The specification pattern allows business rules to be combined using boolean logic to filter objects. It defines a common interface for all specifications to determine if an object meets a certain specification. This loose coupling avoids polluting the repository interface with many filter methods. Specifications can be written for individual rules, then combined to check complex business rules. For example, in recruitment, specifications for a candidate's test score and interview result can be ANDed or ORed to check eligibility.

1. Specifications pattern
TungNT @ Septeni Technolgy

2. Specification Pattern by TungNT 2
Introduction
“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.”
- Wikipedia -

3. Specification Pattern by TungNT 3
Hmmm...

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Let's start by example
● Let's assume we need to build a recruitment
system and we have a user story look like this:
– As recruiter I can check on the status of candidate
– As recruiter I can get list of candidate who passed
Note: Candidate passed if:
– Scala skill test score >= 80
– Interview passed
– ...

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Go! go! go!...

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OOP way
● In “As recruiter I can check on the status of candidate”
~> Created Candidate class which has isPassed
method to check status of candidate

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OOP way (cont.)
● In “As recruiter I can get list of candidate who passed”
~> Created Candidate Repository to get all
candidate who passed from persitence

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But...

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Is there a better way to do this?

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Specification way
● In “As recruiter I can check on the status of candidate”
~>

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Specification way (cont.)
● In “As recruiter I can get list of candidate who passed”
~>

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As you'll soon realize...
● specifications are:
– cheap to write
– easy to test
– encapsulate the business rule
– use the Specification Object anywhere
– can be combined to represent very complex
business rules

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How it work?

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By DDD sample
● Create a Specification interface:

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By DDD sample (cont.)
● Create a Specification abstract:

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By DDD sample (cont.)
● Create a AND Specification:

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By DDD sample (cont.)
● Create a OR Specification:

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By DDD sample (cont.)
● Create a NOT Specification:

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Using combined specs
● And operator

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Conclusion
● Some key benefits of the Specification Pattern
are:
– Loose coupling of the filter logic from the objects
being filtered
– Single responsibility: Interface of repository/data
provider isn’t polluted with lots of filter methods
– Encapsulate the business rule and...
– Can be combined to represent very complex
business rules

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References
● Links:
1. https://en.wikipedia.org/wiki/Specification_pattern
2. http://marcaube.ca/2015/05/specifications/
3. Is the Specification Pattern obsolete?

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Thank you!