The document discusses projections in event sourcing, including terminology, designing projections, authoring projections, and testing projections. Projections transform event data stored in an event log into read models optimized for querying. The document covers topics like defining projection handlers, dispatching events to handlers, and testing projections with sample events.

1. PROJECTIONS
EXPLAINED
{ YVES REYNHOUT }

2. AGENDA
Meet the domain
Projections
Terminology
Designing
Terminatomy
Authoring
Testing
Retrospective

3. WELCOME @
BEAUFORMA (FR)

4. YOU ARE THE PRODUCT

5. ON THE OUTSIDE

6. ON THE INSIDE

7. THAT IS ...
IF YOU CAN AFFORD US
;-)

10. CHALLENGES

11. BUSINESS ACTIVITIES

12. REWARD

13. WHAT'S NOT TO LIKE ABOUT
BEAUFORMA?

14. BUT, DUDE, BRO ...
WE'RE HERE FOR
PROJECTIONS,
REMEMBER?

15. TERMINOLOGY
Event: a fact, something that happened, a message, a
datastructure
Stream: a sequence of events, partitioned by something
Event store: a collection of streams (simplified)
Disclaimer: not authoritive, just my take

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open System
type GuestStartedShopping = {
SubsidiaryId : Guid;
FeelGoodCartId : Guid;
GuestId : Guid;
StartDateAndTime : DateTime;
}
type GuestCheckedOutCart = {
FeelGoodPackageId : Guid;
FeelGoodCartId : Guid;
GuestId : Guid;
Items : Guid array;
}
type GuestAbandonedCart = {
FeelGoodCartId : Guid;
GuestId : Guid;
LastSeenDateAndTime : DateTime;
}

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type ItemWasAddedToCart = {
FeelGoodCartId : Guid;
ItemId : Guid;
}
type ItemWasRemovedFromCart = {
FeelGoodCartId : Guid;
ItemId : Guid;
}

20. VANILLA CQRS+ES

21. DESIGNING
PROJECTIONS
consumer driven (by screen, api, model, ...)
affected by the choice of store (e.g. required querying
capabilities, non-functional requirements, ...)

23. DATASTRUCTURES

25. EXERCISE

26. Define a datastructure for this widget

27. POSSIBLE SOLUTION
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type GuestsArrivingRecord = {
RecordId: string;
SubsidairyId: Guid;
Date: DateTime;
AppointmentId : Guid;
GuestName : string;
AppointmentDateAndTime : DateTime
}

28. POSSIBLE SOLUTION
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type Appointment = {
AppointmentId : Guid;
GuestName : string;
AppointmentDateAndTime : DateTime
}
type GuestsArrivingDocument = {
DocumentId: string;
SubsidairyId: Guid;
Date: DateTime;
Appointments : Appointment array
}

29. EXERCISE

30. Define 1 or more datastructures for this screen

31. OBSERVATIONS
Not all data comes from one projection,
Not all data is owned by one model

32. EVENTS

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type GuestBookedAppointment = {
AppointmentId : Guid;
GuestId : Guid;
AppointmentDateAndTime : DateTime;
FeelGoodPackageId : Guid;
SubsidiaryId : Guid
}
type GuestRescheduledAppointment = {
AppointmentId : Guid;
GuestId : Guid;
AppointmentDateAndTime : DateTime;
FeelGoodPackageId : Guid;
SubsidiaryId : Guid
}
type GuestSwappedAppointmentFeelGoodPackage = {
AppointmentId : Guid;
GuestId : Guid;
FeelGoodPackageId : Guid;
SubsidiaryId : Guid
}

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type GuestCancelledAppointment = {
AppointmentId : Guid;
GuestId : Guid;
Reason : string;
SubsidiaryId : Guid
}
type SubsidiaryCancelledAppointment = {
AppointmentId : Guid;
GuestId : Guid;
Reason : string;
SubsidiaryId : Guid
}

37. EXERCISE

38. Fill your datastructure using these events
- booking.fshttps://goo.gl/aM40kQ

39. OBSERVATIONS
Not all events are useful,
Information might be missing from events,
Not all data is for viewing
Events challenge the datastructure
What about your observations?

40. EXERCISE

41. ONE EXTRA EVENT TO TAKE INTO ACCOUNT
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type GuestRegistered = {
GuestId : Guid;
FullName : string;
DateOfRegistration: DateTime;
}

42. Extend your projection with the event
- booking|guests.fshttps://goo.gl/aM40kQ

43. POSSIBLE SOLUTION
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type GuestDocument = {
DocumentId: string;
GuestId: Guid;
FullName: string;
}

44. TERMINATOMY
ProjectionHandler: a function that projects an event
Projection: a collection of handlers that form a unit
Projector: a function that dispatches an event or a batch
of events to the matching handler(s)
[Optional] ProjectionHandlerResolver: a function that
returns the handlers that match an event
Disclaimer: not authoritive, just my take

48. EXAMPLE - F# - FUNCTIONAL
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open StackExchange.Redis
let inline (!>) (x:^a) : ^b =
((^a or ^b) : (static member op_Implicit : ^a ­> ^b) x)
type ActiveShoppersEvents =
| GuestStartedShopping of GuestStartedShopping
| GuestCheckedOutCart of GuestCheckedOutCart
| GuestAbandonedCart of GuestAbandonedCart
let activeShoppersProjection (connection:IDatabase) (message:ActiveShoppe
match message with
| GuestStartedShopping _ ­>
connection.StringIncrement(!> "ActiveShoppers", 1L) |> ignore
| GuestAbandonedCart _ ­>
connection.StringDecrement(!> "ActiveShoppers", 1L) |> ignore
| GuestCheckedOutCart _ ­>
connection.StringDecrement(!> "ActiveShoppers", 1L) |> ignore

49. EXAMPLE - C# - OBJECT ORIENTED
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public interface IHandler<TMessage>
{ void Handle(TMessage message); }
public class ActiveShoppersProjection : IHandler<GuestStartedShopping>,
IHandler<GuestAbandonedCart>, IHandler<GuestCheckedOutCart>
{
public readonly IDatabase _connection;
public ActiveShoppersProjection(IDatabase connection)
{ this.connection = connection; }
public void Handle(GuestStartedShopping message)
{ this.connection.StringIncrement("ActiveShoppers"); }
public void Handle(GuestAbandonedCart message)
{ connection.StringDecrement("ActiveShoppers"); }
public void Handle(GuestCheckedOutCart message)
{ connection.StringDecrement("ActiveShoppers"); }
}

50. EXAMPLE - C# - PROJAC
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public class ActiveShoppersProjection : ConnectedProjection<IDatabase>
{
public ActiveShoppersProjection()
{
When<GuestStartedShopping>((connection, message) =>
connection.StringIncrementAsync("ActiveShoppers"));
When<GuestAbandonedCart>((connection, message) =>
connection.StringDecrementAsync("ActiveShoppers"));
When<GuestCheckedOutCart>((connection, message) =>
connection.StringDecrementAsync("ActiveShoppers"));
}
}

51. VANILLA CQRS+ES

52. VANILLA DDD

53. AUTHORING
PROJECTIONS

54. EXERCISE

55. Express the projection in your language and store of choice
- booking|guests.fshttps://goo.gl/aM40kQ

56. RECIPE?
define message types in code (for statically typed
languages)
define and implement projection handlers for each
message type
define and implement a dispatcher to those projection
handlers
test drive in the program's main

57. NOT CHALLENGING ENOUGH?

58. There's an online environment for you to play with:
EventStore
Redis
Elasticsearch
Postgres
- online-
environment.md
https://goo.gl/aM40kQ

59. TESTING PROJECTIONS

62. EXAMPLE - F# - FUNCTIONAL
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let ``when multiple active shoppers`` =
Given [
GuestStartedShopping {
SubsidiaryId = BonifacioId;
FeelGoodCartId=RandomCartId();
GuestId = OliverMartinezId;
StartDateAndTime= 6 <pm>; }
GuestStartedShopping {
SubsidiaryId = BonifacioId;
FeelGoodCartId=RandomCartId();
GuestId = RodriguezId;
StartDateAndTime= 4 <pm>; }
] |>
Expect [
{ Key = "ActiveShoppersCount"; Value = 2 }
]

63. EXAMPLE - C# - OBJECT ORIENTED - PROJAC
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public class ActiveShoppersProjectionScenarios
{
public Task when_multiple_active_shoppers()
{
return MemoryCacheProjection.For(new ActiveShoppersProjection())
.Given(
new GuestStartedShopping {
SubsidiaryId = BonifacioId,
FeelGoodCartId = RandomCartId(),
GuestId = OliverMartinezId,
StartDateAndTime = Today.At(6.PM())
},
new GuestStartedShopping {
SubsidiaryId = BonifacioId,
FeelGoodCartId = RandomCartId(),
GuestId = RodriguezId,
StartDateAndTime = Today.At(4.PM())
})
.Expect(new CacheItem("ActiveShoppersCount", 2));
}
}

64. EXERCISE

65. Express an integration test for your projection.
- booking|guests.fshttps://goo.gl/aM40kQ

66. RETROSPECTIVE

67. FURTHER
EXPLORATION
Mechanical Sympathy (know your stores)
Checkpointing & Failure Recovery & High Availability
Event delivery & Idempotency
Batching & Partitioning & (Data) Composition
Deploying & Upgrading
Parallel Test Execution