event sourcing
DESCRIPTION
Presentation November 13, 2014 at 010PHP, Rotterdam, http://www.meetup.com/010PHP/events/212147112/ about Event Sourcing: In many applications the current state is constantly updated, overwriting previous states. Information about the history is lost then. With Event Sourcing you store all events that have led to a change in application state. The current and previous states can always be reconstituted from the series of events that has been stored. That gives lots of possibilities. I gave an overview of the background of Event Sourcing, with a sidestep to some thoughts about 'identity'. Related concepts, like CQRS and other DDD-jargon, were explained and I briefly mentioned Functional Programming. After my talk we looked at Broadway, the open sourced project from Qandidate, that provides a PHP infrastructure for Event Sourcing and CQRS. FritsJan Bakker, one of the builders of Broadway, told something more about it and how they used Event Sourcing at Qandidate. ----- Summary of slides: * 2-3: while updating, history is lost * 4: an n:m relationship where info of the past was stored * 5-7: DDD-jargon: Aggregate and Aggregate Root * 8-10: Demeter's Law as applied to the Aggregate Root * 11-17: Event Sourcing, storing past events, like in accounting * 18-25: about identity (you are the realisation of changes) * 26: Functional Programming (same paradigm as ES) * 27-30: CQRS and the bigger picture * 31-33: Broadway * 34: Pool RequestTRANSCRIPT
Herman Peeren010PHP
November 2014
Event Sourcing
Update
Update
Entities - Relationships
DDD-jargon: Aggregate
Aggregate: house of cooperating entities (consistency boundaries)
DDD-jargon: Aggregate Root
Aggregate root: the entrance of the house, the interface to the outside world
DDD-jargon: Aggregate Root
Aggregate protects its own invariants: keep your house clean
Demeter’s Lawonly talk to your immediate friends
Demeter’s LawCommand the dog, not the dog’s legs
Demeter’s LawCommand the dog, not the dog’s legs
Event Sourcing• you don’t store the state, • but the proces that caused the change
Accounting: same principle• you don’t store the changed balance, • but the journal entries that caused the change
Event Sourcing• you don’t add an item to a collection of jobs, • but you take or quit a job
Event Sourcing• you don’t add an item to a collection of jobs, • but you hire or fire someone
Emphasis on the action, using words used in the domain, not on the data
Event Store
Event Store
Event Sourcing
EntityEquality is decided by its identity, not by its values
never too old to run
Theseus’ship paradox
Theseus’ship paradox
Theseus’ship paradox
Identity
Identity
Identity
IdentityYou are who you were + the difference since then
Functional ProgrammingData are what they were + the difference since then
Event Sourcing & CQRSCQRS = Command Query Responsibility Segregation: separate Read & Write Model
Event Sourcing & CQRS
FromtheAxonFrameworkdocumentation
Event Sourcing & CQRS
Event Sourcing+• lossles, extra read-models can always later be added• more verbs than data centric, closer to mental model and domain language
-• increased complexity when state is rarely updated
CQRS+• loosely coupled, specialised read & write side• scalability
-• more complexity when project is not complex or doesn’t need scaling
Broadway
Broadway: namespaces
Broadway: examples
Other interesting PHP framework (less production-ready?): https://github.com/szjani/predaddy
Pool RequestJoomlaWorldConference,Mexiconovember2014
Thank you!
hermanpeeren.nl@HermanPeeren
November 13, 2014
Credits:Sorry! For this presentation I’ve used all kinds of images I collected from the web during the past months. I didn’t note down the various sources, so I cannot give credit to the owners of the images. I can only thank you all very much for making them available on the internet and I hope I did not violate your rights too much borrowing them. If you own the rights to an image used here, please let me know and I will mention it (or take the image out of these slides if you want to).