Presented in Thessaloníki .NET Meetup. Touches on DDD Aggregates and CQRS. Shows when and how to do to async Command/Event handling with MassTransit over RabbitMQ.

1. MESSAGING in CQRS with
MASSTRANSIT
DDD GREECE / Thessaloníki .NET Meetup

2. www.meetup.com/dddgreece/
DDD Greece Meetup
DDD @ Slack
https://ddd-cqrs-es.herokuapp.com
DDD Greece Channel
#dddgr

3. DDD

4. DDD
DOMAIN MODEL
▸ structured knowledge about the domain
▸ knowledge crunching
▸ ubiquitous language
▸ typically an OO model
▸ no UI or persistence concerns
▸ != data model

5. DDD
BUILDING BLOCKS 1/2
▸ Entities
▸ id
▸ mutable
▸ Value Objects
▸ no id
▸ immutable
▸ diff state = diff value object

6. DDD
BUILDING BLOCKS 2/2
▸ Domain Services
▸ stateless
▸ Domain Events
▸ immutable
▸ timestamped
▸ ref to entities

7. DDD
AGGREGATE
▸ object graph of Entities and Value objects
▸ root = single entity
▸ global identity
▸ external entities refer ONLY to the aggregate root
▸ root enforces invariants via root entity’s interface
▸ consistency boundary

8. CARGO TRACKER

9. CARGO TRACKER
ABOUT
▸ Eric Evans “Big Blue Book”
▸ Published 2003
▸ Imaginary Shipping Company

10. CARGO TRACKER
KEY FUNCTIONALITY 1/2
▸ Book a Cargo based on Spec
▸ Get a Tracking Identifier
▸ (Re-)Assign an Itinerary
▸ Register a Handling Event (Load, Unload, Customs …)

11. CARGO TRACKER
KEY FUNCTIONALITY 2/2
▸ Get Transport Status (In Port, On Board Vessel …)
▸ Get Routing Status (Not Routed, Routed, …)
▸ Calculate Next Expected Handling Activity
▸ Get Delivery History

12. CARGO TRACKER
INVARIANT
▸ Transport Status >= On Board Vessel
AND
▸ Routing Status >= Routed

13. CARGO TRACKER
DOMAIN MODEL - CARGO AGGREGATE 1/2
▸ Cargo Entity as Root
▸ HandlingEvent
▸ Root’s Methods:
▸ New(TrackingId, DeliverySpec)
▸ AssignItinerary(Itinerary)
▸ RegisterHandlingEvent(HandlingEvent)
WRITE /
COMMANDS

14. CARGO TRACKER
DOMAIN MODEL - CARGO AGGREGATE 2/2
▸ Root’s State:
▸ TransportStatus (Enum)
▸ RoutingStatus (Enum)
▸ NextExpected: HandlingActivity
▸ DeliveryHistory:
IEnumerable<HandlingEvent>
READ / QUERIES

15. CARGO TRACKER
PROBLEMS 1/2
▸ Delivery History
▸ 1:M with Handling Events
▸ “Cargo Aggregate involved in the transaction”
▸ “ability to enter Handling Events without contention”

16. CARGO TRACKER
PROBLEMS 2/2

17. CARGO TRACKER
REFACTORING DELIVERY HISTORY
▸ separate HandlingEvent Aggregate:
▸ no domain logic (anemic)
▸ only repository + event class

18. CQRS

19. CQRS
ABOUT
▸ Command-Query Responsibility Segregation
▸ keep separate read (query) and write (command) models
▸ DeliveryHistory: Read/Query
▸ Cargo: Write/Command
▸ typical constructs
▸ Commands
▸ Events

20. CQRS
COMMANDS
▸ Capture Intent
▸ BookNewCargo, AssignItinerary etc.
▸ App Layer
▸ Command Handlers
▸ BookNewCargoHandler, AssignItineraryHandler etc.
▸ practically an App Service

21. CQRS

22. CQRS
EVENTS 1/2
▸ Convey State Transition
▸ NewCargoBooked, DeliveryChanged etc.
▸ != Domain Events (e.g Handling Event)
▸ Domain Layer
▸ Emitted by Aggregate Root after applying a command

23. CQRS
EVENTS 2/2
▸ EventHandler
▸ e.g. NewCargoBookedHandler, DeliveryChangedHandler
▸ App Layer
▸ Typical Usage:
▸ Aggregate / Bounded Context (=SubSystem) Integration
▸ wired to another Command Handler
▸ Read Model Update

24. CQRS
READ MODEL UPDATE PATTERN
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25. CQRS
MULTIPLE EVENT HANDLERS
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26. CQRS
MISCONCEPTIONS 1/2
▸ needs two datastores/databases
▸ must be NoSQL
▸ requires async processing
▸ needs messaging/queueing
▸ consistency always eventual
TRADE OFFS

27. CQRS
MISCONCEPTIONS 2/2
▸ needs a CQRS framework
▸ works only with event sourcing
▸ applied globally to a system

28. CQRS
SYNC PROCESSING
▸ single process (e.g. Web App)
▸ reduces availability
▸ affects UX
▸ in-process event dispatcher for decoupling
▸ immediate consistency

29. CQRS
ASYNC PROCESSING
▸ multiple processes (e.g. WebApp and WorkerApp)
▸ higher availability, scaling out
▸ consistency eventual
▸ also affects UX
▸ typically implemented with async messaging
▸ message bus

30. CQRS
CQRS ASYNC PROCESSING ARCHITECTURE 1/2
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31. CQRS
CQRS ASYNC PROCESSING ARCHITECTURE 2/2
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32. ASYNC
MESSAGING

33. ASYNC MESSAGING
MESSAGE BUS 1/2
▸ frameworks: NServiceBus, MassTransit, Rebus etc.
▸ transports: RabbitMQ, Azure Service Bus etc.
▸ at least-once delivery
▸ FIFO
▸ reliable messaging
▸ automatic transport configuration setup
▸ pub/sub, send/receive

34. ASYNC MESSAGING
ASYNC & RELIABLE MESSAGING
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35. ASYNC MESSAGING
MESSAGE BUS 2/2
▸ provides
▸ transport topology setup
▸ transport connection management
▸ persistent sagas/process managers
▸ retrying policy on subscriber/receiver errors
▸ alarm service
▸ (de)serialization

36. ASYNC MESSAGING
CQRS & MESSAGING PATTERNS
Command Handler Receiver
Command Issuer Sender
Event Handler Subscriber
Event Emitter Publisher

37. ASYNC MESSAGING
COMMAND FLOW
1. Process A
A. Issue
B. Send
2. Process B
C. Receive
D. Handle - Create Events

38. ASYNC MESSAGING
EVENT FLOW
1. Process B
A. Emit
B. Publish
2. Process C
C. Subscribe
D. Handle

40. IMPLEMENTATION

41. IMPLEMENTATION
WARNING
ASYNC MESSAGING
!=
ASYNC/AWAIT

42. STEP 1:
COMMAND ISSUING/SENDING

43. IMPLEMENTATION
COMMAND ISSUING - (WEB)APP LAYER
▸ IssueAsync implemented with Bus.Send<T>

44. IMPLEMENTATION
COMMAND SENDING / BOOTSTRAPPING MASSTRANSIT
▸ bus is a Single Instance / AppDomain

45. IMPLEMENTATION
MASSTRANSIT / BUS.SEND<T>
▸ Send(er)/Receive(r) Pattern

46. IMPLEMENTATION
COMMAND INTEGRITY 1/2

47. IMPLEMENTATION
COMMAND INTEGRITY 2/2

48. IMPLEMENTATION
COMMAND ISSUING/SENDING SUMMARY
▸ Should use Domain Structures in Command
▸ Command Integrity
▸ ICommandIssuer abstraction
▸ MassTransit implementation
▸ Converts Commands to Messages (TbC)
▸ Encapsulate in an App Service
▸ Validation

49. STEP 2:
COMMAND RECEIVING/
HANDLING

50. IMPLEMENTATION
COMMAND RECEIVING / BOOTSTRAPPING MASSTRANSIT

51. IMPLEMENTATION
MASSTRANSIT / CONSUMER<T>

52. IMPLEMENTATION
MASSTRANSIT RETRING FLOW (SIMPLIFIED)
1. message deserialised
2. identity consumer from message type
A. if consumer identified
‣ run handler in try/catch
‣ on exception check retry policy
‣ either retry on ignore
B. if consumer is NOT identified (TbC)

53. IMPLEMENTATION
COMMAND HANDLING / EVENT DISPATCHING - APP LAYER

54. IMPLEMENTATION
AGGREGATE EVENTS
▸ A/Rs should inherit from BaseAggregateRoot
▸ exposes Events collection

55. IMPLEMENTATION
EVENT DISPATCHER 1/2
▸ wires SYNChronously (in-process) command with event handlers
▸ is the “Emitter”
▸ wired handlers can be
▸ an actual, in-process one (e.g. read model update)
▸ a forwarder (event->message + publish) to the bus
▸ both
▸ even for the same event
▸ infra free implementation

56. IMPLEMENTATION
EVENT DISPATCHER 2/2
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FORWARD
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57. STEP 3:
EVENT EMITTING/PUBLISHING

58. IMPLEMENTATION
EVENT EMITTING/PUBLISHING
▸ as with Commands, Events should use domain structures
▸ IEventEmitter abstraction similar to ICommandIssuer
▸ MassTransit implementation with Bus.Publish<T>
▸ Converts Events to Messages (TbC)

59. IMPLEMENTATION
MASSTRANSIT / BUS.PUBLISH<T>
▸ EndPoint agnostic
▸ Publisher(er)/Subscribe(r) Pattern

60. STEP 4:
EVENT SUBSCRIBING/
HANDLING

61. IMPLEMENTATION
EVENT SUBSCRIBING / BOOTSTRAPPING MASSTRANSIT
▸ bootstrapping identical to command receiving
▸ need a Consumer<T>
▸ translates the message to event
▸ instantiates and invokes event handler

62. IMPLEMENTATION
MASSTRANSIT / CONSUMER<T> (AGAIN)

63. IMPLEMENTATION
EVENT HANDLING

64. BEST PRACTICES

65. BEST PRACTICES
MESSAGE DESIGN
▸ messages are POCOS. (period)
▸ do NOT reuse domain structures
▸ type repetition and mapping are unavoidable
▸ Automapper FTW !
▸ messages might change (aka message versioning)
▸ subscribe to interfaces
▸ ensure correct version or serialiser assembly deployed
▸ JSON .NET

66. BEST PRACTICES
SYSTEM ARCHITECTURE
▸ apply selectively not globally
▸ always forward (i.e. publish) the events
▸ bounded context interface
▸ rely on the retrying facilities
▸ idempotency
▸ fence “expensive” services
▸ perhaps even with sync send/receive (e.g. Routing Service)

67. BEST PRACTICES
SCALING OPTIONS
▸ scale up: can invoke Message Handlers in parallel using a
ThreadPool = Round Robin
▸ bus.SetConcurrencyLimit
▸ scale out: “competing consumers”
▸ same EndPointID
▸ probable concurrency issues
▸ UsePartitioner
▸ aggregate versioning !

68. BEST PRACTICES
COMPETING CONSUMERS
1 1
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69. INDICATIONS

70. INDICATIONS
▸ command/event handlers that are:
▸ resource intensive/blocking
▸ involve multiple aggregate instances
▸ e.g. reroute all cargos unloading @ Hong Kong
▸ error prone
▸ involve external service calls
▸ e.g. payment service
▸ multiple bounded contexts

71. CHALLENGES

72. CHALLENGES
EVENTUAL CONSISTENCY
▸ heavily affects UI/UX
▸ contradicts user mentality
▸ need to notify on command handling error
▸ makes UI implementation more complex

73. CHALLENGES
IMPLEMENTATION-RELATED
▸ a LOT of type repetition
▸ more physical processes
▸ changes MIGHT need orchestration
▸ concurrency issues
▸ transport = additional infra

74. RESOURCES

75. RESOURCES
DDD
▸ Patterns, Principles and Practices of Domain-Driven Design by
Scott Millett, Nick Tune
http://amzn.to/2hsNbLg
▸ Domain Driven Design by Eric Evans
http://amzn.to/2uhbOSc
▸ Effective Aggregate Design (3 Part Series) by Vaughn Vernon
https://vaughnvernon.co/?p=838
▸ Awesome DDD by Nick Chamberlain
https://github.com/heynickc/awesome-ddd

76. RESOURCES
CQRS
▸ CQRS Journey by Microsoft Patterns & Practises
cqrsjourney.github.io
▸ CQRS by Edument
cqrs.nu
▸ Clarified CQRS by Udi Dahan
udidahan.com/2009/12/09/clarified-cqrs

77. RESOURCES
SAMPLES
▸ Cargo Tracker
▸ github.com/SzymonPobiega/DDDSample.Net
(.ΝΕΤ) by Szymon Pobiega
▸ DDD Greece Sample (TBA)
▸ CQRS, The example - Mark Nijhof
leanpub.com/cqrs

78. Q&A