CQRS & Event Sourcing — LavaJUG

CQRS & Event Sourcing — LavaJUG

F135ce7f204af6fac8075b469685c91d?s=128

Matthieu Moquet

May 25, 2016
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  1. Event Sourcing @MattKetmo CQRS &

  2. How can it help to maintain your app? How can

    it help to scale horizontally?
  3. Matthieu Moquet @MattKetmo @ BlaBlaCar

  4. {Disclaimer Oriented for long lived apps.
 Overkill for prototypes /

    RAD / CRUD apps. Use those patterns in moderation.
 There is no silver bullet. This talk aims to open your mind.
 Think different!
  5. I/O

  6. 90% ViewModels / Flexible Eventually consistent 10% Validation / Business

    rules Coherence (ACID) Read Write Fast & Scale
  7. Use case bank accounts Account #1 1337€ Account #2 42€

    Account #3 10€ + Transfer money
  8. Facade (Controller) Models (ORM) DB

  9. //  Open  new  account   $account  =  new  Account();  

    $account-­‐>setAmount(0,  'EUR');   $account-­‐>setUser($user);   $em-­‐>persist($account);   $em-­‐>flush();   //  Credit  100  EUR   $account-­‐>setAmount(100,  'EUR');   $em-­‐>flush();   //  Debit  20  EUR   $account-­‐>setAmount(80,  'EUR');   $em-­‐>flush();
  10. does not reflect user intent CRUD does not scale well

  11. Facade (Controller) Models (ORM) DB Service Layer (Business)

  12. interface  BankAccountService     {          /**

     @return  int  */          public  function  open();          /**  @return  Account  */          public  function  get($accountId);          /**  @return  Account[]  */          public  function  findAll($userId);          /**  @return  void  */          public  function  credit($accountId,  $balance);          /**  @return  void  */          public  function  debit($accountId,  $balance);   }  
  13. CQS Command Query Separation }Each operation is either a Command

    or a Query, not both
  14. Commands Queries Change the state of the system. Do not

    return anything. Does not change the state of the system. Return data. Write only Read only
  15. Commands /**  @return  Account  */   public  function  get($accountId);  

    /**  @return  Account[]  */   public  function  findAll($userId); /**  @return  int  */   public  function  open();   /**  @return  void  */   public  function  credit($accountId,  $balance);   /**  @return  void  */   public  function  debit($accountId,  $balance); Should be void Queries
  16. /**      *  @param  Uuid  $accountId    *  

     *  @return  void      */   public  function  open(Uuid  $accountId);   Let the client generate the identifier, not the infrastructure (database)
  17. CQRS Command Query Responsibility Segregation }Like CQS but separate queries

    and commands in different classes
  18. CQRS challenges the assumption that reading and writing are sharing

    the same abstractions Databases Models Apps
  19. Command Facade Query Facade Command Handler Query Repository Command Models

    DB Read Models Command (DTO) Segregation of read and write is a radical form of decoupling Query (DTO)
  20. None
  21. Now that the workflow of Read & Write has been

    separated. We can maintain & optimize each side separately.
  22. asynchronously Since Command Handlers do NOT return anything, 
 then

    do the execution
  23. Command Facade Command Handlers Commands Queueing Ack (validation)

  24. if  (/*  user  is  allowed  to  open  account  */)  {

             $accoundId  =  AccountId::generate();          $command  =  new  OpenAccount($accountId);          $commandBus-­‐>handle($command);          return  new  Response(                  null,                  $async  ?  202  :  201,  //  Accepted  :  Created                  ['Location'  =>  "/accounts/$accountId"]          );     }
  25. //  Fetch  data  from  request   $accountId  =  ...;  

    $debit  =  new  Money(20,  'EUR');   if  (/*  account  balance  >=  $debit  */)  {          $command  =  new  Debit(                  $accountId,                    $debit          );          $commandBus-­‐>handle($command);          return  new  Response(null,  204);   }
  26. Command tracking Scale workers Horizontally by instantiating more workers (subscribers)

    Generate an identifier per command to let the client tracks its status Rabbitmq, Kafka, Gearman, ……… make your choice on queues.io
  27. Projections On the Read side you should prepare you data

    as it will be requested
  28. Having only 1 data store for reads & writes does

    not scale (well) different usage different needs
  29. ………and use different data stores Denormalize your data... UI View

    models Statistic models Search index models API Read models ... Read models are faster than JOIN different usage different needs
  30. None
  31. {      "user_id":  133742      "from":  "Paris",  

       "to":  "Luxembourg",      "by":  [          "Reims",            "..."      ],      "date":  "2015-­‐05-­‐11",   } Input (mysql)
  32. [{      "trip":  {          "from":

     "Paris",          "to":  "Luxembourg",          "date":  "2015-­‐05-­‐11",          "..."      },      "user":  {          "name":  "John  D",          "age":  29,          "grade":  "beginner",          "..."      }   }] Output (Elasticsearch) denormalized searchable
  33. None
  34. SELECT  tweets.*,  users.*      FROM  tweets   JOIN  users

         ON  users.id  =  tweet.sender_id   JOIN  follows      ON  follows.followee_id  =  user.id   WHERE  follows.follower_id  =  $userId   ORDER  BY  tweets.time  DESC   LIMIT  100
  35. None
  36. Tweets stream DB Aggs Timelines {      "user_id":  1234567890

         "status":  "Hello  World"      "timestamp":  1430491773   } [{      "tweet_id":  1234567890876543,      "username":  "MattKetmo",      "name":  "Matthieu  Moquet",      "timestamp":  1430491773,      "status":  "Hello  World",   },  {      "tweet_id":  1234567890886445,      ...   }]  
  37. SELECT  tweets      FROM  timelines    WHERE  timeline_id  =

     $userId    LIMIT  500
  38. PageViewEvents DB Aggs Increment counters /month Google Analytics /day /hour

    total {      "eventType":  "PageViewEvent"      "timestamp":  1430491773,      "ipAddress":  "12.34.56.78",      "sessionId":  "abcd1234567890",      "pageUrl":  "/hello-­‐world",      "..."   }
  39. Using Cassandra you need to: ‣ know the read requests

    before creating your data models ‣ create as many tables (ie. KeySpaces) than you have views ‣ denormalize the data (no join allowed) ’s keypoints
  40. C* C* C* Primary Data C* Read Data User post

    message Endpoint
  41. Multi Data Center Replication + Sharding + Cache

  42. Command Command Handler Primary DB Projections View Models Message Bus

    Write Read Eventual Consistency {...} {...} {...} {...}
  43. Event Sourcing change the way we store our primary data

  44. Event Sourcing Register only a series of events. Reconstitute the

    state of current "entity" by reading the past events. If we know the events of the past we can reconstitute the present
  45. the standard way Something happen State A State B Something

    happen (delta) (dropped) (stored) (delta) Time
  46. the event sourcing way Something happen State A State B

    Something happen (stored) (reconstituted) (reconstituted) (stored) Time
  47. [{        "uuid":  "110e8400-­‐e29b-­‐11d4-­‐a716-­‐446655440000",        "type":

     "AccountWasOpen",        "recorded_on":  "2015-­‐05-­‐11T13:37:00Z",        "payload":  {}   },  {        "uuid":  "110e8400-­‐e29b-­‐11d4-­‐a716-­‐446655440000",        "type":  "AccountWasCredited",        "recorded_on":  "2015-­‐05-­‐11T14:42:00Z",        "payload":  {  "amount":  100,  "currency":  "EUR"  }   },  {      "..."   }] Event Store
  48. Events are the of the system

  49. Aggregates Isolated domain models which run the business logic (via

    an ubiquitous language) through events
  50. //  Open  a  new  account   $accountId  =  AccountId::generate();  

    $account  =  BankAccount::open($accountId);   //  Add  some  money   $account-­‐>credit(new  Money(100,  'EUR'));
  51. Generates events Aggregates class  BankAccount  implements  AggregateRootInterface   {  

           //  ...   }
  52. interface  AggregateRoot   {          /**  

             *  @return  DomainEventStream            */          public  function  getUncommittedEvents();          /**            *  @return  string            */          public  function  getAggregateRootId();   }
  53. $accountId  =  AccountId::generate();   $account  =  BankAccount::open($accountId);   $account-­‐>credit(new  Money(100,

     'EUR'));   //  Retrieve  event  stream   $events  =  $account-­‐>getUncommittedEvents();   //  -­‐  AccountWasOpen   //  -­‐  AccountWasCredited
  54. class  BankAccount  extends  EventSourcedAggregateRoot   {        

     private  $accountId;          public  function  getAggregateRootId()          {                  return  $this-­‐>accountId;          }          public  static  function  open(AccountId  $accountId)          {                  $account  =  new  self();                  $account-­‐>apply(new  AccountWasOpen($accountId));                  return  $account;          }          public  function  credit(Money  $balance)          {                  $this-­‐>apply(new  AccountWasCredited(                          $this-­‐>accountId,                          $balance-­‐>getAmount(),                          $balance-­‐>getCurrency()                  );          }   }
  55. Reconstitute from events Aggregates

  56. class  BankAccount  extends  EventSourcedAggregateRoot   {        

     private  $accountId;          private  $amount;          //  ...              protected  function  applyAccountWasOpen(AccountWasOpen  $event)          {                  $this-­‐>accountId  =  $event-­‐>getAccountId();          }          protected  function  applyAccountWasCredited(AccountWasCredited  $event)          {                  $this-­‐>amount  +=  $event-­‐>getBalance();          }          protected  function  applyAccountWasDebited(AccountWasDebited  $event)          {                  $this-­‐>amount  -­‐=  $event-­‐>getBalance();          }   }
  57. Aggregates Protect invariants

  58. class  BankAccount  extends  EventSourcedAggregateRoot   {        

     //  ...          public  function  debit(Money  $balance)          {                  if  ($this-­‐>amount  <  $balance-­‐>getAmount())  {                          throw  new  NotEnoughMoneyException(                                  'Cannot  debit  more  than  current  amount'                          );                  }                  return  $this-­‐>apply(new  AccountWasCredited(                          $this-­‐>accountId,                          $balance-­‐>getAmount(),                          $balance-­‐>getCurrency()                  ));                        }   }
  59. Command + Event Sourcing

  60. Command User intent Events Event Store Apply changes on system

    Primary DB Transactional append
  61. class  BankAccountCommandHandler  extends  CommandHandler   {        

     public  function  handleOpenAccount(OpenAccount  $command)          {                  $account  =  BankAccount::open($command-­‐>getAccountId());                  $this-­‐>repository-­‐>save($account);          }          public  function  handleCreditAccount(CreditAccount  $command)          {                  $account  =  $this-­‐>repository-­‐>load($command-­‐>getAccountId());                  $account-­‐>credit($command-­‐>getBalance());                  $this-­‐>repository-­‐>save($account);          }   }
  62. class  EventSourcingRepository  implements  RepositoryInterface   {        

     public  function  save(AggregateRoot  $aggregate)          {                  $events  =  $aggregate-­‐>getUncommittedEvents();                  $this-­‐>eventStore-­‐>append(                          $aggregate-­‐>getAggregateRootId(),                            $events                  );          }          public  function  load($id)          {                  try  {                          $events  =  $this-­‐>eventStore-­‐>load($id);                          return  $this-­‐>aggregateFactory-­‐>create($events);                  }  catch  (EventStreamNotFoundException  $e)  {                          throw  AggregateNotFoundException::create($id,  $e);                  }          }   }
  63. Event Sourcing PROS

  64. Append Only / Immutable Very fast data update Deleting data

    = new events
  65. No Database migration when updating the data DBA will love

    it
  66. Sharding Aggregates are self-reliant and don’t have external references No

    Joins
  67. Complete historical data Being able to replay history is a

    major benefit both technically and for the business BI team will love it
  68. Event Sourcing CONS Well, not really

  69. Ensure consistency How to avoid concurrent updates?

  70. Load aggregate from EventStore: 100€ Create AccountWasDebited(100) event Append event

    in datastore Example Two debits of 100€ must not be accepted if only 100€ left Load aggregate from EventStore: 100€ Create AccountWasDebited(100) event Append event in datastore Event must NOT be appended twice
  71. abstract  class  EventSourcedAggregateRoot  implements  AggregateRootInterface   {      

       private  $uncommittedEvents  =  array();          private  $playhead  =  -­‐1;            /**          *  Applies  an  event.            *  The  event  is  added  to  the  list  of  uncommited  events.          */          public  function  apply($event)          {                  $this-­‐>playhead++;                  $this-­‐>uncommittedEvents[]  =  DomainMessage::recordNow(                          $this-­‐>getAggregateRootId(),                          $this-­‐>playhead,                          new  Metadata(array()),                          $event                  );          }   }
  72. DBAL EventStore (MySQL) CREATE  TABLE  EventStore  (      `agg_uuid`

           UUID,      `playhead`        INT(11),      `type`                TEXT,      `payload`          JSON,      `metadata`        JSON,      `recorded_on`  DATETIME,      UNIQUE  KEY  `UNIQ_PLAYHEAD`  (`agg_uuid`,  `playhead`)   );
  73. E_TOO_MANY_EVENTS When your Aggregates are "long lived" it may be

    slow to read the full history eg. BankAccount
  74. Snapshotting is a solution

  75. Snapshotting Store the state of the aggregate and read only

    the events created after
  76. Snapshotting Snapshot Snapshot Events States

  77. TL;DR

  78. Command Handler Events Event Store Command Auditing Read Model Event

    Listener Event Bus Projector Write Read Business rules Historical data Read / search index Side effects User intent View models Process Managers
  79. Frameworks C# / Java nCQRS Fohjin NEventStore LiteCQRS Lokad.CQRS Agr.CQRS

    Axon Framework JDON PHP Prooph Broadway Predaddy EventCentric.Core litecqrs-php
  80. cqrs.nu #MustRead

  81. Thank You!

  82. Thank You! Slides available at moquet.net/talks/lavajug-2016 Leave feedbacks @MattKetmo