TOTAL RECALL - The application that never forgets

Building an application upon CQRS and Event Sourcing offers three pretty nice benefits.

1. Well structured code and business logic, separated by read and write concerns.
2. The source of truth - A reliable and recoverable view into the past of your application state.
3. The opportunity to answer future questions that you don't know about today.

This talk tries to show you examples for these benefits and how to give your application a memory.

At #PHPkonf 20th of May 2017 in Istanbul, Turkey

Transcript

1. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   1 MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   THE APPLICATION THAT NEVER FORGETS
   

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2. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   2
   IT’S MY WIFE’S
   BIRTHDAY
   HELP!
   

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3. HOLGER WOLTERSDORF
   CIO • FATHER • HUSBAND • PHP DEV WITH ♥
   github.com/hollodotme
   @hollodotme
   

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4. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   4
   

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5. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   5
   SEPTEMBER 22nd
   ARNE
   BLANKERTS
   thePHPcc
   SEBASTIAN
   HEUER
   kartenmacherei
   JAN
   BURKL
   ZEND
   MARCO
   PIVETTA
   Ocramius
   STEPHAN
   HOCHDÖRFER
   bitExpert
   ALEXANDER
   MIERTSCH
   prooph software
   CHRISTIAN
   LÜCK
   reactPHP
   

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6. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   6
   CQRS
   (COMMAND QUERY RESPONSIBILITY SEGREGATION)
   

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7. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   CQS => CQRS
   7
   ๏ CQRS EVOLVED FROM THE CQS IMPLEMENTATION PATTERN
   ๏ First stated by Bertrand Meyer in the 1990’s 

   when working on the Eiffel programming language

   ๏ Describes a semantic implementation pattern for object methods
   ๏ Queries: return results, but do not change the observable state (free of side effects)
   ๏ Commands: change the state of objects, but do not return values
   Bertrand Meyer
   

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8. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   CQS => CQRS
   8
   class Number
   {
   /** @var int */
   private $number = 0;
   public function getIncrementedNumber() : int
   {
   $this->number++;
   return $this->number;
   }
   }
   BAD
   

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9. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
   CQS => CQRS
   9
   class Number
   {
   /** @var int */
   private $number = 0;
   public function getNumber() : int # Query
   {
   return $this->number;
   }
   public function increment() : void # Command
   {
   $this->number++;
   }
   }
   GOOD
   

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10. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    CQS => CQRS
    10
    ๏ CQRS IS AN ARCHITECTURE PATTERN
    ๏ Described by Greg Young’s CQRS documents in 2010
    ๏ One part responsible to represent the current state of the application
    ๏ One part responsible to change the state of the application
    ๏ Command and Query have a broader semantic meaning than in CQS
    ๏ Key concept is to split your application into:
    ๏ A read side that is able to answer queries (read-only)
    ๏ A write side that is able to process commands (write-only)
    ๏ Give each side an appropriate storage
    GREG YOUNG
    

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11. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    11
    CQRS DEMO
    

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12. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    12
    THREE APPLICATION LAYERS
    REQUEST HANDLING LAYER
    BUSINESS LOGIC LAYER
    PERSISTENCE LAYER
    CQRS CAN BE APPLIED TO ALL OF THEM
    KNOWS
    HTTP
    KNOWS
    BUSINESS
    RULES
    KNOWS
    DATABASE
    

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13. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    13
    THREE APPLICATION LAYERS
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    STATE
    AFTER CQRS WAS APPLIED
    VIEW LOGIC
    

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14. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    14
    THREE APPLICATION LAYERS
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    STATE
    HOW DO WE GET FROM HERE… … TO THERE?
    VIEW LOGIC
    

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15. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    15
    THREE APPLICATION LAYERS
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    STATE
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    VIEW LOGIC
    

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16. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    REPRESENTATION
    16
    THREE APPLICATION LAYERS
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    STATE
    VIEW LOGIC
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    REPRESENTATION
    MESSAGE
    BROKER
    ASYNC
    EVENTUAL
    CONSISTENCY
    !
    

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17. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    REPRESENTATION
    17
    THREE APPLICATION LAYERS
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    STATE
    VIEW LOGIC
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    REPRESENTATION
    PHP FASTCGI
    CLIENT
    ASYNC
    EVENTUAL
    CONSISTENCY
    !
    

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18. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    18
    CQRS (REQUEST HANDLING LAYER)
    ๏ GET-REQUEST
    ๏ NO VIEW LOGIC
    ๏ COULD BE A STATIC

    HTML PAGE
    ๏ SUBMIT TRIGGERS

    POST REQUEST
    

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19. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    19
    CQRS (REQUEST HANDLING LAYER)
    ๏ GET-REQUEST 

    AFTER REDIRECT
    ๏ CHECK FOR ERRORS 

    IN SESSION
    ๏ ERRORS RENDERED
    INTO VIEW
    ๏ ONLY SUBMIT 

    TRIGGERS POST
    REQUEST
    ๏ RELOAD DOESN’T
    

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20. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    CQRS (REQUEST HANDLING LAYER)
    20
    class POSTRequestHandler
    {
    public function handle( $request, $session ) : void
    {
    #...
    $validator->checkUserInput( $request );
    if ( $validator->failed() )
    {
    $session->addErrors( $validator->getErrors() );
    (new Redirect())->respond( '/newsletter-form' );
    return;
    }
    #...
    }
    }
    

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21. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    CQRS (REQUEST HANDLING LAYER)
    21
    class GETRequestHandler
    {
    public function handle( $request, $session ) : void
    {
    $page = new TemplatePage('/newsletter-form');
    if ( $session->hasErrors() )
    {
    $page->addErrors( $session->getErrors() );
    }
    $page->display();
    }
    }
    

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22. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    22
    CQRS (REQUEST HANDLING LAYER)
    ๏ GET-REQUEST
    ๏ NO VIEW LOGIC
    ๏ COULD BE A STATIC

    HTML PAGE
    ๏ CAN BE DELETED 

    AFTER SUBSCRIPTION

    IS FINISHED
    

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23. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    CQRS (REQUEST HANDLING LAYER)
    23
    class POSTRequestHandler
    {
    public function handle( $request, $session ) : void
    {
    #... input validation passed ...
    $command = new InitSubscriptionCommand(
    new Fullname( $request->get( 'fullname' ) ),
    new Email( $request->get( 'email' ) )
    );
    $handler = new InitSubscriptionCommandHandler();
    $result = $handler->handle( $command );
    if ( $result->succeeded() )
    {
    # $publisher->publish( $result );
    (new Redirect())->respond(
    '/success-' . $result->getSubscription()->getId()
    );
    return;
    }
    }
    }
    

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24. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    CQRS (REQUEST HANDLING LAYER)
    24
    class InitSubscriptionResult
    {
    public function succeeded() : bool
    public function getErrors() : array
    public function getSubscription() : NewsletterSubscription
    }
    class Subscriber
    {
    public function notify( InitSubscriptionResult $result ) : void
    {
    $subscription = $result->getSubscription();
    $page = new TemplatePage('/newsletter-init-success');
    $page->assign( 'subscription', $subscription );
    $page->saveAsHTML( '/newsletter-' . $subscription->getId() );
    }
    }
    

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25. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    25
    CQRS (REQUEST HANDLING LAYER)
    ๏ E-MAIL
    ๏ CALL TO ACTION

    FOR A STATE CHANGE
    ๏ POST REQUESTS

    NOT AVAILABLE 

    FROM E-MAILS!
    

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26. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    26
    RESPECT HTTP VERBS!
    CQRS (REQUEST HANDLING LAYER)
    

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27. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    27
    […] the convention has been established
    that the GET and HEAD methods
    SHOULD NOT have the significance
    of taking an action other than retrieval.
    These methods ought
    to be considered "safe".
    https://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
    CQRS (REQUEST HANDLING LAYER)
    

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28. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    28
    CQRS (REQUEST HANDLING LAYER)
    ๏ DON’T CHANGE STATE
    FROM GET REQUESTS!
    ๏ GET-REQUEST
    ๏ NO VIEW LOGIC
    ๏ COULD BE A STATIC

    HTML PAGE, PREPARED
    BASED ON SUCCESSFUL 

    INIT RESULT
    ๏ SUBMIT TRIGGERS 

    POST REQUEST
    ๏ CAN BE DELETED 

    AFTER SUBSCRIPTION

    IS FINISHED
    

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29. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    29
    CQRS (REQUEST HANDLING LAYER)
    ๏ GET-REQUEST

    AFTER REDIRECT
    ๏ NO VIEW LOGIC
    ๏ COULD BE A STATIC

    HTML PAGE, PREPARED
    BASED ON SUCCESSFUL 

    FINISH RESULT
    

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30. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    30
    BTW ;-)
    icehawk.github.io
    PHP7 micro framework - respecting CQRS
    

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31. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    31
    PERSISTENCE LAYER & STATE
    SubscriberID Fullname Email Interval Subscribed Confirmed Unsubscribed
    ‹UUID-HW›
    Holger
    Woltersdorf
    hollodotme
    @PHPkonf.org
    ONLY WHEN
    PHPkonf
    happens
    2017-05-19 2017-05-20 2017-05-21
    ‹UUID-JD› Jane Doe jane@doe.com Monthly 2017-03-01 NULL NULL
    SubscriberID Topic
    ‹UUID-HW› HIPSTER JEANS
    ‹UUID-HW› NERD CAPS
    ‹UUID-HW› MEME SHIRTS
    

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32. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    32
    PERSISTENCE LAYER & STATE
    WHAT HAPPENS, IF…
    I subscribe again
    for MEME SHIRTS only
    with WEEKLY interval
    

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33. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    33
    PERSISTENCE LAYER & STATE
    SubscriberID Fullname Email Interval Subscribed Confirmed Unsubscribed
    ‹UUID-HW›
    Holger
    Woltersdorf
    hollodotme
    @PHPkonf.org
    Weekly 2017-05-22 2017-05-23 NULL
    ‹UUID-JD› Jane Doe jane@doe.com Monthly 2017-03-01 NULL NULL
    SubscriberID Topic
    ‹UUID-HW› MEME SHIRTS
    

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34. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    34
    PERSISTENCE LAYER & STATE
    ‹‹ MY E-MAIL
    DUPLICATE CHECK
    WORKED OUT! ››
    DEVELOPER:
    YES!
    ADMIN:
    YES!
    ‹‹ YOU KEPT OUR
    DATABASE SLIGHT
    AND FAST! ››
    

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35. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    ‹‹ ARE YOU INSANE?
    YOU JUST ERASED
    VALUABLE B.I. DATA! ››
    35
    PERSISTENCE LAYER & STATE
    BUSINESS ANALYST:
    

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36. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    CQRS (PERSISTENCE LAYER - STATE)
    36
    What have we lost?
    ACTUALLY ALL INFORMATION
    ABOUT THE FIRST SUBSCRIPTION
    except e-mail & full name
    

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37. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    CQRS (PERSISTENCE LAYER - STATE)
    37
    What else have we lost?
    ANALYSABLE DATA
    ABOUT THE USER’S DECISION
    TO REACTIVATE AND CHANGE SUBSCRIPTION
    

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38. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    38
    CQRS (PERSISTENCE LAYER - STATE)
    The application state is observable,
    BUT ONLY AT THE EXACT POINT OF TIME
    we look at it.
    

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39. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    39
    + EVENT SOURCING
    

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40. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING
    40
    ๏ EVENT SOURCING IS AN ANALYSIS PATTERN
    ๏ Article by Martin Fowler in 2005
    ๏ CQRS + Event Sourcing also part of Greg Young’s CQRS documents
    ๏ Pattern was already used in accounting software way earlier
    ๏ Key concept: 

    

    "Record all changes to application state as a sequence of events."

    — Martin Fowler
    GREG YOUNG
    MARTIN FOWLER
    

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41. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    41
    INSTALLING EVENT SOURCING
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    STATE
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    VIEW LOGIC
    

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42. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    42
    INSTALLING EVENT SOURCING
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    STATE
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    VIEW LOGIC
    

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43. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    43
    INSTALLING EVENT SOURCING
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    EVENT STORE
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    VIEW LOGIC
    READ HISTORY OF CHANGES
    RECORD NEW CHANGES
    

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44. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING
    44
    BUSINESS ENTITY
    EVENTSTORE
    COMMAND
    TRIGGER CHANGES
    EMITS EVENTS
    (ALL NEW CHANGES)
    BUILT FROM
    EVENT STREAM
    (HISTORY OF CHANGES)
    PUBLISH NEWLY RECORDED EVENTS
    

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45. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    45
    EVENT STORE
    Seq StreamType StreamID StreamSeq EventID Payload Occurred MetaX Meta…
    10
    Newsletter

    Subscription
    ‹UUID-HW› 1
    Subscription was
    initiated
    {"json"}
    2017-05-21
    T12:13:14
    SHOP TY …
    11 Newsletter

    Subscription
    ‹UUID-HW› 2 Interval was
    changed
    {"json"} 2017-05-22
    T10:09:08
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46. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    Seq StreamType StreamID StreamSeq EventID Payload Occurred MetaX Meta…
    10
    Newsletter

    Subscription
    ‹UUID-HW› 1
    Subscription was
    initiated
    {"json"}
    2017-05-21
    T12:13:14
    SHOP TY …
    11 Newsletter

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    changed
    {"json"} 2017-05-22
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    12 Newsletter

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    46
    EVENT STORE
    SEQUENCE SORTING ALL EVENTS IN THE EVENT STORE
    (AUTO-INCREMENT)
    

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47. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    Seq StreamType StreamID StreamSeq EventID Payload Occurred MetaX Meta…
    10
    Newsletter

    Subscription
    ‹UUID-HW› 1
    Subscription was
    initiated
    {"json"}
    2017-05-21
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    changed
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    12 Newsletter

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    reactivated
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    51 Newsletter

    Subscription
    ‹UUID-HW› 9 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    52 Newsletter
    Subscription
    ‹UUID-HW› 10 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    53 Newsletter

    Subscription
    ‹UUID-HW› 11 Interval was
    changed
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    54 Newsletter

    Subscription
    ‹UUID-HW› 12 Subscription was
    confirmed
    {"json"} 2017-06-02
    T16:17:18
    SHOP DE …
    47
    EVENT STORE
    StreamType
    NAME OF THE BUSINESS ENTITY
    StreamID
    UNIQUE ID OF SPECIFIC BUSINESS ENTITY
    StreamSeq
    VERSION OF THE SPECIFIC BUSINESS ENTITY INSTANCE
    (USED TO SORT THE ENTITY’S EVENT STREAM)
    

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48. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    Seq StreamType StreamID StreamSeq EventID Payload Occurred MetaX Meta…
    10
    Newsletter

    Subscription
    ‹UUID-HW› 1
    Subscription was
    initiated
    {"json"}
    2017-05-21
    T12:13:14
    SHOP TY …
    11 Newsletter

    Subscription
    ‹UUID-HW› 2 Interval was
    changed
    {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    12 Newsletter

    Subscription
    ‹UUID-HW› 3 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    13 Newsletter
    Subscription
    ‹UUID-HW› 4 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    14 Newsletter

    Subscription
    ‹UUID-HW› 5 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    15 Newsletter

    Subscription
    ‹UUID-HW› 6 Subscription was
    confirmed
    {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    33 Newsletter

    Subscription
    ‹UUID-HW› 7 Subscription was
    canceled
    {"json"} 2017-05-25
    T07:09:09
    SHOP DE …
    50 Newsletter

    Subscription
    ‹UUID-HW› 8 Subscription was
    reactivated
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    51 Newsletter

    Subscription
    ‹UUID-HW› 9 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    52 Newsletter
    Subscription
    ‹UUID-HW› 10 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    53 Newsletter

    Subscription
    ‹UUID-HW› 11 Interval was
    changed
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    54 Newsletter

    Subscription
    ‹UUID-HW› 12 Subscription was
    confirmed
    {"json"} 2017-06-02
    T16:17:18
    SHOP DE …
    48
    EVENT STORE
    EventID
    NAME OF THE CHANGE
    (USED TO IDENTIFY EVENT CLASS)
    Payload
    ALL RELEVANT DATA THAT IS NEEDED
    TO REPRODUCE THE CHANGE
    USE ONE TECHNOLOGY INDEPENDENT
    DATA FORMAT!
    Occurred
    TIMESTAMP WHEN THE CHANGE WAS
    RECORDED
    DO NOT USE THIS FOR SORTING
    EVENT STREAMS
    DO NOT RELY ON TIMESTAMPS!
    IT’S JUST META DATA
    

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49. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    Seq StreamType StreamID StreamSeq EventID Payload Occurred MetaX Meta…
    10
    Newsletter

    Subscription
    ‹UUID-HW› 1
    Subscription was
    initiated
    {"json"}
    2017-05-21
    T12:13:14
    SHOP TY …
    11 Newsletter

    Subscription
    ‹UUID-HW› 2 Interval was
    changed
    {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    12 Newsletter

    Subscription
    ‹UUID-HW› 3 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    13 Newsletter
    Subscription
    ‹UUID-HW› 4 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    14 Newsletter

    Subscription
    ‹UUID-HW› 5 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    15 Newsletter

    Subscription
    ‹UUID-HW› 6 Subscription was
    confirmed
    {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    33 Newsletter

    Subscription
    ‹UUID-HW› 7 Subscription was
    canceled
    {"json"} 2017-05-25
    T07:09:09
    SHOP DE …
    50 Newsletter

    Subscription
    ‹UUID-HW› 8 Subscription was
    reactivated
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    51 Newsletter

    Subscription
    ‹UUID-HW› 9 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    52 Newsletter
    Subscription
    ‹UUID-HW› 10 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    53 Newsletter

    Subscription
    ‹UUID-HW› 11 Interval was
    changed
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    54 Newsletter

    Subscription
    ‹UUID-HW› 12 Subscription was
    confirmed
    {"json"} 2017-06-02
    T16:17:18
    SHOP DE …
    49
    EVENT STORE
    ADDITIONAL META DATA
    BUSINESS IRRELEVANT
    DEPENDS ON YOUR DOMAIN
    AND INFRASTRUCTURE
    FOR EXAMPLE
    SHOP INSTANCE
    SERVER INSTANCE
    AUTH-USER (IF NOT BUSINESS RELEVANT)
    APPLICATION VERSION
    ETC.
    ADD WHATEVER YOU NEED
    

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50. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    Seq StreamType StreamID StreamSeq EventID Payload Occurred MetaX Meta…
    10
    Newsletter

    Subscription
    ‹UUID-HW› 1
    Subscription was
    initiated
    {"json"}
    2017-05-21
    T12:13:14
    SHOP TY …
    11 Newsletter

    Subscription
    ‹UUID-HW› 2 Interval was
    changed
    {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    12 Newsletter

    Subscription
    ‹UUID-HW› 3 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    13 Newsletter
    Subscription
    ‹UUID-HW› 4 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    14 Newsletter

    Subscription
    ‹UUID-HW› 5 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    15 Newsletter

    Subscription
    ‹UUID-HW› 6 Subscription was
    confirmed
    {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    33 Newsletter

    Subscription
    ‹UUID-HW› 7 Subscription was
    canceled
    {"json"} 2017-05-25
    T07:09:09
    SHOP DE …
    50 Newsletter

    Subscription
    ‹UUID-HW› 8 Subscription was
    reactivated
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    51 Newsletter

    Subscription
    ‹UUID-HW› 9 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    52 Newsletter
    Subscription
    ‹UUID-HW› 10 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    53 Newsletter

    Subscription
    ‹UUID-HW› 11 Interval was
    changed
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    54 Newsletter

    Subscription
    ‹UUID-HW› 12 Subscription was
    confirmed
    {"json"} 2017-06-02
    T16:17:18
    SHOP DE …
    50
    EVENT STORE
    FIRST SUBSCRIPTION
    PLEASE NOTE:
    GIVING BIRTH TO A NEW BUSINESS ENTITY
    IS A CHANGE TO APPLICATION STATE
    

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51. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    Seq StreamType StreamID StreamSeq EventID Payload Occurred MetaX Meta…
    10
    Newsletter

    Subscription
    ‹UUID-HW› 1
    Subscription was
    initiated
    {"json"}
    2017-05-21
    T12:13:14
    SHOP TY …
    11 Newsletter

    Subscription
    ‹UUID-HW› 2 Interval was
    changed
    {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    12 Newsletter

    Subscription
    ‹UUID-HW› 3 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    13 Newsletter
    Subscription
    ‹UUID-HW› 4 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    14 Newsletter

    Subscription
    ‹UUID-HW› 5 Topic was assigned {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    15 Newsletter

    Subscription
    ‹UUID-HW› 6 Subscription was
    confirmed
    {"json"} 2017-05-22
    T10:09:08
    SHOP TY …
    33 Newsletter

    Subscription
    ‹UUID-HW› 7 Subscription was
    canceled
    {"json"} 2017-05-25
    T07:09:09
    SHOP DE …
    50 Newsletter

    Subscription
    ‹UUID-HW› 8 Subscription was
    reactivated
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    51 Newsletter

    Subscription
    ‹UUID-HW› 9 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    52 Newsletter
    Subscription
    ‹UUID-HW› 10 Topic was removed {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    53 Newsletter

    Subscription
    ‹UUID-HW› 11 Interval was
    changed
    {"json"} 2017-06-01
    T14:15:16
    SHOP DE …
    54 Newsletter

    Subscription
    ‹UUID-HW› 12 Subscription was
    confirmed
    {"json"} 2017-06-02
    T16:17:18
    SHOP DE …
    51
    EVENT STORE
    SECOND SUBSCRIPTION
    PLEASE NOTE:
    EXPRESS EXACTLY WHAT YOU ARE DOING
    INITIATED !== REACTIVATED
    AVOID AGGREGATION OF CHANGES
    YOU’LL LOSE VALUABLE INFORMATION
    Topics were changed !== Topic was added + Topic was removed
    

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52. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT STORE PAYLOAD
    52
    {
    "SubscriptionID": "‹UUID-HW›",
    "Fullname": "Holger Woltersdorf",
    "Email": "hollodotme@PHPkonf.org"
    }
    "Subscription was initialised"
    {
    "SubscriptionID": "‹UUID-HW›"
    }
    vs.
    "Subscription was reactivated"
    

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53. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT STORE PAYLOAD
    53
    DO NOT ADD PREVIOUS VALUES TO PAYLOAD
    IT SHOULD HAVE BEEN RECORDED BEFORE
    STOP THINKING RELATIONAL
    START THINKING TEMPORAL
    

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54. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING
    54
    interface EventInterface
    {
    public function getStreamID() : StreamIDInterface
    public function getEventID() : EventID
    public function getPayload() : Payload
    public static function fromPayload( Payload $payload ) : EventInterface
    }
    interface EventEnvelopeInterface
    {
    public function getHeader() : EventHeaderInterface
    public function getEvent() : EventInterface
    }
    

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55. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING
    55
    interface EventHeaderInterface
    {
    public function getStreamType() : StreamType
    public function getStreamID() : StreamIDInterface
    public function getStreamSeq() : StreamSequence
    public function getOccurred() : Timestamp
    public function getMetaX() : ShopName
    public function getMeta…() : …
    }
    

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56. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING
    56
    ๏ EVENT STORE IS IMMUTABLE AND APPEND-ONLY
    ๏ EVENT STORE IS THE ONLY SOURCE OF TRUTH
    ๏ USE TECHNOLOGY INDEPENDENT PAYLOAD FORMAT (XML/JSON/ETC.)

    DO NOT USE OBJECT SERIALISATION!
    ๏ EVENTS ARE IMMUTABLE
    ๏ SEPARATE PAYLOAD FROM META DATA (EVENT ENVELOPE)
    ๏ VALUE OBJECTS ARE IMMUTABLE
    ๏ EVENT STREAM IS A SORTED LIST OF EVENTS
    ๏ BUSINESS ENTITIES CAN NOT BE INSTANTIATED WITHOUT EVENT STREAM
    

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57. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (BUSINESS ENTITY)
    57
    class NewsletterSubscription
    {
    private $streamType;
    private $streamSeq;
    private $changes;
    # ...
    private function __construct()
    {
    $this->streamType = new StreamType( 'Newsletter Subscription' );
    $this->streamSeq = new StreamSeq( 0 );
    $this->changes = new EventStream();
    }
    # ...
    }
    

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58. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (BUSINESS ENTITY)
    58
    class NewsletterSubscription
    {
    public static function initiate(
    Email $email,
    Fullname $fullname
    ) : NewsletterSubscription
    {
    $subscription = new self();
    $subscription->recordThat(
    new SubscriptionWasInitiated(
    SubscriptionID::generate(),
    $email,
    $fullname
    )
    );
    return $subscription;
    }
    # ...
    }
    ๏ DO NOT CREATE ENTITIES
    WITHOUT RECORDING AN
    EVENT
    

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59. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (BUSINESS ENTITY)
    59
    class NewsletterSubscription
    {
    #...
    private function recordThat( EventInterface $event ) : void
    {
    $envelope = EnvelopeBuilder::build(
    $this->getStreamType(),
    $this->streamSeq->increment(),
    $event
    );
    $this->changes->append( $envelope );
    $this->apply( EventEnvelopeInterface $envelope );
    }
    #...
    }
    

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60. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (BUSINESS ENTITY)
    60
    class NewsletterSubscription
    {
    #...
    private function apply( EventEnvelopeInterface $envelope ) : void
    {
    $header = $envelope->getHeader();
    $event = $envelope->getEvent();
    $this->streamSeq = $header->getStreamSeq();
    $methodName = 'when' . $event->getEventID()->toUpperCamelCase();
    $this->$methodName( $event );
    # $this->whenSubscriptionWasInitiated( $event );
    }
    #...
    }
    

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61. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (BUSINESS ENTITY)
    61
    class NewsletterSubscription
    {
    #...
    protected function whenSubscriptionWasInitiated(
    SubscriptionWasInitiated $event
    ) : void
    {
    $this->email = $event->getEmail();
    $this->fullname = $event->getFullname();
    # This is your "real" constructor
    # Init properties here
    }
    #...
    }
    

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62. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (BUSINESS ENTITY)
    62
    class NewsletterSubscription
    {
    #...
    public static function reconstitute( EventStream $eventStream ) : self
    {
    $subscription = new self();
    foreach ( $eventStream as $envelope )
    {
    $subscription->apply( $envelope );
    }
    return $subscription;
    }
    #...
    }
    

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63. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (BUSINESS ENTITY)
    63
    abstract class AbstractBusinessEntity
    {
    final protected function __construct()
    final protected function recordThat( EventInterface $event ): void
    private function apply( EventEnvelopeInterface $envelope ) : void
    final public static function reconstitute( EventStream $eventStream )
    }
    

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64. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (BUSINESS ENTITY)
    64
    class NewsletterSubscription extends AbstractBusinessEntity
    {
    private $topics;
    public function assignTopic( Topic $topic ) : void
    protected function whenTopicWasAssigned( TopicWasAssigned $event ) : void
    }
    class NewsletterSubscription extends AbstractBusinessEntity
    {
    use NewsletterSubscriptionChanging;
    public function assignTopic( Topic $topic ) : void
    }
    USE A TRAIT TO EXPOSE PUBLIC API ONLY
    trait NewsletterSubscriptionChanging
    {
    private $topics;
    protected function whenTopicWasAssigned( TopicWasAssigned $event ) : void
    }
    

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65. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING
    65
    ๏ QUESTIONS WE CAN ASK NOW:
    ๏ WHAT WAS THE (UN-)SUBSCRIBE RATIO IN 2016?
    ๏ HOW MANY USERS REACTIVATED THEIR SUBSCRIPTION IN 2017?
    ๏ HOW MANY USERS REACTIVATED MORE THAN ONCE?
    ๏ WHICH TOPIC WAS REMOVED THE MOST?
    ๏ HOW MANY USERS CHANGED INTERVAL BEFORE SUBSCRIPTION WAS CANCELED?
    

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66. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    66
    EVENT SOURCING
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    EVENT STORE
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    VIEW LOGIC
    READ HISTORY OF CHANGES
    RECORD NEW CHANGES
    

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67. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    RECORD NEW CHANGES
    READ HISTORY OF CHANGES
    67
    EVENT SOURCING
    READ REQUESTS
    BUSINESS LOGIC
    REPRESENTATION
    WRITE REQUESTS
    EVENT STORE
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    VIEW LOGIC
    

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68. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    RECORD NEW CHANGES
    READ HISTORY OF CHANGES
    68
    EVENT SOURCING (PROJECTION)
    READ REQUESTS
    BUSINESS LOGIC
    PROJECTION
    WRITE REQUESTS
    EVENT STORE
    PUBLISH SUBSCRIBE
    REDIRECT (+ SESSION)
    VIEW LOGIC
    

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69. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    RECORD NEW CHANGES
    READ HISTORY OF CHANGES
    69
    EVENT SOURCING (PROJECTION)
    READ REQUESTS
    BUSINESS LOGIC
    WRITE REQUESTS
    REDIRECT (+ SESSION)
    VIEW LOGIC
    REPRESENTATION
    REPRESENTATION
    EVENT STORE
    PUBLISH SUBSCRIBE
    PROJECTION
    MESSAGE
    BROKER
    ASYNC
    EVENTUAL
    CONSISTENCY
    !
    

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70. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (PROJECTING)
    70
    ๏ PUSH VIEWS
    ๏ Gets notified about a new change (event)
    ๏ Can directly project this information
    ๏ PULL VIEWS
    ๏ Gets a query and pulls up appropriate event streams to gather information
    ๏ Can respond or project the result
    ๏ MIXED VIEWS
    ๏ Gets notified and gathers missing information from event store
    ๏ Projects this information
    

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71. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING (PROJECTING)
    71
    PLEASE NOTE:
    PROJECTIONS SHOULD CONVERT STATE INFORMATION
    TO THE BEST DATA FORMAT FOR YOUR READ-USE-CASE
    THIS COULD ALSO BE ANOTHER EVENT STORE
    WITH A SUBSET OF STREAMS / EVENTS
    

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72. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    CQRS ADVANTAGES
    72
    ๏ WHAT CAN WE GAIN FROM APPLYING CQRS?
    ๏ Separate input / output handling
    ๏ Opportunity to optimise each side for its operational purpose
    ๏ Depends on your application domain
    ๏ e.g. eCommerce Shops usually have a read / write ratio of 95% / 5%

    so optimisation for reading is a good idea
    ๏ Clean code, reusable queries and commands, decoupling, simpler tests
    ๏ No unintended side effects
    ๏ Technology independence for each side
    ๏ Good prerequisite for individual horizontal scalability
    

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73. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    EVENT SOURCING ADVANTAGES
    73
    ๏ WHAT CAN WE GAIN FROM USING EVENT SOURCING?
    ๏ High scalability
    ๏ Simplification of storing application state ("one size fits all")
    ๏ Traceability of all changes ever made to application state
    ๏ Retroactive data analysis at any time
    ๏ Adapting new business requirements with ease
    ๏ Restoring state representation at any given point in time
    ๏ Replay after bug fixes to reestablish view consistency
    ๏ Easy backup
    

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74. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    74
    QUESTIONS?
    

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75. THANK YOU!
    github.com/hollodotme
    @hollodotme / @F9T3ch
    fortuneglobe.com
    phpug-dresden.org
    @phpugdd
    HOLGER WOLTERSDORF
    icehawk.github.io
    speakerdeck.com/hollodotme
    Slides available at:
    

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76. TOTAL RECALL - THE APPLICATION THAT NEVER FORGETS • MAY 20th 2017 • PHPkonf. • ISTANBUL HOLGER WOLTERSDORF
    LINKS / REFERENCES
    76
    ๏ CQS:
    ๏ https://martinfowler.com/bliki/CommandQuerySeparation.html
    ๏ https://en.wikipedia.org/wiki/Command%E2%80%93query_separation
    ๏ Bertrand Meyer:
    ๏ https://de.wikipedia.org/wiki/Bertrand_Meyer
    ๏ https://bertrandmeyer.com
    ๏ Book "Object-Oriented Software Construction 2Ed" by Bertrand Meyer: http://amzn.to/2qdnc2i
    ๏ Greg Young’s CQRS documents: https://cqrs.files.wordpress.com/2010/11/cqrs_documents.pdf
    ๏ GET/HEAD methods: https://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
    ๏ Article about Event Sourcing by Martin Fowler: https://martinfowler.com/eaaDev/EventSourcing.html
    ๏ PHP FastCGI Client: https://github.com/hollodotme/fast-cgi-client
    ๏ IceHawk micro framework: https://icehawk.github.io
    

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