My first year with Event-sourcing

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My first year with event-sourcing Froscon 25-08-2018

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Tim Huijzers @Dragem Developer @ drukwerkdeal.nl Founder of DeventerPHP Deventer NL

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First Try ● ● ● ● Limited knowledge No experience No ES framework Doomed from the start

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Second Try ● ● ● ● ● ● Limited knowledge Limited experience New Framework New DI manager New ES Framework Doomed To Fail

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Third Try ● ● ● ● ● ● Some knowledge Some experience Known framework known database known ES framework Still doomed

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BeerWarehouse

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Why use Event Sourcing

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CRUD

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We will save a new entry in our system because we just bought it and will store it in the fridge for later.

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If we change the location the system only knows about that location.

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We drank it so it’s not in the system anymore

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We want to keep a history of everything we drank.

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I want to know when I drank this in my history.

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But that’s only for new beers.

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Events

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Same Information as before + Explicit action about what happened

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Make Small Events

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Removed Location and changed name because in the real world you might not know this yet.

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When returning home I put the beer in my fridge

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I need room in my fridge so I take it out. Using the same Event

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And at last a event about when I consumed it.

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Crud ● ● ● I know what beer I have. I know when it was consumed. I know where it is. Event-Sourcing ● ● ● ● ● ● ● ● ● I know what beer I have. I know when it was consumed. I know where it is. I know where it was before. I know when it was moved. I know where it was at any point in time I know how many times it was moved. I know when it was added to the system. I know what else was moved in that day.

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“Every software program relates to some activity or interest of its user.” Eric Evans, Domain-Driven Design: Tackling Complexity in the Heart of Software

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When To Use Event Sourcing

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You need an audit log You like scalability You want to separate the read and write of an application You want to replay event on a dev machine to get an accurate situation of what the state was at a point in time. ● You want reporting but don’t know what yet. ● You are done with mapping objects to tables ● ● ● ●

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When NOT To Use Event Sourcing

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● ● ● ● You only need a simple CRUD system. You are processing a lot of personal data. You just want to query a lot of things on the DB You are starting on a big project for production

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Event Sourcing in code

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Prooph

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http://getprooph.org/

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Command

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Command Handler

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Aggregate

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Event

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Back to the Aggregate

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Think About Side Effects

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Structuring your application

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Understanding the DB

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How many beers do I have?

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How many different styles do I have?

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How many beers have I drank last 30 days?

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Projection

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A Projection allows you to loop through all event (past and present) and build your own views. ● Read Model ○ ● Projection ○ ● Define the data you would like to use. Loops through the events and applies that data to your view Finder ○ Helps you find data from that view.

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Pitfalls

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Refactoring is harder, think about your architecture

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Versioning ● ● ● Change an Event but support the old version Make a new Event Make the Event right from the start

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Something wrong with the event

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Event are immutable, So don’t change them ● ● ● ● ● Try solving it another way first. Correct errors with new events Try a upcaster Make a new stream and fill it with mutated events (and test) Change the events in the database

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But what if I have like 100 trillion gazillion events?

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Snapshots

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You Do Not Need Snapshots From The start

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Trigger on Event Count

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Pure Event Sourcing Is Not A Holy Grail

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Do Not Save Personal Data In Events

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Make Projections For All You Lists

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Try It In A Hackathon First

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Most Of The Time Your DB Is Not Holy

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What Now?

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http://getprooph.org/

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Source https://github.com/prooph/proophessor-do-symfony http://getprooph.org/

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Other Tools ● Broadway ○ ○ ○ ● Axon ○ ○ ○ ● No Upcaster, No Snapshots, No Replaying Upcasting by MessageFactory, Snapshots by Trigger on event count, Replaying by Example code for replay Akka ○ ○ ○ Upcasting by Event Adapter, Snapshots decided by actor, Replaying

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Thanks, Any Questions? Example code from talk on: https://github.com/webbaard/BeerWarehouse