Specification pattern


In computer programming, the specification pattern is a particular software design pattern, whereby business rules can be recombined by chaining the business rules together using Boolean logic. The pattern is frequently used in the context of domain-driven design.
A specification pattern outlines a business rule that is combinable with other business rules. In this pattern, a unit of business logic inherits its functionality from the abstract aggregate Composite Specification class. The Composite Specification class has one function called IsSatisfiedBy that returns a Boolean value. After instantiation, the specification is "chained" with other specifications, making new specifications easily maintainable, yet highly customizable business logic. Furthermore, upon instantiation the business logic may, through method invocation or inversion of control, have its state altered in order to become a delegate of other classes such as a persistence repository.
As a consequence of performing runtime composition of high-level business/domain logic, the Specification pattern is a convenient tool for converting ad-hoc user search criteria into low level logic to be processed by repositories.
Since a specification is an encapsulation of logic in a reusable form it is very simple to thoroughly unit test, and when used in this context is also an implementation of the humble object pattern.

Code examples

C#


public interface ISpecification
public abstract class CompositeSpecification : ISpecification
public class AndSpecification : CompositeSpecification
public class AndNotSpecification : CompositeSpecification
public class OrSpecification : CompositeSpecification
public class OrNotSpecification : CompositeSpecification
public class NotSpecification : CompositeSpecification

C# 6.0 with generics


public interface ISpecification
public abstract class LinqSpecification : CompositeSpecification
public abstract class CompositeSpecification : ISpecification
public class AndSpecification : CompositeSpecification
public class AndNotSpecification : CompositeSpecification
public class OrSpecification : CompositeSpecification
public class OrNotSpecification : CompositeSpecification
public class NotSpecification : CompositeSpecification

Python


from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import Any
class BaseSpecification:
@abstractmethod
def is_satisfied_by -> bool:
raise NotImplementedError
def __call__ -> bool:
return self.is_satisfied_by
def __and__ -> "AndSpecification":
return AndSpecification
def __or__ -> "OrSpecification":
return OrSpecification
def __neg__ -> "NotSpecification":
return NotSpecification
@dataclass
class AndSpecification:
first: BaseSpecification
second: BaseSpecification
def is_satisfied_by -> bool:
return self.first.is_satisfied_by and self.second.is_satisfied_by
@dataclass
class OrSpecification:
first: BaseSpecification
second: BaseSpecification
def is_satisfied_by -> bool:
return self.first.is_satisfied_by or self.second.is_satisfied_by
@dataclass
class NotSpecification:
subject: BaseSpecification
def is_satisfied_by -> bool:
return not self.subject.is_satisfied_by

C++


template
class ISpecification
template
class CompositeSpecification : public ISpecification
template
class AndSpecification final : public CompositeSpecification
template
ISpecification* CompositeSpecification::And const
template
class AndNotSpecification final : public CompositeSpecification
template
class OrSpecification final : public CompositeSpecification
template
class OrNotSpecification final : public CompositeSpecification
template
class NotSpecification final : public CompositeSpecification
template
ISpecification* CompositeSpecification::AndNot const
template
ISpecification* CompositeSpecification::Or const
template
ISpecification* CompositeSpecification::OrNot const
template
ISpecification* CompositeSpecification::Not const

TypeScript


export interface ISpecification
export abstract class CompositeSpecification implements ISpecification
export class AndSpecification extends CompositeSpecification
export class AndNotSpecification extends CompositeSpecification
export class OrSpecification extends CompositeSpecification
export class OrNotSpecification extends CompositeSpecification
export class NotSpecification extends CompositeSpecification

Example of use

In the next example, invoices are retrieved and sent to a collection agency if:
  1. they are overdue,
  2. notices have been sent, and
  3. they are not already with the collection agency.
This example is meant to show the result of how the logic is 'chained' together.
This usage example assumes a previously defined OverdueSpecification class that is satisfied when an invoice's due date is 30 days or older, a NoticeSentSpecification class that is satisfied when three notices have been sent to the customer, and an InCollectionSpecification class that is satisfied when an invoice has already been sent to the collection agency. The implementation of these classes isn't important here.
Using these three specifications, we created a new specification called SendToCollection which will be satisfied when an invoice is overdue, when notices have been sent to the customer, and are not already with the collection agency.

var overdue = new OverdueSpecification;
var noticeSent = new NoticeSentSpecification;
var inCollection = new InCollectionSpecification;
// Example of specification pattern logic chaining
var sendToCollection = overdue.And.And);
var invoices = InvoiceService.GetInvoices;
foreach