Patterns

11 min. read

Introduction
In software engineering, a software design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. It 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. Design patterns are formalized best practices that the programmer can use to solve common problems when designing an application or system

Design patterns may be viewed as a structured approach to computer programming intermediate between the levels of a programming paradigm and a concrete algorithm.

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Documents
Pattern Name and Classification: A descriptive and unique name that helps in identifying and referring to the pattern.
Intent: A description of the goal behind the pattern and the reason for using it.
Also Known As: Other names for the pattern.
Motivation (Forces): A scenario consisting of a problem and a context in which this pattern can be used.
Applicability: Situations in which this pattern is usable; the context for the pattern.
Structure: A graphical representation of the pattern. Class diagrams and Interaction diagrams may be used for this purpose.
Participants: A listing of the classes and objects used in the pattern and their roles in the design.
Collaboration: A description of how classes and objects used in the pattern interact with each other.
Consequences: A description of the results, side effects, and trade offs caused by using the pattern.
Implementation: A description of an implementation of the pattern; the solution part of the pattern.
Sample Code: An illustration of how the pattern can be used in a programming language.
Known Uses: Examples of real usages of the pattern.
Related Patterns: Other patterns that have some relationship with the pattern; discussion of the differences between the pattern and similar patterns.

Patterns
1 Creational Patterns
Abstract factory
Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
Builder
Separate the construction of a complex object from its representation, allowing the same construction process to create various representations.
Dependency Injection
A class accepts the objects it requires from an injector instead of creating the objects directly.
Factory method
Define an interface for creating a single object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
Creates other objects
This pattern helps you avoid the problem of complex object instantiation by keeping these complex configurations in a single place, rather than scattered around in your code
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. This pattern appears in the GoF catalog as “virtual proxy”, an implementation strategy for the Proxy pattern.
Multiton
Ensure a class has only named instances, and provide a global point of access to them.
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.
Prototype
Specify the kinds of objects to create using a prototypical instance, and create new objects from the ‘skeleton’ of an existing object, thus boosting performance and keeping memory footprints to a minimum.

Resource acquisition is initialization (RAII)
Ensure that resources are properly released by tying them to the lifespan of suitable objects.
Singleton
Ensure a class has only one instance, and provide a global point of access to it.

2 Structural Patterns
Adapter, 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.
Bridge
Decouple an abstraction from its implementation allowing the two to vary independently.
Composite
Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
Decorator
Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality.
Extension object
Adding functionality to a hierarchy without changing the hierarchy.
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.
Flyweight
Use sharing to support large numbers of similar objects efficiently.
Front controller
The pattern relates to the design of Web applications. It provides a centralized entry point for handling requests.
Marker
Empty interface to associate metadata with a class.
Module
Group several related elements, such as classes, singletons, methods, globally used, into a single conceptual entity.
Proxy
Provide a surrogate or placeholder for another object to control access to it.
Twin
Twin allows modeling of multiple inheritance in programming languages that do not support this feature.

3 Behavioral patterns
Blackboard
Artificial intelligence pattern for combining disparate sources of data (see blackboard system)
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.
Command
Encapsulate a request as an object, thereby allowing for the parameterization of clients with different requests, and the queuing or logging of requests. It also allows for the support of undoable operations.
Interpreter
Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
Iterator
Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
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 allows their interaction to vary independently.
Memento
Without violating encapsulation, capture and externalize an object’s internal state allowing the object to be restored to this state later.
Null object
Avoid null references by providing a default object.
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.

Servant
Define common functionality for a group of classes.
Specification
Recombinable business logic in a Boolean fashion.

Strategy
Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
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.
Visitor
Represent an operation to be performed on the elements of an object structure. Visitor lets a new operation be defined without changing the classes of the elements on which it operates.

4 Concurrency patterns
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.
Balking
Only execute an action on an object when the object is in a particular state.
Binding properties
Combining multiple observers to force properties in different objects to be synchronized or coordinated in some way
Blockchain
Decentralized way to store data and agree on ways of processing it in a Merkle tree, optionally using digital signature for any individual contributions.
Compute kernel
The same calculation many times in parallel, differing by integer parameters used with non-branching pointer math into shared arrays, such as GPU-optimized Matrix multiplication or Convolutional neural network.
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 locking logic proceed.
Can be unsafe when implemented in some language/hardware combinations. It can therefore sometimes be considered an anti-pattern.
Event-based asynchronous
Addresses problems with the asynchronous pattern that occur in multithreaded programs
Guarded suspension
Manages operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed.
Join
Join-pattern provides a way to write concurrent, parallel and distributed programs by message passing. Compared to the use of threads and locks, this is a high-level programming model.
Lock
One thread puts a “lock” on a resource, preventing other threads from accessing or modifying it
Messaging design pattern (MDP)
Allows the interchange of information (i.e. messages) between components and applications.
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.
Reactor
A reactor object provides an asynchronous interface to resources that must be handled synchronously.
Read-write lock
Allows concurrent read access to an object, but requires exclusive access for write operations.
Scheduler
Explicitly control when threads may execute single-threaded code.
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.
Thread-specific storage
Static or “global” memory local to a thread.