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Progress! so i have moved into working on aggregates. Which are a grouping of events that replayed on an object set the current state of the object. I came up with this little bit of generic wonder.
(cont.)
Just to give some context on some of the components around the code structure.. I wrote this up around an earlier version of aggregate code. This generic bit simplifies things by removing the need of the Crud functions for each aggregate.
Domain Objects
A domain object can be used as an aggregate by adding the event.AggregateRoot struct and finish implementing event.Aggregate. The AggregateRoot implements logic for adding events after they are either Raised by a command or Appended by the eventstore Load or service ApplyFn methods. It also tracks the uncommitted events that are saved using the eventstore Save method.
type User struct {
Identity string ```json:"identity"`
CreatedAt time.Time
event.AggregateRoot
}
// StreamID for the aggregate when stored or loaded from ES.
func (a *User) StreamID() string {
return "user-" + a.Identity
}
// ApplyEvent to the aggregate state.
func (a *User) ApplyEvent(lis ...event.Event) {
for _, e := range lis {
switch e := e.(type) {
case *UserCreated:
a.Identity = e.Identity
a.CreatedAt = e.EventMeta().CreatedDate
/* ... */
}
}
}
Events
Events are applied to the aggregate. They are defined by adding the event.Meta and implementing the getter/setters for event.Event
type UserCreated struct {
eventMeta event.Meta
Identity string
}
func (c *UserCreated) EventMeta() (m event.Meta) {
if c != nil {
m = c.eventMeta
}
return m
}
func (c *UserCreated) SetEventMeta(m event.Meta) {
if c != nil {
c.eventMeta = m
}
}
Reading Events from EventStore
With a domain object that implements the event.Aggregate the event store client can load events and apply them using the Load(ctx, agg) method.
// GetUser populates an user from event store.
func (rw *User) GetUser(ctx context.Context, userID string) (*domain.User, error) {
user := &domain.User{Identity: userID}
err := rw.es.Load(ctx, user)
if err != nil {
if err != nil {
if errors.Is(err, eventstore.ErrStreamNotFound) {
return user, ErrNotFound
}
return user, err
}
return nil, err
}
return user, err
}
OnX Commands
An OnX command will validate the state of the domain object can have the command performed on it. If it can be applied it raises the event using event.Raise() Otherwise it returns an error.
// OnCreate raises an UserCreated event to create the user.
// Note: The handler will check that the user does not already exsist.
func (a *User) OnCreate(identity string) error {
event.Raise(a, &UserCreated{Identity: identity})
return nil
}
// OnScored will attempt to score a task.
// If the task is not in a Created state it will fail.
func (a *Task) OnScored(taskID string, score int64, attributes Attributes) error {
if a.State != TaskStateCreated {
return fmt.Errorf("task expected created, got %s", a.State)
}
event.Raise(a, &TaskScored{TaskID: taskID, Attributes: attributes, Score: score})
return nil
}
Crud Operations for OnX Commands
The following functions in the aggregate service can be used to perform creation and updating of aggregates. The Update function will ensure the aggregate exists, where the Create is intended for non-existent aggregates. These can probably be combined into one function.
// Create is used when the stream does not yet exist.
func (rw *User) Create(
ctx context.Context,
identity string,
fn func(*domain.User) error,
) (*domain.User, error) {
session, err := rw.GetUser(ctx, identity)
if err != nil && !errors.Is(err, ErrNotFound) {
return nil, err
}
if err = fn(session); err != nil {
return nil, err
}
_, err = rw.es.Save(ctx, session)
return session, err
}
// Update is used when the stream already exists.
func (rw *User) Update(
ctx context.Context,
identity string,
fn func(*domain.User) error,
) (*domain.User, error) {
session, err := rw.GetUser(ctx, identity)
if err != nil {
return nil, err
}
if err = fn(session); err != nil {
return nil, err
}
_, err = rw.es.Save(ctx, session)
return session, err
}