyên biệt miền dựa trên ghi chú (annotation-based domain-specific language (aDSL)), được phát triển trong OOPL, để không chỉ biểu diễn một mô hình miền hợp nhất thiết yếu mà còn để xây dựng phần mềm có tính mô-đun từ mô hình miền này.

Thứ nhất, luận án đề xuất một aDSL, tên là ngôn ngữ đặc tả lớp miền (domain class specification language (DCSL)), bao gồm một tập các ghi chú để biểu diễn các ràng buộc cấu trúc thiết yếu và các hành vi thiết yếu của lớp miền Tác giả đã cẩn

kỹ nghệ phần mềm và kỹ nghệ hệ thống và lập luận rằng các đặc trưng này tạo thành một không gian thiết kế tối giản cho lớp miền.

Thứ hai, luận án đề xuất một phương thức tiếp cận mô hình hóa miền hợp nhất, trong đó sử dụng DCSL để biểu diễn các thành phần mô hình hóa cấu trúc và hành

vi Luận án đã chọn ngôn ngữ biểu đồ hoạt động UML cho mô hình hóa hành vi và trình bày cách biểu diễn các đặc trưng chuyên biệt trạng thái của ngôn ngữ này bằng DCSL Để chứng tỏ tính thực tiễn của cách tiếp cận, luận án định nghĩa một tập mẫu

mô hình hóa miền hợp nhất cho các bài toán thiết kế liên quan trực tiếp đến năm luồng hoạt động UML cơ bản.

Thứ ba, luận án đề xuất một mô tả đặc điểm gồm bốn tính chất cho phần mềm được xây dựng trực tiếp từ mô hình miền Bốn tính chất này được định nghĩa dựa trên

mô hình khái niệm phần mềm dạng phân lớp, bao gồm mô hình miền ở lớp lõi, một lớp mô-đun trực tiếp bao quanh lớp lõi và một lớp phần mềm ở ngoài.

Thứ tư, luận án đề xuất một aDSL thứ hai, tên là ngôn ngữ lớp cấu hình mô-đun (module configuration class language (MCCL)), dùng để thiết kế các lớp cấu hình mô-đun (module configuration classes (MCCs)) trong một kiến trúc phần mềm dựa trên mô-đun Mỗi MCC cung cấp một định nghĩa dạng lớp cho một tập các cấu hình mô-đun của một lớp mô-đun Các MCC có thể dễ dàng sử dụng lại để tạo ra các biến thể của một lớp mô-đun mà không cần sửa thiết kế bên trong của mô-đun.

Thứ năm, luận án phát triển một bộ công cụ dành cho DCSL, MCCL và các bộ sinh mã của các ngôn ngữ này, dưới dạng các thành phần của một phần mềm khung, tên là JDOMAINAPP Để đánh giá các kết quả trên, luận án trước hết trình diễn tính thực tiễn của phương pháp bằng cách áp dụng vào một trường hợp nghiên cứu tương đối phức tạp về phát triển phần mềm, liên quan đến quản lý quy trình tổ chức Tiếp theo, luận án đánh giá DCSL từ khía cạnh một ngôn ngữ đặc tả và đánh giá hiệu quả việc sử dụng MCCL trong xây dựng mô-đun phần mềm một cách tự động Chúng tôi cho rằng, các đóng góp của luận án giúp phương pháp DDD trở nên cụ thể và đầy đủ hơn Một mặt, phương pháp trở nên cụ thể hơn với các giải pháp giúp áp dụng một cách hiệu quả vào các nền tảng OOPL Mặt khác, phương pháp trở nên đầy đủ hơn

I am deeply grateful for my home university (Hanoi University) for providing the PhDstudentship and a gracious teaching arrangement, that has enabled me to have the time tocomplete the required course works and research I am also very grateful for the financialsupport that I have additionally received from the MOET’s 911 fund and the NAFOSTEDproject (grant number 102.03-2015.25), led by Assoc Prof Nguyen Viet Ha.

I would also like to thank all of my colleagues and fellow PhD students for the manymeaningful and entertaining discussions Last but not least, I wish to thank my family for thesacrifices that they have made and for all the love and encouragement that they have given

me during my PhD study

1.1 Problem Statement 3

1.1.1 Motivating Example 3

1.1.2 Domain-Driven Design Challenges 5

1.1.3 Research Statement 7

1.2 Research Aim and Objectives 7

1.3 Research Approach 8

1.4 Dissertation Structure 12

2 State of the Art 13 2.1 Background 13

2.1.1 Model-Driven Software Engineering 13

2.1.2 Domain-Specific Language 15

2.1.3 Meta-Modelling with UML/OCL 17

2.1.4 Domain-Driven Design 22

2.1.5 Model-View-Controller Architecture 27

2.1.6 Comparing and Integrating MDSE with DDD 28

2.1.7 A Core Meta-Model of Object-Oriented Programming Language 29

2.1.8 Using Annotation in MBSD 33

2.2 Domain-Driven Software Development with aDSL 35

2.2.1 DDD with aDSL 36

2.2.2 Behavioural Modelling with UML Activity Diagram 36

2.2.3 Software Module Design 40

2.2.4 Module-Based Software Architecture 41

2.3 Summary 45

3 Unified Domain Modelling with aDSL 46 3.1 Introduction 46

3.2 DCSL Domain 47

3.2.1 Essential State Space Constraints 47

3.2.2 Essential Behaviour Types 48

3.3 DCSL Syntax 49

3.3.1 Expressing the Pre- and Post-conditions of Method 56

3.3.2 Domain Terms 57

3.4 Static Semantics of DCSL 57

3.4.1 State Space Semantics 58

3.4.2 Behaviour Space Semantics 64

3.4.3 Behaviour Generation for DCSL Model 68

3.5 Dynamic Semantics of DCSL 71

3.6 Unified Domain Model 72

3.6.1 Expressing UDM in DCSL 73

3.6.2 UD Modelling Patterns 76

3.7 Summary 87

4 Module-Based Software Construction with aDSL 88 4.1 Introduction 88

4.2 Software Characterisation 89

4.2.1 An Abstract Software Model 90

4.2.2 Instance-based GUI 91

4.2.3 Model reflectivity 92

4.2.4 Modularity 92

4.2.5 Generativity 94

4.3 Module Configuration Domain 95

4.3.1 One Master Module Configuration 95

4.3.2 The ‘Configured’ Containment Tree 95

4.3.3 Customising Descendant Module Configuration 96

4.4 MCCL Language Specification 97

4.4.1 Specification Approach 97

4.4.2 Conceptual Model 98

4.4.3 Abstract Syntax 104

4.4.4 Concrete Syntax 110

4.4.5 Semantics 114

4.5 MCC Generation 114

4.5.1 Structural Consistency between MCC and Domain Class 114

4.5.2 MCCGEN Algorithm 116

4.6 Summary 118

5 Evaluation 119 5.1 Implementation 119

5.1.1 UD Modelling 119

5.1.2 Module-Based Software Construction 121

5.2 Case Study: ProcessMan 122

5.2.1 Method 122

5.2.2 Case and Subject Selection 122

5.2.3 Data Collection and Analysis 123

5.2.4 Results 123

5.3 DCSL Evaluation 127

5.3.1 Evaluation Approach 127

5.3.2 Expressiveness 129

5.3.3 Required Coding Level 132

5.3.4 Behaviour Generation 133

5.3.5 Performance Analysis 134

5.3.6 Discussion 134

5.4 Evaluation of Module-Based Software Construction 135

5.4.1 Module Generativity Framework 135

5.4.2 MP1: Total Generativity 137

5.4.3 MP2–MP4 138

5.4.4 Analysis of MCCGen 143

5.4.5 Discussion 144

5.5 Summary 144

6 Conclusion 145 6.1 Key Contributions 146

6.2 Future Work 147

Bibliography 150 Appendices A Helper OCL Functions for DCSL’s ASM 158 B MCCL Specification 164 B.1 Library Rules of the MCCL’s ASM 164

B.2 Two MCCs of ModuleEnrolmentMgmt 167

C DCSL Evaluation Data 171 C.1 Expressiveness Comparison Between DCSL and the DDD Frameworks 171

C.2 Level of Coding Comparison Between DCSL and the DDD Frameworks 173

aDSL Annotation-Based DSL, page 36

ASM Abstract Syntax Meta-model, page 17

AtOP Attribute-Oriented Programming, page 35

BISL Behaviour Interface Specification Language, page 35CSM Concrete Syntax Meta-model, page 17

DCSL Domain Class Specification Language, page 51DDD Domain-Driven Design, page 22

DDDAL DDD with aDSLs, page 8

DSL Domain-Specific Language, page 15

JML Java Modelling Language, page 36

MCC Module Configuration Class, page 99

MCCL Module Configuration Class Language, page 99MDA Model-Driven Architecture, page 13

MDD Model-Driven Development, page 13

MDE Model-Driven Engineering, page 13

MDSE Model-Driven Software Engineering, page 13

MVC Model-View-Controller, page 27

OCL Object Constraint Language, page 18

OOPL Object-Oriented Programming Language, page 29PIM Platform-Independent Model, page 14

PSM Platform-Specific Model, page 14

SDM Semantic Domain Meta-model, page 17

UDM Unified Domain Model, page 76

UML Unifield Modelling Language, page 18

UML/OCL UML made precise with OCL, page 18

List of Figures

1.1 A partial domain model of CourseMan 3

1.2 An overview of DDDAL (with an emphasis on phases 1 and 2) 9

2.1 The essential ASM of UML (synthesised from seven meta-models of UML [57]) 18

2.2 The OCL expression meta-model (Adapted from §8.3 [56]) 20

2.3 A UI class of the domain class Student (Source: [45]) 28

2.4 The UML-based ASM of OOPL 30

2.5 The meta-model of UML activity modelling language (Adapted from §15.2.2

of the UML specification [57]) 38

2.6 The UML activity models of five basic variants of the CourseMan’s ment management activity 39

enrol-2.7 The MOSA model of CourseMan 43

2.8 A ModuleEnrolmentMgmt’s view containing a child ModuleStudent’s view 44

3.1 The abstract syntax model of DCSL 50

3.2 A DCSL model for a part of the CourseMan domain model 55

3.3 (A: Left) The UML activity and class models of a CourseMan softwarevariant that handles the enrolment management activity; (B: Right) TheUDM that results 75

3.4 The sequential pattern form (top left) and an application to the enrolmentmanagement activity 77

3.5 The sequential pattern form view of enrolment management activity 78

3.6 The decisional pattern form (top left) and an application to the enrolmentmanagement activity 79

3.7 The decisional pattern form view of enrolment management activity 80

3.8 The forked pattern form (top left) and an application to the enrolment

man-agement activity 81

3.9 The forked pattern form view of enrolment management activity 82

3.10 The joined pattern form (top left) and an application to the enrolment man-agement activity 83

3.11 The joined pattern form view of enrolment management activity 84

3.12 The merged pattern form (top left) and an application to the enrolment man-agement activity 85

3.13 The merged pattern form view of enrolment management activity 86

4.1 A detailed view of DDDAL’s phase 2 with software module construction 89

4.2 An abstract UML-based software model: (core layer) domain model, (middle layer) module and (top layer) software 90

4.3 The GUI of CourseMan software prototype generated by jDomainApp: (1) main window, (2-4) the UDM’s GUI for EnrolmentMgmt, Student, and Enrolment 91

4.4 The CM of MCCL 98

4.5 (A-shaded area) The transformed CM (CMT); (B-remainder) Detailed design of the key classes of CMT 105 4.6 The annotation-based ASM 109

4.7 The view of a (stand-alone) ModuleStudent object 111

4.8 The customised view of ModuleEnrolmentMgmt (as configured in Listing 4.2).113 5.1 A partial CourseMan’s GUI that is generated by DomainAppTool 121

5.2 ProcessMan’s domain model 124

5.3 A partial MCC Model for process structure 125

5.4 The view of ModuleEnrolmentMgmt of the software in Figure 5.1 138

5.5 An example CourseMan software that has substantially been customised 141

A.1 Utility class ExprTk 158

A.2 Utility class Tk 160

List of Tables

2.1 Meta-mapping between OOPL and UML/OCL 33

3.1 The essential state space constraints 48

3.2 The essential behaviour types 49

3.3 Well-formedness constraints of DCSL 59

3.4 The boolean state space constraints of DCSL 60

3.5 The non-boolean state space constraints of DCSL 63

3.6 The core structural mapping rules 67

3.7 Translating Domain Field properties into JML’s class invariants 72

4.1 Mapping from CM to CMT 107

5.1 The expressiveness aspects and domain properties of interest 128

5.2 (A-left) Comparing DCSL to DDD patterns; (B-right) Comparing DCSL to AL and XL 130

5.3 (A-left) Summary of max-locs for DCSL, AL and XL; (B-right) Summary of typical-locs for DCSL, AL and XL 132

5.4 BSpaceGen for CourseMan 133

5.5 Module pattern classification 135

5.6 Module generativity values of two CourseMan’s modules in MP2–MP4 140 C.1 Comparing the expressiveness of DCSL to AL, XL 171

C.2 Comparing the max-locs of DCSL to AL, XL 173

C.3 Comparing the typical-locs of DCSL to AL, XL 174

Chapter 1

Introduction

There is no doubt that an important software engineering research area over the last twodecades is what we would generally call model-based software development (MBSD) – the

idea that a software can and should systematically be developed from abtractions, a.k.a models,

of the problem domain MBSD brings many important benefits, including ease of problemsolving and improved quality, productivity and reusability Perhaps a most visible and novelsoftware engineering development that falls under the MBSD umbrella is model-drivensoftware engineering (MDSE) [9,16,38,67] Another more modest and direct developmentmethod is domain-driven design (DDD) [22,62,75]

Very early on, Czarnecki [16] stated that MDSE is a type of generative software opment (GSD) The key benefits of MDSE are to significantly improve reusability and pro-ductivity in producing software for different implementation platforms These are basicallyachieved by constructing the high-level (platform-independent) software models before-handand then very efficiently, typically with the help of proven automated techniques and tools,applying these models to a new platform to generate the software for it This applicationstep makes extensive use of reusable assets, including software frameworks, components,architectures and models [16]

devel-While the MDSE’s goal is ambitiously broad and encompassing, DDD [22] focusesmore specifically on the problem of how to effectively use models to tackle the complexityinherent in the domain requirements DDD’s goal is to develop software based on domainmodels that not only truly describe the domain but are technically feasible for implementation.According to Evans [22], object-oriented programming language (OOPL) is especially suitedfor use with DDD This is not surprising, given that Booch [8] had ealier pointed out two

main reasons why OOPL would result in domain models that are inherently expressive andfeasible First, object naturally represents the (abstract) entities that are conceived to exist inreal-world domains Second, the construct of object used in OOPL is also a basic construct

of high-level analysis and design modelling languages that are used to conceptualise andanalyse the domain

The domain model, which is primarily studied under DDD and a type of model engineered

in MDSE, is in fact the basis for specifying what had been called in the language engineeringcommunity as domain-specific language (DSL) [74] The aim of DSL is to express thedomain using concepts that are familiar to the domain experts A key requirement in DSLengineering is to enable the domain experts and the technical team to focus on building thedomain model, not having to worry about the technicality of language design

A type of DSL, called annotation-based DSL (aDSL) [25, 54], appears to satisfy this

requirement aDSL is an application of the annotation feature of modern OOPLs in DSL engineering Before this, however, annotation (called attribute in C# [33]) was used inattribute-oriented programming (AtOP) [12,13] to express meta-attributes and in behaviourinterface specification language (BISL) [32] to define the specification structure A keybenefit of aDSL is that it is internal to the host OOPL and thus does not require a separatesyntax specification This helps significantly reduce development cost and increase ease-of-learning

In fact, simple forms of aDSL have been used quite extensively in both DDD and MDSEcommunities In DDD, annotation-based extensions of OOPLs have been used to developsoftware frameworks (e.g [17, 60]) that support the development of not only the domainmodel but the final software The design rules in these models are expressed by a set ofannotations In MDSE, annotation sets are used to construct platform-specific models ofsoftware [4,76,77,78]

Our initial research started out with an MBSD-typed investigation into a practical problem

of how to improve the productivity of object-oriented software development [8, 43,49, 51]using a Java-based design language for the domain model [44] and a software architecturalmodel [45] Placing these works in the context of DDD, MDSE and aDSL have given us

an opportunity to advance our research to tackle a broader and more important problemconcerning the DDD method for object-oriented software

1.1 Problem Statement

In this section, we will discuss the key challenges facing DDD for the object-oriented problemdomains and define a research statement that is tackled in this disseration We motivate ourdiscussion using a software example, which we will also use throughout the dissertation toillustrate the concepts that are being presented

Figure 1.1: A partial domain model of CourseMan

Our motivating example is a course management problem domain (abbr CourseMan),

which describes a compact, yet complete, domain that includes both structural and behaviouralaspects Figure1.1shows a partially completed CourseMan domain model expressed in theform of a UML class diagram [57] Notationwise, in this dissertation, when it is necessary

to highlight the type of model element we will use a/an/the with name of the element type

to refer to a particular element, and the plural form of this name to refer to a collection

of elements Further, we will use normal font for high-level concepts and fixed font fordomain-specific and technical concepts

The bottom part of Figure1.1shows four classes and two association classes of Man Class Student represents the domain concept Student, who registers to study in anacademic instituition Class CourseModule represents the Course Modules1that are offered

Course-by the institution Class ElectiveModule represents a specialised type of CourseModule.Class SClass represents the student class type (morning, afternoon, or evening) for students

to choose Association class SClassRegistration captures details about the many-manyassociation between Student and SClass Finally, association class Enrolment capturesdetails about the many-many association between Student and CourseModule

The top part of Figure 1.1 (the area containing a star-like shape with this label “?”)shows six other classes that are intended to capture the design of an activity called enrolmentmanagement We know some design details (the attributes shown in the figure) and thefollowing description about the six classes We will give more details about the activity later

in Chapter2

– HelpRequest: captures data about help information provided to students

– Orientation: captures data about orientation programs for newly-registered students.– Payment: captures data about payment for the intuition fee that a student needs tomake

– Authorisation: captures data about the decision made by an enrolment officer cerning whether or not to allow a student to undertake the registered course modules.– EnrolmentApproval: captures data about the decision made by the enrolment officerconcerning a student’s enrolment

con-– EnrolmentMgmt: represents the enrolment management activity This activity involvesregistering Students, enrolling them into CourseModules and registering them intoSClasses In addition, it allows each Student to raise a HelpRequest

1 we use class/concept name as countable noun to identify instances.

1.1.2 Domain-Driven Design Challenges

Let us now outline the key challenges facing domain modelling in DDD and software opment from the domain model Understanding these challenges leads us to formulate a set

devel-of open issues for the research statement that we tackle in this dissertation

Essential Constraints

The UML note boxes in Figure1.1shows examples of 11 kinds of constraints concerning class,field and association These constraints appear frequently in real-world domain models [34,

48,49,57] We describe these constraints below:

1 Class Payment is immutable, i.e all objects of this class are immutable

2 Field Student.name is mutable, i.e its value can be changed

3 Student.name is not optional, i.e its value must be specified for each object

4 Student.name does not exceed 30 characters in length

5 Student.name is not unique, i.e its value may duplicate among objects

6 Student.id is an id field

7 Student.id is auto, i.e its value is automatically generated

8 The minimum value of CourseModule.semester is 1

9 The maximum value of CourseModule.semester is 8

10 The association constraint, e.g with respect to the enrols-in association, a Student isenrolled in zero or no more than 30 CourseModules, and a CourseModule is enrolled

in by zero or more Students

11 The minimum value of CourseModule.semester in ElectiveModule is 3

These constraints are primitive in the sense that they are applied independently to a singlemodel element (e.g class Student and Student.id) The key design questions concerning

these constraints are: (i) are they essential to real-world domain models? and (ii) how do we

Essential Behaviour Types

When we consider constraints in designing the domain model, we are effectively constrainingthe state space of each domain class in the model For this state space, two orthogonal design

questions that can be raised are: (i) what are the essential types of behaviour that operate on

a constrained state space? and (ii) how do we specify the behaviour types directly in each

domain class, in such a way that can help capture the relationship with the state space?Let us take class Student in Figure 1.1 as an example Given the constrained statespace that is structured around the class and its two attributes (id and name), what are the

Student(Student, String) shown in the figure sufficient for creating Student objects thatthe software needs? What are the basis for defining the other three operations of the class,namely getId, setName and getName? Are there any other essential operations that we need

to add to this class?

Regarding to modelling the behaviour type, how do we specify the behaviours of, say, theconstructor Student(Student, String) and the operations setName and getName so that

they make explicit the connections to the attribute Student.name? Clearly, such connections

help validate the essentiality of the behaviours, while making the design of Student easier

to comprehend

Software Construction from the Domain Model

Software construction from the domain model requires this model to essentially supportboth structural and behavioural modelling elements UML [57] includes three behaviouralmodelling languages: state machine and interaction and activity diagrams The first challenge

is how to factor in the domain model the behavioural modelling structure (e.g activity classand activity graph structure of an activity)? For the CourseMan example in Figure1.1, forinstance, how do we define the activity class EnrolmentMgmt and the activity graph structure

of the concerned activity? This would cover the area labelled ‘?’ in the figure

Another challenge with software construction is what software architecture can be suitableand how to use it to effectively construct software from the domain model? This challengeneeds to be addressed in the context of a well-accepted fact that software construction cannot be fully automated [26] The generally accepted position is that a GUI-based tool isemployed to assist the development team in the construction process

The constructed software plays two important roles First, it is used during domain modeldevelopment to enable the development team to iteratively and interactively build the domainmodel As discussed in [22], this phase is central to DDD For the CourseMan example,for instance, the software would need to present a GUI that truly reflects the domain modelstructure Further, it should support partially completed models, such as the one shown

in Figure1.1, and should evolve with the model as it is updated through the developmentiterations

Second, the software is reused during production software development to quickly developthe production-quality software For instance, the production-quality CourseMan softwarewould be constructed not only from the finalised domain model but from the softwarecomponents (e.g architectural components, view layouts and view fields) that were usedduring the development of the domain model

DDD is a design method that tackles the complexity that lies at the heart of software velopment However, in the context of the challenges presented above, we argue that thereare still important open issues concerning the object-oriented problem domains that need

de-to be addressed These issues fall inde-to two main areas: domain modelling and softwaredevelopment from the domain model First, the domain model does not define the essentialstructural elements and lacks support for behavioural modelling Second, there has been

no formal study of how aDSL is used in DDD This is despite the fact that annotation isbeing used quite extensively in implementations of the method in the existing DDD softwareframeworks Third, there has been no formal study of how to construct software from thedomain model In particular, such a study should investigate generative techniques (similar

to those employed in MDSE) that are used to automate software construction

In this dissertation, we aim to address the issues mentioned in the research statement above

by formally using aDSL to not only construct an essential and unified domain model butgeneratively construct modular software from this model We claim that attaining this aimwould help make the DDD method not only more concrete but more complete for object-

oriented software development purposes On the one hand, the method would become moreconcrete with specific solutions that help the technical team effectively apply the method inthe OOPL platforms On the other hand, the method would become more complete withsolutions for the design aspects that were not originally included We organise our researchwith the following objectives:

1 To investigate and design a unified domain model that includes the essential elementsfor both the structural and behavioural modelling aspects

2 To investigate and characterise the software that is constructed from the domain model.This should include properties that are specific to software construction with DDD

3 To investigate and define a generative and modular software construction approach

4 To investigate and design suitable aDSL(s) for the unified domain model and forsoftware construction

5 To implement a support tool, which includes the aDSL(s) and their generators

We take the view that software is a system and apply the system approach [14, 19] to bothengineer the software and to tackle the stated research problem Churchman [14] genericallydefines system as “ a set of parts coordinated to accomplish a set of goals” This definitionmeans that system refers to both object and abstract construct of the human mind [19].First, to engineer complex software requires a software engineering approach, which,according to Dekkers [19], is a type of system engineering approach A complex software

is one whose behaviour and interaction between the software components are initially notwell-defined A main characteristic of the system engineering approach for such software isthat it is broken down into stages, which are performed in a disciplined manner to build thesystem We adapt the iterative software engineering method [43, 70] to define an enhancedDDD method, shown in Figure1.2and which we call DDD with aDSLs (DDDAL).

In principle, DDDAL consists of three phases that are performed in sequence and eratively All three phases are supported by a software framework named jDomainApp 2

it-2 paper 5 listed in the Publications section.

Figure 1.2: An overview of DDDAL (with an emphasis on phases 1 and 2).

Phases 1 and 2 are the two key phases of the method and are depicted in Figure1.2 with

more details The elements that are drawn using dashed lines in the figure are not within

the research scope of this dissertation Conceptually, phase 1 takes as input the (typicallyincomplete) structural and behavioural requirements from the domain experts and produce

as output a domain model, called UDM The purpose of phase 2 is to generate a softwareprototype that reflects the domain model in such a way that the project team can, in phase 3,intuitively review the model, the software and the associated requirements The subsequentiterations repeat at phase 1 with an update on the domain model The process is stopped whenthe domain model satisfies the requirements From here, it is used to develop the productionsoftware The generated software prototype is reusable for this development

Consequently, our research approach in this dissertation is a system meta-approach, whoseaim is to define a subset of elements for the first two phases of DDDAL We consider thesetwo phases as two sub-problems of our research problem (defined in Section1.1), which aretackled by carrying out the research objectives (defined in the previous section) We willdiscuss in more detail below our research approach for tackling these two sub-problems

Sub-Problem 1: Unified Domain Modelling with aDSL

This sub-problem is tackled by performing the research objectives1, 4and5 We conduct

a literature survey on the essential structural and behavioural features of the domain classand apply modelling and abstraction [19] to construct a unified domain model (UDM) that incorporates both types of features We call the process for constructing UDM UD

high-level modelling languages for expressing the input requirement sets In our method, thedesigner works closely with the domain expert to map the input requirements, together withany accompanied high-level models, to the UDM and expresses this UDM in an aDSL.This collaboration is performed iteratively with the help of a GUI-based software prototypethat is constructed directly from the UDM We design the structural aspect of the UDMusing UML class diagram and the behavioural aspect using UML activity diagram Wechoose UML activity diagram because it has long been demonstrated to be domain-expert-friendly [20] and that it is tightly linked to the other two UML behavioural diagrams (statemachine and interaction diagram [57]) To express UDM, we propose an aDSL named

Domain Class Specification Language (DCSL) We apply modelling to specify DCSL and

implement this language as part of the jDomainApp framework We evaluate DCSL in terms

of expressiveness, required coding level and constructibility

Sub-Problem 2: Module-Based Software Construction with aDSL

This sub-problem is addressed by performing the objectives 2, 3, 4 and 5 It consists of

two smaller problems: (2.a) to construct the software modules from the UDM and (2.b)

to construct software from these modules We perform a literature survey on DDD, DDDframeworks and software design properties We then characterise a software in terms of four

essential properties We focus primarily on tackling problem (2.a) We conduct a literature

survey on modular software development and adopt a module-based software architecture toexplicitly model software in terms of a set of modules Software modules are instantiated frommodule classes The core component of each module class is a domain class To automaticallyconstruct the module class, we focus on the module configurations and capture their design in

a module configuration class (MCC) We then apply modelling and abstraction to specify an aDSL, named Module Configuration Class Language (MCCL), to express the MCCs We

implement MCCL as a component of the jDomainApp framework and evaluate module-based

software construction with MCCL.

The jDomainApp framework has an implementation for the sub-problem (2.b) However,

we do not discuss sub-problem (2.b) in detail in this dissertation.

Contributions

We summarise below five main contributions of our dissertation that concern the two problems presented above As the contributions will show, the term “unified view” in thedissertation’s title refers to our aDSL-centric view of automated software construction from

sub-the domain model: (i) domain model is expressed using an aDSL, named DCSL; (ii) software

module construction is automated by using another aDSL, named MCCL

Unified domain modelling with aDSL:

¬ Domain Class Specification Language (DCSL): an aDSL for designing domain class

in the domain model It consists in a set of annotations that express the essentialstructural constraints and essential behaviour of domain class

­ A unified domain modelling approach: uses DCSL to construct a unified domain

model that incorporates state-specific modelling elements of UML activity diagram

Module-based software construction with aDSL:

® A 4-property software characterisation: to provide a structured guidance for the

construction of software from the domain model

¯ Module Configuration Class Language (MCCL): an aDSL for designing the

config-uration classes for software modules Each of these classes serves as a template forcreating the configurations of a module class

° Tool support: to evaluate the aforementioned contributions and to ease the adoption of our

overall DDDAL method in real-world software projects, we implement DCSL, MCCL andthe associated generators as components in the jDomainApp software framework

1.4 Dissertation Structure

This dissertation is organised into6chapters that closely reflect the stated contributions InChapter 2, we systematically present the background knowledge concerning the concepts,methods, techniques and tools that are most relavant to the research problem and necessary forexplaining our contributions in the rest of this dissertation We also highlight the limitations

of the existing DDD method and the works concerning the use of aDSL with MDSE andDDD

In Chapter3, we describe our contributions concerning UD modelling Taking the viewthat domain class design is the basis for UDM construction, we first specify DCSL forexpressing the essential structural and behavioural features of the domain class We then useDCSL to define UDM After that, we present a set of generic UD modelling patterns that can

be applied to construct UDMs for real-world domains

In Chapter4, we explain our contributions concerning module-based software tion We first set the software construction context by defining a software characterisationscheme We then specify MCCL for expressing the MCCs and present a generator forgenerating the MCCs

construc-In Chapter5, we present an evaluation of our contributions We first describe an mentation of the research contributions as components in the jDomainApp framework Wethen describe a real-world software development case study which we have developed usingthe implemented components After that, we present two evaluations: one is for DCSL andthe other is for module-based software construction

imple-In Chapter6, we conclude the dissertation with a summary of the research problem, thecontributions that we made and the impacts that these have on advancing the DDD method

We also highlight a number of ideas and directions for future research in this area

Chapter 2

State of the Art

In this chapter, we present a methodological study of the literatures that are relevant to thisdissertation We have two main objectives The first objective is to gather authoritativeguidance for and to define the relevant foundational concepts, methods, and techniques Thesecond objective is to identify significant, unresolved issues that can be addressed in ourresearch In particular, we highlight the limitations of the existing DDD method and theworks concerning the use of aDSL with MDSE and DDD

We begin in this section with a review of the relevant background concepts, methods andtechniques concerning MDSE, DDD and OOPL In particular, we highlight the role of aDSL(which is derived from OOPL) as the underlying theme that connects MDSE and DDD

Historically, model-driven software engineering (MDSE) evolves from a general system engineering method called model-driven engineering (MDE) MDE in turn was invented

on the basis of model-driven architecture (MDA) [55] Early works, especially by Kent

et al [38] and Schmidt [67], use the term MDE A recent book by Brambilla et al [9]defines MDSE as a more specialised term In particular, they relate MDSE to a form ofMDE and highlights the differences between the latter and two other related terms (MDA and

MDD is smaller in scope than MDE (the letter ‘D’ means development, while ‘E’ meansengineering) MDA, on the other hand, is a particular realisation of MDD by the ObjectManagement Group (OMG)1.

Since our aim in this dissertation is to study the development of software systems, we will

limit our focus to just MDSE Before reviewing the related work, let us clarify the meaning

of two basic and related terms: domain and domain model We adopt the following recent

definition of both terms from Brambilla et al [9]

Definition 2.1 Domain (a.k.a problem domain) is a field of expertise that needs to be

examined to solve a problem Domain model is the conceptual model of the domain.

The work by Kent et al [38] serves as a good light-weight introductory background forMDSE, not only because of the timing of its publication but because it broadly covers MDEand does so on the basis of an initial draft specification of the MDA by OMG They defineMDA as “ an approach to IT system specification that separates the specification of systemfunctionality from the specification of the implementation of that functionality on a specifictechnology platform” [55] This means that the same system model of the functionality can

be applied to different implementation platforms The two types of specification in the above

definition are represented by two types of model: platform-independent model (PIM) (the system functionality) and platform-specific model (PSM) (the platform).

MDA defines four types of model transformations between PIM and PSM and suggests

that, to ease maintenance, these transformations be automated as much as possible The firstmodel transformation type is PIM-to-PIM, which is used for design refinement The secondtype is PIM-to-PSM, which is for realising PIM in a particular platform The third type

is PSM-to-PSM, which is for platform model refinement The fourth type is PSM-to-PIM,which is for reverse engineering PIMs from PSMs of an existing platform

Next, Kent et al suggest that meta-modelling be used to specify the languages that

are used to express models This is attractive because it effectively reuses MDE to define

itself A meta-model is a model that expresses the shared structure of a set of related

models Among the key points about meta-modelling are: (i) language definitions are just models (called meta-models) with mappings defined between them, (ii) meta-modelling can

be used to specify the abstract and concrete syntax, and the semantics of a language and (iii)

meta-modelling is achievable with an object-oriented modelling language (e.g MOF [58])

1 http://www.omg.org

Schmidt [67] both reinforces the work by Kent et al and discusses another importantcontribution of MDE to software development He argues that MDE had a potential for

addressing the abstraction gap problem, through effective domain modelling This

prob-lem arised from a fact that software languages were primarily concerned with the solutionspaces and, thus, were inadequate for effectively expressing the domain concepts Schmidt

next states that MDE technologies should be developed to use domain-specific modelling

meta-modelling) to construct models and transformation engine and generator to

auto-matically produce other software development artefacts from the models Before Schmidt,Czarnecki [16] also stated that MDA/MDD is a form of generative software development

We will discuss DSL shortly in Section2.1.2

More recently, Brambilla et al [9] has studied MDSE as a method and how it is applied toengineering software They define MDSE as “ a methodology for applying the advantages

of modelling to software engineering activities” The key concepts that MDSE entails

are models and transformations (or “manipulation operations” on models) This definition

then serves as the basis for integrating MDSE into four well-known software developmentprocesses MDSE may be integrated into the traditional development processes (such aswaterfall, spiral, and the like) by making models the primary artefacts and by using modeltransformation techniques to automate (at least partially) the activites that are performedwithin and at the transition between different phases

MDSE can also be integrated into agile, domain-driven design (DDD) and test-driven

development processes Within the scope of this dissertation, we are interested in theintegration capability of MDSE into DDD We will discuss this capability in Section2.1.6

In general, DSL [40,50,74] is a software language that is specifically designed for expressingthe requirements of a problem domain, using the conceptual notation suitable for the domain

An important benefit of DSL is that it raises the level of abstraction of the software model

to the level suitable for the domain experts and thus help them participate more actively andproductively into the development process Despite the fact that different DSLs are designedfor different domains, they share some underlying properties that can be used to classify them.DSLs can be classified based on domain [39] or on the relationship with the target

(a.k.a host) programming language [25,50, 74] From the domain’s perspective, DSLs areclassified as being either vertical or horizontal DSL [39] A vertical DSL, a.k.a business-

oriented DSL, targets a bounded real-world domain For example, course management is areal-world domain of CourseMan and the completed domain model of the one shown inFigure1.1would form the domain model of a vertical DSL for expressing this domain

In contrast, a horizontal DSL (a.k.a technical DSL) targets a more technical domain,

whose concepts describe the patterns that often underlie a class of vertical domains whichshare common features Examples of horizontal DSLs include Structured Query Language(SQL), used for writing executable queries for relational databases, and Hypertext Mark-upLanguage (HTML), used for writing the source code of web pages

Regarding to the relationship with the host language [25,40,50,74], DSLs are classified

as being internal or external In principle, internal DSL has a closer relationship with thehost language than external DSL A typical internal DSL is developed using either the syntax

or the language tools of the host language In contrast, a typical external DSL has its ownsyntax and thus requires a separate compiler to process

MDSE with DSL

It is a little surprising that the idea of combining MDSE with DSL [16,67] only came outabout five years after the OMG’s proposal for MDA/MDD A plausible explanation for this,

as Czarnecki [16] pointed out, is that the MDSE effort until then had only been focusing

on addressing the platform complexity issue It had paid almost no attention to the domaincomplexity problem A recent DSL survey [40] reveals that this issue still remains: there hasbeen insufficient focus in DSL research on domain analysis and language evaluation and onthe integration of DSL engineering into the overall software engineering process

The general method for combining MDSE with DSL is formulated in [9] A technicaldiscussion on how to use meta-modelling to engineer DSLs is presented in [39] The work in[39] proposes two variants of the meta-modelling process: for vertical DSLs, it is the domainmodelling process (discussed in detail in [25]); for horizontal DSLs, it is a pattern-basedmodelling process

In practice, one of the very early works [77,78] had experimented with combining MDSEand DSL in a layered, service-oriented architecture, named SMART-Microsoft A key feature

of this work is that it defines a framework for identifying and combining the DSLs that form

a complete software model More specifically, this entails a 3-step development method: (i) determine the application architecture, (ii) develop the DSLs that fit this architecture and (iii) combine the DSLs by defining transformations between them In this method, the DSLs

are designed to address different parts of the architecture and each DSL is used to create notone monolithic model but multiple partial models

As discussed above, meta-modelling is a modelling approach that is applied to definingDSL More generally, Kleppe [39] suggests that meta-modelling is applicable to any softwarelanguage This includes both DSL and general purpose language (e.g Java [28] and C# [33])

In meta-modelling, a language specification consists in three meta-models and the

rela-tions between them The first meta-model describes the abstract syntax and is called abstract

syntax meta-model (ASM) The second meta-model describes the concrete syntax and is

called concrete syntax meta-model (CSM) The third meta-model describes the semantics and is called semantic domain meta-model (SDM) According to Kleppe [39], the ASMdescribes the conceptual structure that exists in the language’s domain, while the CSM de-scribes the linguistic structure that helps present the conceptual structure to a pre-determinedpopulation of language users The SDM makes clear the semantic structure of the concepts.Technically, if we consider the Kleppe’s view that a (modelling or programming) language is

a set of mograms (a mogram is either a model or a program (resp.) written in the language)

then SDM describes “ what happens in the computer when a mogram of the language isexecuted” Here, “what happens in the computer” is defined based on a representation of thecomputer’s run-time This representation must allow for a precise description of the run-timestate when a mogram is executed

As a general modelling approach, there are a variety of meta-modelling languages that

can be applied A de facto language is Unifield Modelling Language (UML) [57] UMLconsists in a family of languages, one of which is UML class diagram This language, which

we will refer to in this dissertation as class modelling language, is made more precise when combined with the Object Constraint Language (OCL) [56] This combination forms an

enriched language, which we will generally refer to in this dissertation as UML/OCL In

fact, UML/OCL is used in the UML specification itself [56] to define the abstract syntaxes

of the languages in the UML family Later in Section2.2.2, we will discuss an example of

how UML/OCL is used to specify the activity modelling language.

In the remainder of this section, we present an essential review of UML/OCL Our focus is

on the abstract syntax of a core meta-modelling structure of UML/OCL The meta-conceptsthat form this structure will be used later in this dissertation to define aDSLs The informalsemantics of the meta-concepts are given in [56,57] In the spirit of meta-modelling, we adoptthe approach used in the UML specification to use UML/OCL to define itself Specifically,

we first construct the ASM of UML and introduce very briefly the graphical syntax Afterthat, we discuss OCL and, as an example, explains how OCL is used to express some syntaxrules of the ASM We conclude with a brief review of the approaches for specifying SDM

The Essential ASM of UML

Figure 2.1: The essential ASM of UML (synthesised from seven meta-models of UML [57]).The essential ASM for UML consists of the following meta-concepts: Class, Attribute,Association, Association End, Operation, Parameter, Association Class and Generalisation.This ASM, which is shown in Figure2.1, suffices for our research purpose in this dissertation

A meta-concept is represented in the figure by a labelled rectangle A relationship betweentwo meta-concepts is represented by a line segment that connects the two correspondingrectangles On the one hand, the ASM is derived from a combination of the core structures

of seven relevant UML meta-models presented in the UML specification [57] On the otherhand, it adds two new, more specialised, meta-concepts (Attribute and Association End) formodelling the class structure

The referenced meta-models come from the following sections of the UML specification:– Abstract syntax of the root (foundational) concepts of UML (see §7.2.22)

– Abstract syntax of Classifiers (the base concept of UML’s classification) (see §9.2.2)– Abstract syntax of Feature which includes the Parameter concept (see §9.4.2)

– Abstract syntax of Property and the related concepts (see §9.5.2)

– Abstract syntax of Operation and the related concepts (see §9.6.2)

– Abstract syntax of Class – a type of Classifier – and the related concepts (see §11.4.2)– Abstract syntax of Association and the related concepts (see §11.5.2)

To ease look-up, we label each key element of the ASM with the section reference, withinwhose meta-model the element is defined For instance, meta-concept Class is defined in themeta-model of §11.4.2

The ASM presented in Figure2.1includes two new meta-concepts: Attribute and ciation End These meta-concepts, whose rectangles are filled in gray, are sub-types of themeta-concept Property We use the two meta-concepts to differentiate between properties thatare attributes and those that serve as association ends of associations The former participate

Asso-in the contaAsso-inment relationship between Class and Attribute, while the latter participate Asso-inthe containment relationship between Association and Association End Further, Class has acontainment relationship with Association

The Essential OCL

According to the OCL specification [56], OCL is a user-friendly, typed, formal language forconstructing expressions on UML models OCL expressions are side-effect-free and each has

a type Typical examples of OCL expressions are invariants and queries over a UML model.The OCL specification consists in two packages: type (§8.2) and expression (§8.3) The

former defines the valid types for the expressions in the latter In the type package, all OCL

types are derived from Classifier3(see UML specification §9.2.2) Two core OCL types

2 we use the symbol § to denote sections in the UML specification.

3 we use fixed font here to be consistent with the OCL’s notation style below.

Figure 2.2: The OCL expression meta-model (Adapted from §8.3 [56]).

that are commonly used are CollectionType and TupleType CollectionType has foursub-types, namely SetType, OrderedSetType, SequenceType and BagType, representing

sets, order sets, sequences, and bags (resp.) CollectionType is parameterised with an element type and can be nested A nested CollectionType is one whose element type is

another CollectionType

The TupleType conveniently combines different types to form an aggregate type, called

identified by a name Similar to CollectionType, TupleType is also nested This meansthat the attributes in a tuple can have their types defined as TupleTypes

The expression package, whose meta-model is shown in Figure2.2, specifies the structure

of OCL expression Every OclExpression is typed, which is usually not explicitly statedbut derived The core structure of OclExpression is described by VariableExp and theCallExp’s type hierarchy VariableExp is an expression that involves the declaration and(possibly initialisation) of a variable The initialisation consists in another OclExpression,which is typically a LiteralExp (for writing a value) or a CallExp CallExp is used to

“call”, i.e set up, another expression that either, for the subtype named FeatureCallExp,references the value of a model’s feature (e.g attribute, operation and association end) or,for the subtype named LoopExp, performs a loop over a collection This loop involves a

Variablethat points to each element of the collection and one or more OclExpressionsthat define a condition against which the elements are checked.

An OclExpression is defined in a context of a model element (e.g a class, an attribute, a method, etc.) It is via this element that an OclExpression can navigate the model, through

association chains, to access other model elements that are of interest A special keywordselfis available in every OclExpression for referring to the contextual element To easewriting, this keyword is implicit if omitted

Example 2.1 Let us illustrate OCL by showing how it is used to express an invariant

on the UML’s ASM in Figure 2.1 The invariant is taken from §11.8.1.8 of the UMLspecification [57], which states that an aggregation is a binary association:

Line 1 uses the keyword context to specify the context of the invariant (keyword inv)

to be an Association The invariant is interpreted as being true for every Association instance (at all times) Lines 2–6 form the body of the invariant It is an implication A → B, where A is specified at lines 2–3 and B is specified at lines 5–6 The keyword implies

means → Lines 2–3 form a CallExp that performs an operation called exists over the

collection self.memberEnd This operation returns true or false depending on whether

there exists an element of the collection that satisfies the condition: e.aggregation <>AggregationKind::none In this condition, e is a Variable that points to each collectionelement and AggregationKind::none means ‘not-an-aggregation’ Thus, the conditioneffectively means to find all Association Ends of the current Association instance that is

an aggregation of some kind

Lines 5–6 state two conditions that hold for memberEnd (which is a short-form of

self.memberEnd): (line 5) it contains exactly two elements (hence the association is nary) and (line 6) at least one other Association End of the current Association is not an

bi-aggregation The second condition consists in a negated expression of the one used in A.