Year with EventSourcing & CQRS

as web is growing

 

in complexity and size

more core business systems are web 

simple CMS

simple webshops

<html>
<?php
 $mysql = mysql_connect(....);
 $users = mysql_fetch('SELECT * FROM ...');
?>

<table>
<?php for($i=0;$i<count($users);$i++){ ?>
<tr>
  <td><?php echo $users[$i]['first_name']; ?> </td>
...
</tr>

...

<?php

if( $x = 123) {
 echo '<span> .... </span>';
}
?>

Good old days

lots of old school devs 'tought' this was good enough

MVC

CRUD

complexity increases

existing practices 

Feals like

tired of 'I have no idea why that happened'

greping logs

stuff not logged

joining 5 tables to get list of most popular X

looking for data instead of creating it

logging

logging

build state from changes

create data sources for each purpose

separate business complexity and scalability

id 123456789
title Register button doesnt work in IE
assigned null
type BUG
status OPEN
priority URGENT
createdAt 2016-03-11 22:22:11
updatedAt 2017-06-19 11:01:35
closedAt

Issue tracker: Issue

How long is this urgent bug opened?

No one is assigned to it?

Lets check the logs

  • IssueOpened: 123456789 [ 2016-03-11 22:22:11]
  • IssueClosed: 123456789 [2016-03-12 12:00:11]
  • IssueReopened: 123456789, [2017-01-04 11:01:11]
  • We are missing details now :(
  • Lets add logging when ..
  • Lets add logging when ..
  • Lets add logging when ..

What if we record all changes?

IssueCreated(123, 'Something', 'text', BUG, NORMAL, ' 2016..')

IssueAssigned(123, TeamManager, ' 2016-03-12 10:00:00')

PriorityChanged(123,URGENT, ' 2016-08-11 15:46:33')

IssueDellocated(123,'2016...')

IssueClosed(123,'2016...')

IssueReopened(123, ' 2017-06-19 11:01:35')

TitleChanged(123, ' Register button doesnt work in IE', '2016...')

and create state from applying them?

event sourcing

issue

issue

678

number of issues opened today

issue

678

number of issues opened today

456

number of issues opened on day YYYY-MM-DD

issue

678

number of issues opened today

abc123-123-121313

daa-2-2-3-3-2-3-2

opened urgent issue ids

issue

678

1.234

number of issues opened today

number of issues reopened this month

abc123-123-121313

daa-2-2-3-3-2-3-2

opened urgent issue ids

issue

678

1.234

67.890

number of issues opened today

number of issues reopened this month

number of closed issues last year

abc123-123-121313

daa-2-2-3-3-2-3-2

opened urgent issue ids

Command Query Responsibility Segregation

Miro Svrtan

senior developer 


ZgPHP user group organizer


@msvrtan

My ES + CQRS experience:

external concerts on TicketSwap

  • part of existing app
  • imports external concerts
  • admin flow to match to our concerts

project X

  • ver 2 build with full ES + CQRS
  • ver 1 had lot of 'what happened issues'
  • test ground to find when ES makes sense

devboard

  • my pet project
  • tracking github builds, PRs and issues
  • ver 1 had lot of existance issues
  • version 2 going full ES + CQRS

existance issues?

  • GitHub hook
    • branch exists? if not create one
    • commit exists? if not create one
    • author exists? if not create one
  • branch needs commit
  • commit needs author

Thinking of objects and relations

branch

commit

author

branch2

branch3

commit2

After rethinking it a bit

  • in my domain
    • commit is a value object
    • author is a value object
  • I don't care about relationship of branch->commit->author
  • I don't care how it looks in DB
  • I don't care how many commits did author do

No relation needed

branch

commit

author

branch2

branch3

commit2

commit

author

author

1st row

2nd row

3rd row

Defining bounded context

  • my projects don't have domain experts
  • often they are the 'domain'
  • separation is hard

You as a TicketSwap user:

  • can be a seller
  • can be a buyer too
  • just use notifications?

User is a buyer and seller

  • what is a bounded contexts here?
    • user
    • buyer
    • seller
  • I'm still trying to figure it out :)

In "boring" domain:

domain experts will tell you who does what

In "boring" domain:

 

  • buying a computer goes thru 'supply office'
    • fill a request
    • manager says OK
    • supply orders it

In "boring" domain:

 

  • your paycheck is calculated by human resources
    • talk to team lead and ask for more money
    • team lead says OK
    • manager says OK
    • HR does it's magic

In "boring" domain:

both will be paid by accounting

In "boring" domain:

CEO will get expense reports from financial ppl

In startups:

  • airbnb for bicycles
  • we dont know what we want
  • or where the road will take us
  • we might try X out
    • and we need it yesterday

Concentrate on bringing value not perfect code

TicketSwap example:

User, buyer, seller

  • you as a user
    • can be a seller
    • can be a buyer too
    • just use notifications?

TicketSwap example:

User, buyer, seller

  • unclear boundaries
  • too much "core" into the app
  • dont know future direction
  • existing code
    • looks ok
    • works ok

Domain - application - infrastructure separation

  • we often start by defining
    • language
    • framework
    • tools
  • design the ER model
  • 80/20 - 80% time spent on 20% of problem
  • try to make our problem fit into our solution
  • designing a 'house of cards'
  • implementing business logic & rules at crunch time

Saving invoice into RDBMS

  • relation to a person/company
  • company address changes?
  • person last name changes?

blog post + comments in NoSQL

  • building a entity with relations as a document
  • comments are related to a blog post
  • comments are related to a user

Domain - application - infrastructure separation

  • solve the problem
  • locate application usages
  • store data

Saga/process manager

  • connects multiple bounded contexts / aggregates
  • possible inconsistent state if Xth step fails

 

User registration saga

  • aggregates:
    • user
    • buyer
    • seller

User registration saga

  • BuyerSaga
    • listens for UserRegisteredEvent
    • sends a CreateBuyerCommand
  • SellerSaga
    • listens for UserRegisteredEvent
    • sends a CreateSellerCommand

Queues

Your best friend

Queues

Your best friend

And worst enemy

Queues

  • isolation
  • instead of 1 queue per 1 payload you can queue all commands to same queue

Queue: multiple queues & workers

  • 2 simultaneous changes on same aggregate root -> error
  • if no concurrency issues increase worker count > 1 

Queue: multiple queues & workers

  • each external concert I import is 100% sure unique
    • X workers on that queue
  • concurency issues on GitHub notifications 
    • 1 worker per queue
    • queue sharding by starting letter

Eventual consistency

  • workers are ASYNC
  • queues can be clogged

Eventual consistency

  • never blindly trust your read model data
  • aggregates are ONLY source of truth

Performance

  • loading events from an event store is blazing fast
  • unserializing is fast
  • applying is fast

Loading aggregate root

    <10 events                   10 ms

    ~50 events                   20 ms

  ~100 events                   50 ms

~2000 events              1500 ms

Big aggregate root

  • ~700 entities inside
  • 580.000 events

Snapshotting

  • avoid replaying all events for performance reasons
  • record the state of aggregate root and save it 
  • load snapshot + load events afterwards

Snapshotting: DIY

  • had to implement it myself :(
  • simple MySQL table with
    • autoincrement id
    • aggregate id
    • event number
    • payload (serialized object)

Snapshotting: DIY

  • creates a snapshot every ~200 events
  • 17Mb of payload
  • 1000 snapshots is 17 GB of data
  • aggregate load time < 100ms

Snapshotting: warning

Changing aggregate

  • delete all snapshots :(
  • manually generate snapshots for big aggregate roots

No read side logic

  • read side should be "anemic models"
  • putting logic is a design smell

No read side logic example

  • events have price with and without VAT
  • you need VAT amount or percentage in the application
  • don't calculate it on the read side
  • update your domain events
  • design smell that your domain is missing "crucial" data

Testing

  • skip ES + CQRS if no testing experience
  • don't test state of aggregate root
  • concentrate on unit/integration testing
  • do some end-to-end tests

Testing

  • mock the infrastructure
  • use InMemoryRepositories
  • not locked to infrastructure while developing
  • 10-100x faster testing 

Refactoring read side

  • external concert reads were stored in ElasticSearch
  • ElasticSearch is not a database :)
  • lots of issues -> lots of end to end tests

Refactoring read side

  • dropping old ElasticSearch cluster
  • refactored read side use DoctrineORM + MySQL
  • half of days work
  • updating tests -> ~2 days

Refactoring read side: replaying events

  • we had all events stored
  • replay all events to fill up new read models
  • 30 line PHP CLI command
  • 16.000 events ~ 1min 
  • turned off ElasticSearch

UUIDs

  • instead of expecting DB to provide ID
  • commands/aggregates need you to provide ID
  • ramsey/uuid

UUID performance

  • "64c3e987-905a-4426-8dc3-ddb61650b86b" takes more space than "1"
  • instead of 32 chars, you can save them as 16 binary
  • not continous
    • index rebalancing
    • need "createdAt" to sort

UUIDs everywhere

  • I try to avoid autoincrement in CRUD
  • helps avoiding problems where entity might have ID

Use familiar tech

  • ES+CQRS is a big shift in thinking
  • if you have experience using MySQL/Mongo/X/Y/Z try to use those instead of learning new tech as well

Class number explosion

  • UserController
  • User (Entity)
  • UserRepository
  • UserController
  • RegisterUserCommand
  • UserCommandHandler
  • User (AggregateRoot)
  • UserRegisteredEvent
  • UserReadProjector
  • UserReadEntity
  • UserReadRepository

Class number explosion

  • UserController
  • User (Entity)
  • UserRepository
  • UserController
  • RegisterUserCommand
  • ChangePasswordCommand
  • UserCommandHandler
  • User (AggregateRoot)
  • UserRegisteredEvent
  • PasswordChangedEvent
  • UserReadProjector
  • UserReadEntity
  • UserReadRepository

Recap

event sourcing and CQRS is best thing EVER

Thank you!

Please please please leave feedback https://joind.in/talk/b4f6b

@msvrtan

Any questions?

"Microservice" inside monolith

  • separate event store
  • important data for debugging
  • not business important
  • truncate data every X 

Hammer problem

"When you have a hammer, everything looks like a nail"

Hammer problem

  • self-doubt if now really everything looks like a nail

Year with EventSourcing and CQRS

By Miro Svrtan

Year with EventSourcing and CQRS

Slides for my 'Year with EventSourcing and CQRS' on AFP Tour 2017 conference in Nantes,France

  • 528

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