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In this article, we will cover the need for software patterns and describe the most prominent and dominant software architecture patterns.
This article is an excerpt from Architectural Patterns by Pethuru Raj, Anupama Raman, and Harihara Subramanian.
Why software patterns?
There is a bevy of noteworthy transformations happening in the IT space, especially in software engineering. The complexity of recent software solutions is continuously going up due to the continued evolution of the business expectations. With complex software, not only does the software development activity become very difficult, but also the software maintenance and enhancement tasks become tedious and time-consuming. Software patterns come as a soothing factor for software architects, developers, and operators.
Types of software patterns
Several newer types of patterns are emerging in order to cater to different demands. This section throws some light on these.
An architecture pattern expresses a fundamental structural organization or schema for complex systems. It provides a set of predefined subsystems, specifies their unique responsibilities, and includes the decision-enabling rules and guidelines for organizing the relationships between them. The architecture pattern for a software system illustrates the macro-level structure for the whole software solution.
A design pattern provides a scheme for refining the subsystems or components of a system, or the relationships between them. It describes a commonly recurring structure of communicating components that solves a general design problem within a particular context. The design pattern for a software system prescribes the ways and means of building the software components.
There are other patterns, too. The dawn of the big data era mandates for distributed computing. The monolithic and massive nature of enterprise-scale applications demands microservices-centric applications. Here, application services need to be found and integrated in order to give an integrated result and view. Thus, there are integration-enabled patterns. Similarly, there are patterns for simplifying software deployment and delivery. Other complex actions are being addressed through the smart leverage of simple as well as composite patterns.
Software architecture patterns
Let’s look at some of the prominent and dominant software architecture patterns.
Object-oriented architecture (OOA)
Objects are the fundamental and foundational building blocks for all kinds of software applications. Therefore, the object-oriented architectural style has become the dominant one for producing object-oriented software applications. Ultimately, a software system is viewed as a dynamic collection of cooperating objects, instead of a set of routines or procedural instructions.
We know that there are proven object-oriented programming methods and enabling languages, such as C++, Java, and so on. The properties of inheritance, polymorphism, encapsulation, and composition being provided by OOA come in handy in producing highly modular (highly cohesive and loosely coupled), usable and reusable software applications.
The object-oriented style is suitable if we want to encapsulate logic and data together in reusable components. Also, the complex business logic that requires abstraction and dynamic behavior can effectively use this OOA.
Component-based assembly (CBD) architecture
Monolithic and massive applications can be partitioned into multiple interactive and smaller components. When components are found, bound, and composed, we get the full-fledged software applications. CBA does not focus on issues such as communication protocols and shared states. Components are reusable, replaceable, substitutable, extensible, independent, and so on.
Design patterns such as the dependency injection (DI) pattern or the service locator pattern can be used to manage dependencies between components and promote loose coupling and reuse. Such patterns are often used to build composite applications that combine and reuse components across multiple applications.
Aspect-oriented programming (AOP) aspects are another popular application building block. By deft maneuvering of this unit of development, different applications can be built and deployed. The AOP style aims to increase modularity by allowing the separation of cross-cutting concerns. AOP includes programming methods and tools that support the modularization of concerns at the level of the source code.
Agent-oriented software engineering (AOSE) is a programming paradigm where the construction of the software is centered on the concept of software agents. In contrast to the proven object-oriented programming, which has objects (providing methods with variable parameters) at its core, agent-oriented programming has externally specified agents with interfaces and messaging capabilities at its core. They can be thought of as abstractions of objects. Exchanged messages are interpreted by receiving agents, in a way specific to its class of agents.
Domain-driven design (DDD) architecture
Domain-driven design is an object-oriented approach to designing software based on the business domain, its elements and behaviors, and the relationships between them. It aims to enable software systems that are a correct realization of the underlying business domain by defining a domain model expressed in the language of business domain experts. The domain model can be viewed as a framework from which solutions can then be readied and rationalized.
DDD is good if we have a complex domain and we wish to improve communication and understanding within the development team. DDD can also be an ideal approach if we have large and complex enterprise data scenarios that are difficult to manage using the existing techniques.
This pattern segregates the system into two main applications, where the client makes requests to the server. In many cases, the server is a database with application logic represented as stored procedures. This pattern helps to design distributed systems that involve a client system and a server system and a connecting network.
The main benefits of the client/server architecture pattern are:
- Higher security: All data gets stored on the server, which generally offers a greater control of security than client machines.
- Centralized data access: Because data is stored only on the server, access and updates to the data are far easier to administer than in other architectural styles.
- Ease of maintenance: The server system can be a single machine or a cluster of multiple machines. The server application and the database can be made to run on a single machine or replicated across multiple machines to ensure easy scalability and high availability.
However, the traditional two-tier client/server architecture pattern has numerous disadvantages. Firstly, the tendency of keeping both application and data on a server can negatively impact system extensibility and scalability. The server can be a single point of failure. The reliability is the main worry here. To address these issues, the client-server architecture has evolved into the more general three-tier (or N-tier) architecture. This multi-tier architecture not only surmounts the issues just mentioned but also brings forth a set of new benefits.
Multi-tier distributed computing architecture
The two-tier architecture is neither flexible nor extensible. Hence, multi-tier distributed computing architecture has attracted a lot of attention. The application components can be deployed in multiple machines (these can be co-located and geographically distributed).
Application components can be integrated through messages or remote procedure calls (RPCs), remote method invocations (RMIs), common object request broker architecture (CORBA), enterprise Java beans (EJBs), and so on. The distributed deployment of application services ensures high availability, scalability, manageability, and so on. Web, cloud, mobile, and other customer-facing applications are deployed using this architecture.
Thus, based on the business requirements and the application complexity, IT teams can choose the simple two-tier client/server architecture or the advanced N-tier distributed architecture to deploy their applications. These patterns are for simplifying the deployment and delivery of software applications to their subscribers and users.
This pattern is an improvement over the client/server architecture pattern. This is the most commonly used architectural pattern. Typically, an enterprise software application comprises three or more layers: presentation/user interface layer, business logic layer, and data persistence layer. The presentation layer is primarily usded for user interface applications (thick clients) or web browsers (thin clients).
With the fast proliferation of mobile devices, mobile browsers are also being attached to the presentation layer. Such tiered segregation comes in handy in managing and maintaining each layer accordingly. The power of plug-in and play gets realized with this approach. Additional layers can be fit in as needed.
There are model view controller (MVC) pattern-compliant frameworks hugely simplifying enterprise-grade and web-scale applications. MVC is a web application architecture pattern. The main advantage of the layered architecture is the separation of concerns. That is, each layer can focus solely on its role and responsibility. The layered and tiered pattern makes the application:
- Easy to assign specific and separate roles
- Easy to update and enhance layers separately
This architecture pattern is good for developing web-scale, production-grade, and cloud-hosted applications quickly and in a risk-free fashion. When there are business and technology changes, this layered architecture comes in handy in embedding newer things in order to meet varying business requirements.
Event-driven architecture (EDA)
The world is eventually becoming event-driven. That is, applications have to be sensitive and responsive proactively, pre-emptively, and precisely. Whenever there is an event happening, applications have to receive the event information and plunge into the necessary activities immediately. The request and reply notion paves the way for the fire and forgets tenet. The communication becomes asynchronous. There is no need for the participating applications to be available online all the time.
EDA is typically based on an asynchronous message-driven communication model to propagate information throughout an enterprise. It supports a more natural alignment with an organization’s operational model by describing business activities as series of events. EDA does not bind functionally disparate systems and teams into the same centralized management model. EDA ultimately leads to highly decoupled systems. The common issues being introduced by system dependencies are getting eliminated through the adoption of the proven and potential EDA.
We have seen various forms of events used in different areas. There are business and technical events. Systems update their status and condition emitting events to be captured and subjected to a variety of investigations in order to precisely understand the prevailing situations. The submission of web forms and clicking on some hypertexts generate events to be captured. Incremental database synchronization mechanisms, RFID readings, email messages, short message service (SMS), instant messaging, and so on are events not to be taken lightly.
There are event processing engines, message-oriented middleware (MoM) solutions such as message queues and brokers to collect and stock event data and messages. Millions of events can be collected, parsed, and delivered through multiple topics through these MoM solutions. As event sources/producers publish notifications, event receivers can choose to listen to or filter out specific events and make proactive decisions in real-time on what to do next.
EDA style is built on the fundamental aspects of event notifications to facilitate immediate information dissemination and reactive business process execution. In an EDA environment, information can be propagated to all the services and applications in real-time. The EDA pattern enables highly reactive enterprise applications. Real-time analytics is the new normal with the surging popularity of the EDA pattern.
Service-oriented architecture (SOA)
With the arrival of service paradigms, software packages and libraries are being developed as a collection of services. Services are capable of running independently of the underlying technology. Also, services can be implemented using any programming and script languages.
Services are self-defined, autonomous, and interoperable, publicly discoverable, assessable, accessible, reusable, and compostable. Services interact with one another through messaging. There are service providers/developers and consumers/clients.
Every service has two parts: the interface and the implementation. The interface is the single point of contact for requesting services. Interfaces give the required separation between services. All kinds of deficiencies and differences of service implementation get hidden by the service interface.
Precisely speaking, SOA enables application functionality to be provided as a set of services, and the creation of personal as well as professional applications that make use of software services.
In short, SOA is for service-enablement and service-based integration of monolithic and massive applications. The complexity of enterprise process/application integration gets moderated through the smart leverage of the service paradigm.
To summarize, we detailed the prominent and dominant software architecture patterns and how they are used for producing and running any kind of enterprise-class and production-grade software applications.
To know more about patterns associated with object-oriented, component-based, client-server, and cloud architectures, grab the book Architectural Patterns.