聊聊go.cqrs的AggregateRoot

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本文主要研究一下go.cqrs的AggregateRoot

AggregateRoot

//AggregateRoot is the interface that all aggregates should implement
type AggregateRoot interface {
    AggregateID() string
    OriginalVersion() int
    CurrentVersion() int
    IncrementVersion()
    Apply(events EventMessage, isNew bool)
    TrackChange(EventMessage)
    GetChanges() []EventMessage
    ClearChanges()
}

AggregateRoot接口定义了AggregateID、OriginalVersion、CurrentVersion、IncrementVersion、Apply、TrackChange、GetChanges、ClearChanges方法

AggregateBase

// AggregateBase is a type that can be embedded in an AggregateRoot
// implementation to handle common aggragate behaviour
//
// All required methods to implement an aggregate are here, to implement the
// Aggregate root interface your aggregate will need to implement the Apply
// method that will contain behaviour specific to your aggregate.
type AggregateBase struct {
    id      string
    version int
    changes []EventMessage
}

// NewAggregateBase contructs a new AggregateBase.
func NewAggregateBase(id string) *AggregateBase {
    return &AggregateBase{
        id:      id,
        changes: []EventMessage{},
        version: -1,
    }
}

// AggregateID returns the AggregateID
func (a *AggregateBase) AggregateID() string {
    return a.id
}

// OriginalVersion returns the version of the aggregate as it was when it was
// instantiated or loaded from the repository.
//
// Importantly an aggregate with one event applied will be at version 0
// this allows the aggregates to match the version in the eventstore where
// the first event will be version 0.
func (a *AggregateBase) OriginalVersion() int {
    return a.version
}

// CurrentVersion returns the version of the aggregate as it was when it was
// instantiated or loaded from the repository.
//
// Importantly an aggregate with one event applied will be at version 0
// this allows the aggregates to match the version in the eventstore where
// the first event will be version 0.
func (a *AggregateBase) CurrentVersion() int {
    return a.version + len(a.changes)
}

// IncrementVersion increments the aggregate version number by one.
func (a *AggregateBase) IncrementVersion() {
    a.version++
}

// TrackChange stores the EventMessage in the changes collection.
//
// Changes are new, unpersisted events that have been applied to the aggregate.
func (a *AggregateBase) TrackChange(event EventMessage) {
    a.changes = append(a.changes, event)
}

// GetChanges returns the collection of new unpersisted events that have
// been applied to the aggregate.
func (a *AggregateBase) GetChanges() []EventMessage {
    return a.changes
}

//ClearChanges removes all unpersisted events from the aggregate.
func (a *AggregateBase) ClearChanges() {
    a.changes = []EventMessage{}
}

AggregateBase定义了id、version、changes属性；AggregateID方法返回id；OriginalVersion方法返回version；CurrentVersion返回version+len(a.changes)；IncrementVersion递增version；TrackChange方法往changes增加event；GetChanges方法返回changes；ClearChanges会清空changes

EventMessage

// EventMessage is the interface that a command must implement.
type EventMessage interface {

    // AggregateID returns the ID of the Aggregate that the event relates to
    AggregateID() string

    // GetHeaders returns the key value collection of headers for the event.
    //
    // Headers are metadata about the event that do not form part of the
    // actual event but are still required to be persisted alongside the event.
    GetHeaders() map[string]interface{}

    // SetHeader sets the value of the header specified by the key
    SetHeader(string, interface{})

    // Returns the actual event which is the payload of the event message.
    Event() interface{}

    // EventType returns a string descriptor of the command name
    EventType() string

    // Version returns the version of the event
    Version() *int
}

EventMessage接口定义了AggregateID、GetHeaders、SetHeader、Event、EventType、Version方法

EventDescriptor

// EventDescriptor is an implementation of the event message interface.
type EventDescriptor struct {
    id      string
    event   interface{}
    headers map[string]interface{}
    version *int
}

// NewEventMessage returns a new event descriptor
func NewEventMessage(aggregateID string, event interface{}, version *int) *EventDescriptor {
    return &EventDescriptor{
        id:      aggregateID,
        event:   event,
        headers: make(map[string]interface{}),
        version: version,
    }
}

// EventType returns the name of the event type as a string.
func (c *EventDescriptor) EventType() string {
    return typeOf(c.event)
}

// AggregateID returns the ID of the Aggregate that the event relates to.
func (c *EventDescriptor) AggregateID() string {
    return c.id
}

// GetHeaders returns the headers for the event.
func (c *EventDescriptor) GetHeaders() map[string]interface{} {
    return c.headers
}

// SetHeader sets the value of the header specified by the key
func (c *EventDescriptor) SetHeader(key string, value interface{}) {
    c.headers[key] = value
}

// Event the event payload of the event message
func (c *EventDescriptor) Event() interface{} {
    return c.event
}

// Version returns the version of the event
func (c *EventDescriptor) Version() *int {
    return c.version
}

EventDescriptor定义了id、event、headers、version属性；其EventType返回typeOf(c.event)；AggregateID返回id；GetHeaders返回headers；SetHeader会设置header；Event方法返回event；Version返回version

小结

go.cqrs的AggregateRoot接口定义了AggregateID、OriginalVersion、CurrentVersion、IncrementVersion、Apply、TrackChange、GetChanges、ClearChanges方法。

doc

• go.cqrs