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- Herausgeber: Packt Publishing
- Kategorie: Fachliteratur
- Sprache: Englisch
Knowing about design patterns enables developers to improve their code base, promoting code reuse and making their design more robust.
This book focuses on the practical aspects of programming in .NET. You will learn about some of the relevant design patterns (and their application) that are most widely used. We start with classic object-oriented programming (OOP) techniques, evaluate parallel programming and concurrency models, enhance implementations by mixing OOP and functional programming, and finally to the reactive programming model where functional programming and OOP are used in synergy to write better code.
Throughout this book, we’ll show you how to deal with architecture/design techniques, GoF patterns, relevant patterns from other catalogs, functional programming, and reactive programming techniques.
After reading this book, you will be able to convincingly leverage these design patterns (factory pattern, builder pattern, prototype pattern, adapter pattern, facade pattern, decorator pattern, observer pattern and so on) for your programs. You will also be able to write fluid functional code in .NET that would leverage concurrency and parallelism!
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Seitenzahl: 353
Veröffentlichungsjahr: 2017
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Table of Contents
.NET Design Patterns
.NET Design Patterns
Copyright © 2017 Packt Publishing
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First published: January 2017
Production reference: 1250117
Published by Packt Publishing Ltd.
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ISBN 978-1-78646-615-0
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Credits
Authors
Praseed Pai
Shine Xavier
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Reviewers
Soumya Mukherjee
Vidya Vrat Agarwal
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Foreword
We are living in an era where most of the things that we do are getting automated through software. Even though, there is definite trend towards pre-packaged software modules and platforms, there is always a place for custom software development and integration projects. In the Enterprise world, .NET and Java EE are the natural platforms of choice for custom software development and deployments. With Agile software development methods, continuous delivery and Domain-Driven Design dominating the scene, there is a need for standardization and communication about software artifacts in a “platform-agnostic” manner. In this context, software design patterns becomes a primary tool for such communications.
You have in your hands a book that encompasses more than four decades of collective wisdom amassed by these two authors, as part of their continuous pursuit of knowledge and excellence, in the software engineering and technology consulting arena. What is more significant is the fact that these knowledge representations are a direct reflection of a practitioner’s mindset; some of those have been undertaken for our organization under our supervision! This is more than a reference book and should be looked at as a handbook for developers who are aspiring to take their skills to the next level and design state-of-the-art systems independently. This is undoubtedly a book that helps unfold many underlying principles that further educate and help one appreciate the various programming paradigms that are in vogue, as I write this foreword.
We have been continuously disrupting and pushing computing limits way beyond CPUs and GPUs over the past five decades. It is quite amazing to see the ground the authors have covered, in terms of contemplating the evolution of software design, patterns and idioms, to help developers create scalable and consistent code. More than coverage, their focus has been on certain aspects of design that help you create well structured (leveraging GoF patterns), scalable (through concurrent and parallel programming) and reliable (through functional and reactive programming) code with some of the .NET platform tools that transcend conventional imperative programming techniques.
At the core, the book deals with how pattern-based software development helps one to write better code. The authors have deliberately crafted the book to convey how various development patterns that are part of different catalogues can be put to use in a coordinated manner to write good quality code in .NET. The book covers how one can use GoF, POSA, DDD, POEAA, and EIP catalogue in the C# programming language.
Praseed and Shine are part of UST Global’s Technology Consulting group and have been collaborating professionally for the last ten years. They truly complement each other when it comes to software design and development. Their command over computer science first principles, ability to learn new technologies, appetite for challenges, and passion for software design is truly commendable. This makes their technology leadership invaluable to any professional organization.
Leveraging the core platform features (in this case, .NET) in tandem with industry-standard best practices (in this case, proven design pattern catalogues and idioms) is the need of the hour and becomes a clear differentiator when it comes to delivering high quality, scalable, and consistent code. This clearly is any developer’s best hedge in the present day's highly volatile technology landscape. I believe the authors have tried their best in articulating this with anecdotes and detailed code samples, and I hope that, after reading this book, you will feel the same way.
Arun Narayanan
COO, UST Global
About the Authors
Praseed Pai is a software engineering/re-engineering professional with good business acumen, technical competency, and software engineering skills. He has presented on various topics in more than two hundred technical events in the last decade and half. His areas of interest include software architecture, design patterns, domain-specific languages, engineering software development, computer graphics, and machine learning. Currently, he is associated with the UST global as a senior solutions architect in their consulting division. He is also the primary force behind the SLANGFORDOTNET compiler infrastructure, which has been ported to VB.NET (CLR), Java (JVM), C++ (LLVM), Python, JavaScript, and Ruby. He lives in the city of Kochi with his wife and two kids.
I would like to start off thanking my co-author Shine Xavier for undertaking this journey along with me. An important acknowledgment goes to my longtime collaborator and friend Shalvin PD, who introduced us to Denim Pinto of Packt. I would like to acknowledge the contributions of Aneesh P Revi, Nikhil Nair, Sarath Soman, Vaisakh Babu, Haridas Nair, John Thomas, Biju Alapat, Joseph Abraham, Shaji P Dayanandan, and members of the Kerala Microsoft Users Group (KMUG) while working on the content of the book. I would like to thank my parents, Mrs. Mohana T Pai and the late K.J. Thrivikrama Pai, who taught me the value of continuous learning. I whole heartedly thank my wife, Sandhya L Kammath, for giving encouragement and taking care of our domestic issues herself, while I was writing this book. Being a developer herself, I could get different perspectives on the content I was writing. Last, but not least, Sidharth Pai and Karthik Pai, my sons, for allowing me to concentrate on the book, by not bothering much with their usual demands! Finally, I thank the wonderful folks at Packt and the two reviewers who helped us to make the book better.
Shine Xavier is a core software engineering practitioner with an extreme passion for designing/building software solutions, application frameworks, and accelerators that help maintain productivity, code quality, performance, and security. His areas of interest include functional programming, interpreters, JavaScript library development, visual programming, algorithms, performance engineering, automation, enterprise mobility, IoT and machine learning. He is currently associated with UST Global as a senior architect, where he continues to provide technical leadership in customer engagements, pre-sales, practice development, product development, innovation, and technology adoption. He lives with his wife and three kids in Thiruvananthapuram, Kerala, India.
I would like to start off thanking my colleague and mentor Praseed Pai for choosing me to co-author this book. It has been a joy and privilege penning this book along with you, my friend! There are others without whom I wouldn’t have been able to complete this book. In particular, I would like to thank: the folks at Packt (Denim Pinto, Pooja Mhapsekar, Siddhi Chavan, and Sunith Shetty) who were really patient and inspiring us throughout this journey and the technical reviewers Soumya Mukherjee and Vidya Vrat Agarwal, who really pushed us all along to get this book one knot better. This book has become a lot better and taken its present shape because of you all! My teachers and friends in school and college, mentors (Arun Narayanan and Sunil Balakrishnan), colleagues (Jeenu Mathew, Viby Abraham, Renju Ramadevan, Nikhil Nair, John Thomas, Renjith Ramachandran, Jenson Joseph, Ashok Nair, and Abhilash Ponnachan) and members of the computer science fraternity (many whom I paid my respects to in the book). You all continue to teach, motivate, and inspire me! My parents (Xavier and Grace) without whom I would cease to exist. My wife (Kavitha Jose) and kids (Joel, Sera and Johaan) for their whole-hearted love, support, and understanding in helping me realize this dream.
About the Reviewers
Soumya Mukherjee is very passionate about working on open source projects and has extensive experience in architecting enterprise applications in .NET, JAVA, Ruby, and Python.
He has expertise in implementation of Test Driven and Behavioural Driven development and test automation for large teams. He has developed various tools, one of them is the Quantinuous Agile Framework (QAF) for continuous mobile and web automation.
He is a certified MCAD.NET, Certified Scrum Master (CSM), Certified Professional Selenium Agile Tester, a Trainer, and a seasoned author of couple of books on amazon. He is also a speaker across multiple world renowned conferences, an Agile Practitioner, and loves mentoring team's in Open Source Technologies.
He has extensive experience in Software Test Automation using professional tools, such as QTP, QC, LoadRunner, Jmeter, and Neoload GreenHat, and with Open Source Tools, such as Selenium RC & Web driver with Java, .Net, Python, Ruby, FitNess, JMeter, SoapUI, TestNG, ReportNG, JUnit, Cucumber & Capybara, and Tosca & Perfecto.
An ex-employee of JP Morgan, Merill Lynch, he is currently the co-founder of QAAgility Technologies and AppTestPro. He resides in Mumbai, India, with his wife, Garima.
Vidya Vrat Agarwal is passionate about .NET, a Microsoft MVP, a C# Corner MVP, a TOGAF certified architect, a Certified Scrum Master (CSM), an author, a speaker, a certified MCT, MCPD, MCTS, MCSD.NET, MCAD.NET, and MCSD. He is currently working as a .NET Enterprise Architect and Consultant in USA. He is passionate about .NET technology and loves to contribute to the .NET community. He blogs at http://www.MyPassionFor.NET and can be followed on Twitter, @DotNetAuthor. He lives with his wife, Rupali, and two daughters, Pearly and Arshika, in Redmond, WA.
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Preface
Writing modern Enterprise software requires large teams that need close co-ordination to deliver quality products and services. The teams require a common language for communication when dealing with technical solutions and artifacts, in order to work as a cohesive unit for meeting the set objective. The GOF design pattern catalog was a major step in this direction. This was followed by other similar catalogs to empower software engineering teams with a common pool of shared idioms and practices. For all practical purpose, GOF patterns became sine qua non for patterns. But there are other equally important catalogs that you would want to leverage.
There have been attempts to map the pattern catalogs to platforms such as JEE and .NET. The J2EE --design patterns and Microsoft--patterns and practices catalogs were attempts in this direction. The emergence and adoption of functional programming and reactive programming models mandates a fresh look at the patterns, applied to modern .NET software development. That is where this book comes into the picture. This book provides detailed examples written using C#/.NET, which will help developers map different constructs from various pattern catalogs to executable code.
The book you have got in your hand outlines author's perspectives about how design, architecture, and domain patterns can be applied in practice by leveraging the .NET platform. The topics covered deals with various patterns and pattern catalogs, including GOF patterns, concurrent programming, object/functional programming, and functional reactive programming. The final chapter talks about materials which we could not cover in this book, but, which a modern programmer should master to become an effective developer.
Learning, as we all know, is a continuous process. We have made to the best of our ability to impart our learnings on the job so that it benefits any one who is interested. We have leveraged code extensively for illustrating this with very detailed examples. Design illustration in the form of UML and context diagrams accompany the concepts and code. The views and perspectives we have outlined mostly align with the industry references and standards.
What this book covers
Chapter 1, An Introduction to Patterns and Pattern Catalogs, deals with the summary of key pattern catalogs and also deals with the evolution of the C# programming language. The chapter deals with the gist of GOF, POSA, DDD, EIP, J2EE, POEAA, and the Arlow and Neustadt pattern catalog.
Chapter 2, Why We Need Design Patterns?, provides the rationale for applying patterns in the context of software engineering. The chapter deals with some foundational principles such as SOLID, KISS, YAGNI, DRY/WET and so on, that lay the foundation for pattern oriented software development. A tax computation engine that uses the factory method pattern, the template method pattern, and design by contract is given as a running example.
Chapter 3, A Logging Library, implements a logging library which can be used in a multithreaded environment. The implementation leverages the template method pattern, the strategy pattern, and the factory method pattern. A custom Dependency Injection framework loads the classes on the fly.
Chapter 4, Targeting Multiple Databases, shows how one can use different persistent technologies (Oracle, SQL Server, SQLite, and so on) without changing a single line of application code. The library leverages ADO.net and uses the abstract factory pattern, the factory pattern, and the adapter pattern.
Chapter 5, Producing Tabular Reports, shows how one can create a canonical object model based on the composite pattern and traverse the object model through visitor pattern to generate HTML and PDF documents.
Chapter 6, Plotting Mathematical Expressions, deals with a library that uses composite, interpreter, observer, builder, and facade patterns to plot arbitrary mathematical expression on the fly using Windows Presentation Foundation.
Chapter 7, Patterns in the .NET Base Class Library, demonstrates how the designers of .NET base class library has leveraged various GOF patterns to give developers a good programing model.
Chapter 8, Concurrent and Parallel Programming under .NET, steps into the very important topic of concurrent and parallel programming by leveraging constructs provided by the .NET platform. Topics such as concurrency versus parallelism, fork/join parallelism, producer-consumer model, and so on are dealt at depth.
Chapter 9, Functional Programming Techniques for Better State Management, deals with the very important topic of managing state by leveraging closures, lambda, immutability, LINQ and other functional programming techniques.
Chapter 10, Pattern Implementation Using Object/Functional Programming, illustrates Map/Reduce, the template method pattern, the iterator pattern, and the strategy pattern by a judicious mix of functional and object-oriented programming constructs.
Chapter 11, What is Reactive Programming?, gives you the gist of Functional Reactive programming model and its realization on the .NET platform.
Chapter 12, Reactive Programming Using .NET Rx Extensions, carries forward from the previous chapter and shows some concrete examples where Functional Reactive Programming techniques can be put for real world use.
Chapter 13, Reactive Programming Using RxJS, gives a very good introduction to the techniques and internals of RxJS for writing web-based responsive UI.
Chapter 14, A Road Ahead, sheds light on a learning progression path highlighting the importance of polyglot programming, domain-specific languages, ontology, and antipatterns.
What you need for this book
The reader is supposed to have worked with at least a project with the .NET platform using C# as the primary programming language. A good understanding of basic object oriented programming will help you navigate the territory faster. From a software perspective, a laptop, desktop or a virtual machine with Visual Studio 2012 (or above) to test and play around with the source code.
Who this book is for
The book is useful for any C#/.NET developer to understand how enterprise class applications are designed, developed, and deployed. Rather than studying various patterns in isolation, the book deals with scenarios where patterns are often combined to engineer state of the art applications. Developers who work with Java can also follow the book without much difficulty, as both platforms have lot of things in common.
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class Employee { public String name {get;set;} public int age {get;set;} public double salary { get; set; } }When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:
return function () { try { return fn.apply(null, args.slice(1)); }Any command-line input or output is written as follows:
npm install rxNew terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "You could simply do this in JSbin by using the Add library option shown in the web editor."
Note
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Tip
Tips and tricks appear like this.
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Chapter 1. An Introduction to Patterns and Pattern Catalogs
Design patterns have always fascinated software developers, yet true knowledge of their applicability and consequences has eluded many. The various solutions that have been created and applied to solve similar problems have been studied over time by experienced developers and architects. A movement slowly began to catalog such time-tested and successful solutions, which served as a blueprint for software design. The applicability of design patterns exhibited maturity (even though over-engineering was a perceived risk) in solution architecture (in terms of stability, consistency, maintainability, and extensibility), and became a core skill for serious developers and architects. In this introduction to patterns and pattern catalogs, the authors wish to provide a detailed illustration of the movement in the software development industry that led to the discovery and consolidation of the various patterns and pattern catalogs. It is equally important to understand the evolution of patterns, idioms, programming languages, and standards that led to standardization of these technology-agnostic blueprints, which form the basis of enterprise application development today. We will cover the following topics in this regard:
OOP - A short history
OOP is a programming model that is supposed to combine structure (data) and behavior (methods) to deliver software functionality. This was a marked contrast from the procedural programming model, which was mostly in vogue when the OOP model gained prominence. The primary unit of composition in a procedural programming model is a procedure (mostly a function with side-effects). Data is fed into a series of procedures that constitutes the process or algorithm in a solution context. In the case of OOP, the data and related functions are represented together as a class, which acts as a fundamental unit in the programming model. Schematically it is as follows:
Class Test { <------ Static (Class Level) Variables ---------------> <------ Instance (Object Level) Variables ------------> <------ Private Methods ------------------------------> <------ Public Methods -------------------------------> }As a programmer, one can create many instances of a class during the execution of a program. Since class encapsulates data and its associated operations to provide a coherent entity, the problems (or rather side-effects) associated with global variables/data (being used as payload for the procedures) went away all of a sudden. This helped to manage the complexity of developing large software.
OOP revolutionized the way programmers modeled the problem domain, with class compositions leveraging encapsulation, association, inheritance, and polymorphism. Additionally, with the flexibility to model hierarchies (that closely represent the problem domain) with ease, it became natural for developers to think in terms of objects.
Note
The origin of OOP can be traced back to the Simula programming language created by Kristen Nygaard and Ole-Johan Dahl, released in the year 1965. The advent of the Smalltalk system helped the ideas of OOP to percolate to the academia and some consulting circles. Smalltalk was a dynamically typed language, and primarily designed as a message passing system. Later, they added Simula's class-based Object model. Alan Kay, Dan Inaglis, and Adele Goldberg at Xerox PARC designed the language.
The OOP model reached a critical mass in the early 1990s, with the popularity of the C++ programming language. Even though Smalltalk and C++ were OOP languages, Smalltalk was a dynamically typed programming language, and C++ was a statically typed (though weakly enforced) programming language. The C++ programming language was created by Bjarne Stroustrup at the AT&T Bell Laboratories, as an extension of C (for wider adoption). In this regard, C++, as a programming language, has issues in terms of usage because of the compulsion to make it C-compatible. The story of evolution of the language is well chronicled in, The Design and Evolution of C++, a book written by Bjarne himself. The book deals with the rationale of designing the language and the design choices available for him to incorporate features such as single inheritance, multiple inheritance, virtual methods, exception handling, templates (Generics), I/O streams, and so on. Any serious C++ developer should not miss this particular book, as it helps to understand the reason why the C++ programming language is the way it is!
There were attempts to make protocol-based development using middleware technologies like Microsoft's Component Object Model (COM) and OMG's Common Object Request Broker Architecture (CORBA). Both CORBA and COM were very similar, and both facilitated object interoperability at the binary level. Each protocol had its own binary encoding format, and interoperability between these two standards became a problem. Some enterprising companies made a living by writing COM/CORBA bridge to rectify this problem. Also, COM was mostly available only on Microsoft Windows, making it a platform-specific solution.
Then, in 1996, Sun Microsystems came up with a language which was marketed as a programming language to write applications that are hosted in a browser (Applets). They named it Java. However, due to performance and political reasons, applet development did not took off. The language, along with its associated platform, was soon projected as a server-side programming system. This was a tremendous success, and the Java language made a strong comeback, further popularizing the OOP programming model. The primary architect of the Java language was James Gosling.
In the year 2001, Microsoft released C#, a brand new OOP language for their new virtual machine development platform, known as .NET. Later, Microsoft did add support for generics, lambda, dynamic typing, and LINQ, among others, to make C# one of the most powerful programming languages in the world. The primary architect of the language was Anders Hejlsberg.
Meanwhile, languages such as Ruby and Python made an appearance, and are still relevant in certain areas. Then, there were object-functional languages such as F#, Scala, Groovy, Clojure, and so on. However, the OOP model is symbolized by C++, C#, and Java.
Patterns and pattern movement
Programmers of the early 1990s struggled a lot to understand OOP, and how to effectively use them in large projects. Without a viral medium such as the Internet, it was quite a struggle for them. Early adopters published technical reports, wrote in periodicals/journals, and conducted seminars to popularize OOP techniques. Magazines such as Dr. Dobbs Journal and C++ Report carried columns featuring OOP.
A need was felt to transfer the wisdom of the experts to the ever-increasing programming community, but this knowledge propagation was not happening. The legendary German mathematician Carl Friedrich Gauss once remarked, "Always learn from the masters". Even though Gauss had mathematics in mind, it is true for any non-trivial human endeavor. However, there were very few masters of the OOP techniques, and the apprenticeship model was not scaling well.
Note
James Coplien published an influential book titled Advanced C++ Programming Styles and Idioms, which dealt with low-level patterns (idioms) associated with the usage of the C++ programming language. Despite being not widely cited, authors consider this a notable book towards cataloguing the best practices and techniques of OOP.
The following figure illustrates the evolution of design methodologies, programming languages, and pattern catalogs:
Key pattern catalogs
Patterns are most often catalogued in some kind of pattern repository. Some of them are published as books. The most popular and widely used pattern catalog is GoF, named after the four collaborators that produced them. They are Eric Gama, Ralph Johnson, John Vlissides, and Richard Helm.
GoF patterns
The GoF catalog, named after the four creators of the catalog, started the pattern movement. The creators mostly focused on designing and architecting object-oriented software. The ideas of Christopher Alexander were borrowed to the software engineering discipline, and applied to application architecture, concurrency, security, and so on. The GoF divided the catalog into structural, creational, and behavioral patterns. The original book used C++ and Smalltalk to explain the concepts. These patterns have been ported and leveraged in most of the programming languages that exist today.
Sr. no
Pattern type
Patterns
1
Creational patterns
Abstract factory, builder, factory method, prototype, singleton.
2
Structural patterns
Adapter, bridge, composite, decorator, facade, flyweight, proxy.
3
Behavioral patterns
Chain of responsibility, command, interpreter, iterator, mediator, memento, observer, state, strategy, template method, visitor.
We believe that a good understanding of the GoF patterns is necessary for any programmer. These patterns occur everywhere, regardless of the application domain. GoF patterns help us to communicate and reason about systems in a language-agnostic manner. They are widely implemented in the .NET and Java world.
POSA catalog
POSA Volume 5 is an influential book series, which covers most of the applicable patterns while developing mission-critical systems. An element of bias is seen towards native code programming; perhaps C++ was the prevalent OOP language during the time of research. The catalog, which spanned five published volumes, is listed as follows:
Sr. no
Pattern type
Patterns
1
Architectural
Layers, pipes and filters, blackboard, broker, MVC, presentation-abstraction-control, microkernel, reflection.
2
Design
Whole-part, mater-slave, proxy, command processor, view handler, forwarder-receiver, client-dispatcher-server, publisher-subscriber.
3
Service access and configuration patterns
Wrapper façade, component configurator, interceptor, extension interface.
4
Event handling patterns
Reactor, proactor, asynchronous completion token, acceptor-connector.
5
Synchronization patterns
Scoped locking, strategized locking, thread-safe interface, double-checked locking optimization.
6
Concurrency patterns
Active object, monitor object, half-sync/half-async, leader/followers, thread-specific storage.
7
Resource acquisition patterns
Lookup, lazy acquisition, eager acquisition, partial acquisition.
8
Resource lifecycle
Caching, pooling, coordinator, resource lifecycle manager.
9
Resource release patterns
Leasing, evictor.
10
A pattern language for distributive computing
Consolidation of patterns from different catalogs in the context of distributed programming.
11
On patterns and pattern languages
This last volume gives some meta information about patterns, pattern languages, and their usage.
We believe the POSA catalog is very important (to the extent, one of the author feels, that if someone has not heard about the POSA catalog, he or she does not understand patterns) if one is writing middleware servers and scalable web infrastructure. For some reason, it has not got the kind of traction that it deserves. They are very useful for writing server-class software infrastructure such as web containers, application containers, and other middleware components.
POEAA catalog
Martin Fowler, along with some co-authors, published a book entitled Patterns of Enterprise Application Architecture. The book is a treasure trove of patterns, that helps one to structure and organize the design of enterprise applications using .NET and Java. Some of Fowler's pattern has been leveraged in the context of distributed computing by POSA-Volume 4 authors.
Sr. no
Pattern type
Patterns
1
Domain logic
Transaction script, domain model, table module, service layer.
2
Data source architectural patterns
Table data gateway, row data gateway, active record, data mapper.
3
Object-relational behavioral patterns
Unit of work, identity map, lazy load.
4
Object-relational structural patterns
Identity field, foreign key mapping, association table mapping, dependent mapping, embedded value, serialized LOB, single table inheritance, class table inheritance, concrete table inheritance, inheritance mappers.
5
Object-relational metadata mapping patterns
Metadata mapping, query object, repository.
6
Web presentation patterns
Model view controller, page controller, front controller, template view, transform view, two-step view, application controller.
7
Distribution patterns
Remote facade, data transfer object.
8
Offline concurrency patterns
Optimistic offline lock, pessimistic offline lock, coarse grained lock, implicit lock.
9
Session state patterns
Client session state, server session state, database session state.
10
Base patterns
Gateway, mapper, layer supertype, separated interface, registry, value object, money, special case, plugin, service stub, record set.
The POEAA catalog is a rich source of ideas when it comes to enterprise application software development. Some of these patterns are implemented by frameworks such as Spring (including Spring.NET), Nhibernate/Entity Framework, Windows Communication Foundation (WCF), and Windows Presentation Foundation (WPF). Awareness about the POEAA catalog helps one to reason about the architecture of pretty much everything happening in the .NET platform.
EIP catalog
A modern day enterprise requires information to flow from one application to another, in real time or while offline. Since applications use different implementation technologies, we require message passing systems to transfer the data. Most often, these communications happen in an asynchronous manner. The
