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Creating Events

An event is any type that implements the event.Event / event.Of[T] interface. You can create such an event with the event.New() function.

The following example creates a "user.created" event with the string "Bob" as its event data.

go
package example

import (
  "github.com/google/uuid"
  "github.com/modernice/goes/event"
)

func example() {
  evt := event.New("user.created", "Bob")

  evt.ID() == uuid.UUID{...} // randomly generated UUID
  evt.Name() == "user.created"
  evt.Data() == "Bob"
  evt.Time() == time.Now() // roughly
}

Event Data

You can use any type as event data. In the previous example, we created an event with a string as its event data, representing the name of the user. Now, when registering a user, you typically need to put more information in the event data, like the user's email address and password. You can use structs as event data, like so:

go
package myapp

// UserRegisteredData is the event data for the "user.registered" event.
type UserRegisteredData struct {
  Name string
  Email string
  Password string
}

Then create the event with that event data:

go
package myapp

import "github.com/modernice/goes/event"

func example() {
  evt := event.New("user.registered", UserRegisteredData{
    Name: "Bob",
    Email: "bob@example.com",
    Password: "<some-encrypted-password>",
  })

  evt.Data() == UserRegisteredData{
    Name: "Bob",
    Email: "bob@example.com",
    Password: "<some-encrypted-password>",
  }
}

Event Options

The event.New() function accepts functional options to override the default ID and Time of the event.

go
package example

import (
  "time"
  "github.com/google/uuid"
  "github.com/modernice/goes/event"
)

func example() {
  id := uuid.New()
  t := time.Now().Add(-time.Hour) // 1 hour ago

  evt := event.New(
    "user.registered", 
    "Bob",
    event.ID(id),
    event.Time(t),
  )

  evt.ID() == id
  evt.Time() == t
}

Aggregate Events

TIP

Typically, you will create aggregate events using aggregate.Next() instead of event.New(). Read the Aggregate documentation for more information.

An Aggregate Event is an event that additionally provides its position in the event stream of an aggregate. To create an such an event, you can pass the event.Aggregate() option to event.New(). The option accepts the id and name of the aggregate as well as the position in the event stream, starting at position 1.

INFO

An event's position in the event stream of an aggregate is denoted by the Aggregate Version of the event.

go
package example

import "github.com/modernice/goes/event"

func example() {
  id := uuid.New()
  name := "auth.user"
  version := 1 // position in the event stream

  evt := event.New(
    "user.registered",
    "Bob",
    event.Aggregate(id, name, version),
  )

  aid, aname, aversion := evt.Aggregate()

  aid == id
  aname == name
  aversion == version
}

Custom Implementation

event.New() returns an event.Evt[T], which implements the event.Of[T] / event.Event interface. If you don't want to depend on the implemenation provided by goes, you can create a custom implementation:

go
package myapp

type UserRegistered struct {
  Name string
  Email string
  Password string

  userID uuid.UUID

  evtID uuid.UUID
  evtTime time.Time
  evtVersion int
}

func (evt UserRegistered) ID() uuid.UUID {
  return evt.evtID
}

func (evt UserRegistered) Time() time.Time {
  return evt.evtTime
}

func (evt UserRegistered) Data() UserRegistered {
  return evt
}

func (evt UserRegistered) Aggregate() (uuid.UUID, string, int) {
  return userID, "auth.user", evt.evtVersion
}