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September 02, 2022

Software Design Patterns

Taken from Wikipedia, personal adaptations and additions made, shortened to be used as a small reference card, links back to wikipedia kept for details.

See also Software Anti-Patterns.

Architectural patternsChoice: How is the software system (as a whole) organized?
LayersSeparation of the system in layers, each layer knowing only of the layer above and under itself
MVCSeparation of the view (user interface) from the model (underlying data) and the controller (user interaction)
MVPModel View Presenter: all presentation logic is pushed to the presenter, mainly used for GUI development
MVVMModel View ViewModel: The view model handles the view's display logic, is responsible for exposing the data objects from the model in such a way that the objects are easily managed and consumed
Multitier architectureThe presentation, the application processing, and the data management are logically separate processes
PipelineChain of processing elements
Implicit invocationEvent broadcasting, the caller doesn't know who is called
Blackboard systemA common knowledge base, is iteratively updated by a diverse subsystems
Peer-to-peerNo central server, each peer is supplier and consumer
Service-oriented architectureLoose coupling of services within the system
Naked objectsObjects and their representation are not biased by adapters or proxies


[View] -down-> [Controller]
[Controller] -down-> [Model]
[Model] -> [View]


[View] `<-down->` [Presenter]
[Presenter] `<-down->` [Model]


[View] `<-down->` [Data Binding]
[Data Binding] `<-down->` [ViewModel]
[ViewModel] `<-down->` [Model]
Creational patternsChoice: How to create the objects?
Abstract factoryInterface for creating families of related or dependent objects without specifying their concrete classes
Factory methodCreating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses
BuilderSeparate the construction of a complex object from its representation. The same construction process can create different representations
Lazy initializationTactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed
Object poolAvoid expensive acquisition and release of resources by recycling objects that are no longer in use
PrototypeSpecify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. (Cloning)
SingletonEnsure a class has only one instance, and provide a global point of access to it
MultitonEnsure a class has only named instances, and provide global point of access to them
Resource acquisition is initializationEnsure that resources are properly released by tying them to the lifespan of suitable objects
Structural patternsChoice: What relationship have the objects?
Adapter or WrapperConvert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.
BridgeDecouple an abstraction from its implementation so that the two can vary independently
CompositeCompose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly
DecoratorAttach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality
FacadeProvide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use
FlyweightUse sharing to support large numbers of fine-grained objects efficiently
ProxyProvide a surrogate or placeholder for another object to control access to it
Behavioral patternsChoice: How are the objects communicating?
Chain of responsibilityAvoid 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
CommandEncapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undo-able operations
InterpreterGiven a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language
IteratorProvide a way to access the elements of an aggregate object sequentially without exposing its underlying representation
MediatorDefine 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
RestorerAn alternative to the existing Memento pattern
MementoWithout violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later
Null ObjectDesigned to act as a default value of an object, usually to avoid special treatment for null pointers
BlackboardGeneralized observer, which allows multiple readers and writers. Communicates information system-wide
StateAllow an object to alter its behavior when its internal state changes. The object will appear to change its class
StrategyDefine a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it
SpecificationRecombinable business logic in a boolean fashion
Template methodDefine 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
VisitorRepresent 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
Concurrency patternsChoice: How to synchronize concurrent access to the object?
Active ObjectDecouples 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
Binding PropertiesCombining multiple observers to force properties in different objects to be synchronized or coordinated in some way
Event-Based AsynchronousAddresses problems with the Asynchronous Pattern that occur in multithreaded programs
BalkingExecutes an action on an object only when the object is in a particular state
Guarded suspensionIn concurrent programming, managing operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed
Monitor objectApproach to synchronize two or more computer tasks that use a shared resource, usually a hardware device or a set of variables
SchedulerExplicitly control when threads may execute single-threaded code
Thread poolA 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
Thread-specific storageThread-local storage (TLS) is a computer programming method that uses static or global memory local to a thread
ReactorHandling service requests delivered concurrently to a service handler by one or more inputs. The service handler then demultiplexes the incoming requests and dispatches them synchronously to the associated request handlers
LockOne thread puts a "lock" on a resource, preventing other threads from accessing or modifying it
Double checked locking"double-checked locking optimization". 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((In some language/hardware combinations, can be unsafe. It can therefore sometimes be considered an anti-pattern))
Read write lockAllows concurrent read access to an object but requires exclusive access for write operations
Unit Test patternsChoice: Better choices for implementing or organizing tests?
AAA, Arrange/Act/AssertAAA is a pattern for organizing unit tests. Arrange: Perform the setup and initialization required for the test. Act: Take action(s) required for the test. Assert: Verify the outcome(s) of the test.
GWT, Given/When/ThenBehavior Driven Development point of view. Given: describe the state of the world before you begin (pre-conditions). When: the behavior you're specifying. Then: section describes the expected changes.
Result testchecks the returned value
State testchecks the state of the object under test. i.e. a length for a vector, state variable on the object
Unit testTest using a single component in complete isolation.
Integration testTest using a working environment, not a single component in isolation.
E2E (End to End) testTest a driver simulating user interaction and triggering communication from the front-end to the backend and back.
StubA stub is an object that provides (canned) hardcoded values to method calls. It always returns the same output regardless of the input.
SpyA spy lets us verify what functions were called, with what arguments, when, and how often.
MockA mock is similar to a stub, but the behaviour of the mocked interface can be changed dynamically based on scenarios. It is also similar to a spy as it allows us to verify that a method was called. However, the assertion is in the verify method in a mock.
FakeA Fake is an object with a concrete implementation that works similarly to the actual implementation. It is a simplified version of production code.
DoubleGeneric term for Fake, Mock or Stub

Functional programming principles

  • functions are things
  • composition everywhere
  • types are NOT classes


  • Strive for totality (total functions cover the domain)
  • Use static types for domain modelling and documentation
  • Parametrize everything (values, functions, ...)
  • Function types are interfaces
  • Partial application (e.g. when working with lists, dependency injection)
  • Hollywood principle: continuations ("Dont't call us, we'll call you")
  • Chaining callbacks with continuations
  • Use bind to chain options or tasks or error handlers (monadic bind)
  • If you write own generic type add a map to it (functor)
  • simplify aggregation code with monoids
  • convert non monoids to monoids
  • convert expensive monoids to cheap monoids (using monoid to incrementally update)