1.
PROJECTIONS
EXPLAINED
{ YVES REYNHOUT }

2.
AGENDA
Meet the domain
Projections
Terminology
Designing
Authoring
Testing

3.
WELCOME @
BEAUFORMA (FR)

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YOU ARE THE PRODUCT

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ON THE OUTSIDE

6.
ON THE INSIDE

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

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CHALLENGES

9.
CONTEXT MAP

10.
REWARD

11.
WHAT'S NOT TO LIKE ABOUT
BEAUFORMA?

12.
BUT, DUDE ... WE'RE
HERE FOR
PROJECTIONS,
REMEMBER?

13.
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;
}

16.
VANILLA CQRS+ES

17.
TERMINOLOGY
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

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EXAMPLE
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open StackExchange.Redis
let inline (!>) (x:^a) : ^b =
((^a or ^b) : (static member op_Implicit : ^a ­> ^b) x)
let activeShoppersProjection (connection:IDatabase, message:Object) =
match message with
| :? GuestStartedShopping ­>
connection.StringIncrement(!> "ActiveShoppers", 1L) |> ignore
| :? GuestAbandonedCart ­>
connection.StringDecrement(!> "ActiveShoppers", 1L) |> ignore
| :? GuestCheckedOutCart ­>
connection.StringDecrement(!> "ActiveShoppers", 1L) |> ignore
| _ ­> ()

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EXAMPLE
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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"));
}
}

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EXAMPLE
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public interface IHandler<TMessage>
{
void Handle(IDatabase connection, TMessage message);
}
public class ActiveShoppersProjection : IHandler<GuestStartedShopping>,
IHandler<GuestAbandonedCart>, IHandler<GuestCheckedOutCart>
{
public void Handle(IDatabase connection, GuestStartedShopping message)
{
connection.StringIncrement("ActiveShoppers");
}
public void Handle(IDatabase connection, GuestAbandonedCart message)
{
connection.StringDecrement("ActiveShoppers");
}
public void Handle(IDatabase connection, GuestCheckedOutCart message)
{
connection.StringDecrement("ActiveShoppers");
}
}

21.
VANILLA CQRS+ES

22.
VANILLA DDD

23.
DESIGNING
PROJECTIONS
consumer driven (by screen, api, model, ...)
affected by the choice of store (e.g. required querying
capabilities, non-functional requirements, ...)

24.
DATASTRUCTURES

25.
EXERCISE
Define a datastructure for this widget

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

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

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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
}

31.
EXERCISE
Fill your datastructure using these events
- booking.fshttps://goo.gl/BTBTfi

32.
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?

33.
OBSERVATIONS
Not all data comes from one projection,
Not all data is owned by one model

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

35.
EXERCISE
Extend your projection with the event
- booking|guests.fshttps://goo.gl/BTBTfi

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

37.
AUTHORING
PROJECTIONS

38.
EXERCISE
Express the projection in your language and store of choice
- booking|guests.fshttps://goo.gl/BTBTfi

39.
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

40.
NOT CHALLENGING ENOUGH?

41.
EVENTSTORE
IP Address: 178.62.229.196
Http Port: 2113
Tcp Port: 1113
Login: admin
Password: changeit
REDIS
IP Address: 178.62.229.196
Port: 6379
ELASTICSEARCH
IP Address: 178.62.229.196
Port: 9200

42.
POSTGRES
IP Address: 46.101.161.64
Port: 5432
Login: dddeu16
Password: dddeu16
Database: dddeu16
SslMode: required
Server Certificate: see online environment file
- online-
environment.md
https://goo.gl/BTBTfi

43.
TESTING PROJECTIONS

44.
EXAMPLE
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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));
}
}