Post on 18-Jul-2015
Object Invariants, Immutability & You
Making Objects Do Your Bidding NijmegenPHP 15-04-2015
Jeroen v.d. Gulik
• CTO / Co-Founder Isset (http://isset.nl)
• Builder of Artificial Stupidity
• @n0x13
• Likes Cats … a lot
Isset Internet Professionals
• We build custom internet technology based solutions
• We helped design and build several online video platforms (Pathé Thuis, Videoland, NPO Plus/NLZiet)
• Currently focussing on video-transcoder.com
• We like a challenge so if you have interesting problems to solve, come and talk to me
• Still Hiring
“Every Great Talk should start with a Great Quote”
Ross Tuck
-House M.D.
Telephone Game
“Over time, the original intent and purpose gets
lost”- Jeroen v.d. Gulik
“A Customer should always have an email
address”
Testing shows the presence, not the absence of bugs.
- Kanye West
“We should enforce (business) rules when creating objects”
- Justin Bieber
“A Customer should have a valid email address”
Outside-in validation should be avoided
- Albert Einstein
Objects should be responsible for their own internal consistency
- Barack Obama
Best Solution ?
● Single Responsibility Principle
● Duplicate code
Design by contract
Value Object
● Small Objects
● Equality based on value
● Immutable
Example Value Objects
• Money
• Currency
• Time
• DateRange / Period
Entity Object
● Usually big(-ger)
● Equality based on Identity
● Mutable
● Can change over time
Immutability● Simplifies code
● Less cognitive load
● Guaranteed consistent
● Referencing without side effects
● Easy to test
● Caching (profit!)
Immutable Object, Best Object
- The Hulk
“A Customer should have a valid email address
and a password”
Make the Implicit, Explicit !
“As an Author I want to be able to Create an Article. Before Publishing it, I want to have a Draft. I also would like to be able
to Retract an Article with a Reason”
Objects as dumb data stores a.k.a.
Anemic Domain Model
This is our industry standard
Published ?
Objects should have behaviour attached
to them- Lex Luther
Objects should tell a tale about its current state and all the possible
ways to change that state- Bruce Lee
Schrödinger's cat
UnpaidOrder PaidOrder
“Sounds like a State Machine”- Igor Wiedler
Immutabilitybut of a different kind
UnpublishedArticle PublishedArticle
UnpublishedArticle PublishedArticleDraft Article
Draft Article RetractedArticle
Objects should only expose methods relevant to it's current
state
- George Bush
Objects should …
• Be responsible for their own internal consistency
• Have specific behaviour towards its responsibility
• Broadcast their state
• Only care about the state it’s currently in.
But there is still a piece of the puzzle missing…
As a Publisher I want to know which Article has been retracted more than X times
As an Editor I want to see all the changes that have
been made
Store all the Changes (Δ delta’s),that have
happened
EventSourcing!But that is a talk on its own
Magic is bad, hmmmkay- Saddam Hussein