software design patterns

In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into source or machine code. It is a description or template for how to solve a problem that can be used in many different situations. Patterns are formalized best practices that the programmer must implement themselves in the application.  Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved. Many patterns imply object-orientation or more generally mutable state, and so may not be as applicable in functional programming languages, in which data is immutable or treated as such.

Design patterns reside in the domain of modules and interconnections. At a higher level there are architectural patterns that are larger in scope, usually describing an overall pattern followed by an entire system.

There are many types of design patterns, for instance

  • Algorithm strategy patterns addressing concerns related to high-level strategies describing how to exploit application characteristics on a computing platform.
  • Computational design patterns addressing concerns related to key computation identification.
  • Execution patterns that address concerns related to supporting application execution, including strategies in executing streams of tasks and building blocks to support task synchronization.
  • Implementation strategy patterns addressing concerns related to implementing source code to support
    1. program organization, and
    2. the common data structures specific to parallel programming.
  • Structural design patterns addressing concerns related to high-level structures of applications being developed.

Design patterns can speed up the development process by providing tested, proven development paradigms.  Effective software design requires considering issues that may not become visible until later in the implementation. Reusing design patterns helps to prevent subtle issues that can cause major problems, and it also improves code readability for coders and architects who are familiar with the patterns.

In order to achieve flexibility, design patterns usually introduce additional levels of indirection, which in some cases may complicate the resulting designs and hurt application performance.

By definition, a pattern must be programmed anew into each application that uses it. Since some authors see this as a step backward from software reuse as provided by components, researchers have worked to turn patterns into components. Meyer and Arnout were able to provide full or partial componentization of two-thirds of the patterns they attempted.

Software design techniques are difficult to apply to a broader range of problems. Design patterns provide general solutions, documented in a format that does not require specifics tied to a particular problem.

Structure

Design patterns are composed of several sections . Of particular interest are the Structure, Participants, and Collaboration sections. These sections describe a design motif: a prototypical micro-architecture that developers copy and adapt to their particular designs to solve the recurrent problem described by the design pattern. A micro-architecture is a set of program constituents (e.g., classes, methods…) and their relationships. Developers use the design pattern by introducing in their designs this prototypical micro-architecture, which means that micro-architectures in their designs will have structure and organization similar to the chosen design motif.

In addition to this, patterns allow developers to communicate using well-known, well understood names for software interactions. Common design patterns can be improved over time, making them more robust than ad-hoc designs.

Domain-specific patterns

Efforts have also been made to codify design patterns in particular domains, including use of existing design patterns as well as domain specific design patterns. Examples include user interface design patterns, information visualization, secure design, “secure usability”, Web design [14] and business model design.

The annual Pattern Languages of Programming Conference proceedings [16] include many examples of domain specific patterns.

Classification and list

Design patterns were originally grouped into the categories: creational patterns, structural patterns, and behavioral patterns, and described using the concepts of delegation, aggregation, and consultation. For further background on object-oriented design, see coupling and cohesion, inheritance, interface, and polymorphism. Another classification has also introduced the notion of architectural design pattern that may be applied at the architecture level of the software such as the Model–View–Controller pattern.

Creational patterns
Name Description In Design Patterns In Code Complete Other
Abstract factory Provide an interface for creating families of related or dependent objects without specifying their concrete classes. Yes Yes N/A
Builder Separate the construction of a complex object from its representation allowing the same construction process to create various representations. Yes No N/A
Factory method Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses (dependency injection[18]). Yes Yes N/A
Lazy initialization Tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed. No No PoEAA[19]
Multiton Ensure a class has only named instances, and provide global point of access to them. No No N/A
Object pool Avoid expensive acquisition and release of resources by recycling objects that are no longer in use. Can be considered a generalisation of connection pool and thread pool patterns. No No N/A
Prototype Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Yes No N/A
Resource acquisition is initialization Ensure that resources are properly released by tying them to the lifespan of suitable objects. No No N/A
Singleton Ensure a class has only one instance, and provide a global point of access to it. Yes Yes N/A
Structural patterns
Name Description In Design Patterns In Code Complete[17] Other
Adapter or Wrapper or Translator. Convert the interface of a class into another interface clients expect. An adapter lets classes work together that could not otherwise because of incompatible interfaces. The enterprise integration pattern equivalent is the translator. Yes Yes N/A
Bridge Decouple an abstraction from its implementation allowing the two to vary independently. Yes Yes N/A
Composite Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. Yes Yes N/A
Decorator Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality. Yes Yes N/A
Facade Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use. Yes Yes N/A
Flyweight Use sharing to support large numbers of similar objects efficiently. Yes No N/A
Front Controller The pattern relates to the design of Web applications. It provides a centralized entry point for handling requests. No Yes N/A
Module Group several related elements, such as classes, singletons, methods, globally used, into a single conceptual entity. No No N/A
Proxy Provide a surrogate or placeholder for another object to control access to it. Yes No N/A
Behavioral patterns
Name Description In Design Patterns In Code Complete[17] Other
Blackboard Generalized observer, which allows multiple readers and writers. Communicates information system-wide. No No N/A
Chain of responsibility Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. Yes No N/A
Command Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. Yes No N/A
Interpreter Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. Yes No N/A
Iterator Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. Yes Yes N/A
Mediator Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently. Yes No N/A
Memento Without violating encapsulation, capture and externalize an object’s internal state allowing the object to be restored to this state later. Yes No N/A
Null object Avoid null references by providing a default object. No No N/A
Observer or Publish/subscribe Define a one-to-many dependency between objects where a state change in one object results in all its dependents being notified and updated automatically. Yes Yes N/A
Servant Define common functionality for a group of classes No No N/A
Specification Recombinable business logic in a Boolean fashion No No N/A
State Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. Yes No N/A
Strategy Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. Yes Yes N/A
Template method Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure. Yes Yes N/A
Visitor Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. Yes No N/A
Concurrency patterns
Name Description In POSA2[20] Other
Active Object Decouples method execution from method invocation that reside in their own thread of control. The goal is to introduce concurrency, by using asynchronous method invocation and a scheduler for handling requests. Yes N/A
Balking Only execute an action on an object when the object is in a particular state. No N/A
Binding properties Combining multiple observers to force properties in different objects to be synchronized or coordinated in some way.[21] No N/A
Double-checked locking Reduce the overhead of acquiring a lock by first testing the locking criterion (the ‘lock hint’) in an unsafe manner; only if that succeeds does the actual lock proceed.Can be unsafe when implemented in some language/hardware combinations. It can therefore sometimes be considered an anti-pattern. Yes N/A
Event-based asynchronous Addresses problems with the asynchronous pattern that occur in multithreaded programs. No N/A
Guarded suspension Manages operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed. No N/A
Lock One thread puts a “lock” on a resource, preventing other threads from accessing or modifying it.[23] No PoEAA[19]
Messaging design pattern (MDP) Allows the interchange of information (i.e. messages) between components and applications. No N/A
Monitor object An object whose methods are subject to mutual exclusion, thus preventing multiple objects from erroneously trying to use it at the same time. Yes N/A
Reactor A reactor object provides an asynchronous interface to resources that must be handled synchronously. Yes N/A
Read-write lock Allows concurrent read access to an object, but requires exclusive access for write operations. No N/A
Scheduler Explicitly control when threads may execute single-threaded code. No N/A
Thread pool A number of threads are created to perform a number of tasks, which are usually organized in a queue. Typically, there are many more tasks than threads. Can be considered a special case of the object pool pattern. No N/A
Thread-specific storage Static or “global” memory local to a thread. Yes N/A
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