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Golang client for Eventuate API

Install

To install this package run

go get https://github.com/eventuate-clients/eventuate-client-golang

or clone/create this package as git submodule for your projects vendor directory.

Usage

To use the library, import it with the statement:

import  eventuate "https://github.com/eventuate-clients/eventuate-client-golang"

Definitions

Aggregates

In the Eventuate programming model, the majority of your application's business logic is implemented by aggregates. An aggregate does two things:

  • Processes commands and returns events, which leaves the state of the aggregate unchanged.
  • Consumes events, which updates its state.

The Eventuate client framework for Golang relies on a simple way to define aggregates.

It has to be a public type, like so:

type FooBarAggregate struct {
	state string
}

accompanied by an instantiation function:

func NewFooBarAggregate() *FooBarAggregate {
	return &FooBarAggregate{
		state: "New"}
}

and its own business logic:

func (todo *FooBarAggregate) Foo(foo string) {
	todo.state = fmt.Sprintf("%v;%v:%v", todo.state, "foo", foo)
}
func (todo *FooBarAggregate) Bar(bar string) {
	todo.state = fmt.Sprintf("%v;%v:%v", todo.state, "bar", bar)
}

to top it all, you have to prepare a fully-qualified name (FQN) for this aggregate:

const FOOBAR_ENTITY = "golang.FooBarApplication.examples.FooBarEntity"

Commands and events

As mentioned above, the Aggregate processes commands (returning events and leaving its state unchanged) and consumes events (which update its state).

A command is a desired action to be performed on the instance of an up-to-date entity. Once processed, it generates one or several, or none, events ('increased this', 'decreased that', 'signaled an error', etc.). A command is not supposed to get serialized or passed outside of the boundaries of a local code.

Events, on the other hand, are designed to be serialized and passed on to the Eventuate server. They carry bits of data, which, when applied in order, recreate an entity's instance in its current state.

Commands

Commands are defined as simple structs whose names end with Command:

type FooCommand struct {
	Foo string
}

type BarCommand struct {
	Bar string
}

Your aggregate needs to be able to process them. Either with specific methods:

func (foobar *FooBarAggregate) ProcessFooCommand(cmd *FooCommand) []eventuate.Event {
    // logic based of the foobar's current state
	return []eventuate.Event{
		newFooEvent(cmd.Foo)}
}
func (foobar *FooBarAggregate) ProcessBarCommand(cmd *BarCommand) []eventuate.Event {
    // logic based of the foobar's current state
	return []eventuate.Event{
		newBarEvent(cmd.Bar)}
}

func newFooEvent(foo string) *FooEvent {
	return &FooEvent{
		Foo: foo}
}

func newBarEvent(bar string) *BarEvent {
	return &BarEvent{
		Bar: bar}
}

.. or with a general one:

func (foobar *FooBarAggregate) ProcessCommand(command eventuate.Command) []eventuate.Event {
	switch cmd := command.(type) {

	case *FooCommand:
		{
			return []eventuate.Event{
				newFooEvent(cmd.Foo)}
		}
	case *BarCommand:
		{
			return []eventuate.Event{
				newBarEvent(cmd.Bar)}
		}
	}

	return []eventuate.Event{}
}

Important! Note, how specific command-processing methods are named ProcessXYZCommand and return slices of eventuate.Event. Also note how general command-processing method is named ProcessCommand and accepts a type which is an alias to interface{}, eventuate.Command. If specific methods are present, they take precedence over the general one. Thus, there is no need to have the latter if all specific methods are defined.

Events

Events are defined as simple structs whose names end with Event and whose fields (as a rule) have JSON tags as (de)serialization guidelines:

type FooEvent struct {
	Foo string `json:"foo"`
}

type BarEvent struct {
	Bar string `json:"bar"`
}

alongside with the defined Event types there must be prepared their fully-qualified names (FQN):

const FOOBAR_FOO_EVENT = "golang.FooBarApplication.examples.FooBarEntity.Events.FooEvent"
const FOOBAR_BAR_EVENT = "golang.FooBarApplication.examples.FooBarEntity.Events.BarEvent"

Your aggregate needs to be able to apply these events with either specific methods:

func (foobar *FooBarAggregate) ApplyFooEvent(evt *FooEvent) *FooBarAggregate {
	foobar.Foo(evt.Foo)
	return todo
}
func (foobar *FooBarAggregate) ApplyBarEvent(evt *BarEvent) *FooBarAggregate {
	foobar.Bar(evt.Bar)
	return todo
}

or with a general one:

func (foobar *FooBarAggregate) ApplyEvent(evt eventuate.Event) *FooBarAggregate {
	switch t := evt.(type) {
	case *FooEvent:
		{
			foobar.Foo(t.Foo)
		}
	case *BarEvent:
		{
			foobar.Bar(t.Bar)
		}
	}
	return todo
}

Important! Note, how specific event-application methods are named ApplyXYZEvent and return a reference to the same receiver (*FooBarAggregate here). Also note how general event-application method is named ApplyEvent and accepts a type which is an alias to interface{}, eventuate.Event. If specific methods are present, they take precedence over the general one. Thus, there is no need to have the latter if all specific methods are defined.

Configuring metadata for an aggregate

The rules outlined above (marked with the 'Important!') are enforced and used during command-processing and event-application with the help of an instance of eventuate.AggregateMetadata. This metadata-holding object is crucial for all operations with the Eventuate store, so it is important to have it handy prior to establishing connection to Eventuate:

aggregateMetadata, metaErr := eventuate.CreateAggregateMetadata(NewFooBarAggregate, FOOBAR_ENTITY)
// check for and handle errors

Note that we pass the aggregate's instantiation function and its fully-qualified name. Refer back to the section Aggregates.

Event (de)serialization hints

Since events need to be passed over the network and be correctly serialized and de-serialized along with their fully-qualified names, we need to provide FQN-Type correspondence. Thus:

aggregateMetadata.RegisterEventType(FOOBAR_FOO_EVENT, &FooEvent{})
aggregateMetadata.RegisterEventType(FOOBAR_BAR_EVENT, &BarEvent{})

Failure to do so will prevent a correct deserialization of events and, as a result, their application. We pass a sample of the instantiated event type following its FQN.

Working with the Eventuate server's API

Creating a REST client

To create a REST Client, prepare a variable of eventuate.AggregateCrud type:

var client eventuate.AggregateCrud

and use Eventuate client builder API:

client, _ = eventuate.ClientBuilder().BuildREST()
// check for and handle errors

This defaults to connecting to the Eventuate REST API server's address http://api.eventuate.io (and STOMP API server's address https://dev.eventuate.io:61615), picking credentials from the environment variables EVENTUATE_API_KEY_ID and EVENTUATE_API_KEY_SECRET, setting a namespace for operations to "default", and silent log mode. If these defaults need to be changed use the following chainable builder option methods:

WithUrl(serverUrl)

To change default Eventuate REST API server's address. (Default is http://api.eventuate.io)

client, _ = eventuate.ClientBuilder().WithUrl("https://dev.eventuate.io").BuildREST()
// check for and handle errors

WithSpace(space)

To change default operations namespace. (Default is "default")

const FOOBAR_NS = "FooBarNamespace"
client, _ = eventuate.ClientBuilder().WithSpace(FOOBAR_NS).BuildREST()
// check for and handle errors

WithCredentials(apiKeyId, apiKeySecret)

To provide credentials directly instead of relying on reading the environment variables EVENTUATE_API_KEY_ID and EVENTUATE_API_KEY_SECRET

client, _ = eventuate.ClientBuilder().WithCredentials("ABCD0987..", "FEDCBA010...01234").BuildREST()
// check for and handle errors

WithDevMode(devMode)

client, _ = eventuate.ClientBuilder().WithDevMode(eventuate.Verbose).BuildREST()
// check for and handle errors

Create an AggregateRepository

Once the REST API Client instance is ready, an Aggregate repository instantiation is due:

// var repo *AggregateRepository
repo := eventuate.NewAggregateRepository(&client)

The creation of an entity, its retrieving and updating is done against the repository instance (repo here):

Saving aggregate

entity, _ := repo.Save(aggregateMetadata, &FooCommand{
    Foo: "FooString"})
// check for and handle errors

entityId := entity.EntityId // id is used to reference a newly created entity

Updating aggregate

entity, _ = repo.Update(aggregateMetadata, entityId, &BarCommand{
    Bar: "BarString"})
// check for and handle errors

Finding aggregate

locatedEntity, _ := repo.Find(aggregateMetadata, entityId)
// check for and handle errors

entityInstance := locatedEntity.EntityInstance

STOMP

Creating a STOMP client

Establishing a STOMP connection uses the same eventuate.ClientBuilder() API with the only difference of calling BuildSTOMP() in the end.

BuildSTOMP() (for STOMP)

To instantiate Eventuate STOMP API Client instance:

var stomp *StompClient
stomp, _ = eventuate.ClientBuilder().BuildSTOMP()
// check for and handle errors
WithStompUrl(serverUrl) (for STOMP)

To change default Eventuate STOMP API server's address. (Default is https://api.eventuate.io:61614)

stomp, _ := eventuate.ClientBuilder().WithStompUrl("https://dev.eventuate.io:61615").BuildSTOMP()
// check for and handle errors

Subscription manager

Subscription manager instantiation:

var (
        sm *eventuate.SubscriptionManager
)
sm, _ := eventuate.NewSubscriptionManager(stomp)
// check for and handle errors

Subscription

Subscribing for events:

subscriberId := "..." // generate UID or use your constant
entityEventTypeMap := eventuate.EventResultHandlerMap{
    FOOBAR_ENTITY: {
        FOOBAR_FOO_EVENT: testResult.generalResultEventHandler,
        FOOBAR_BAR_EVENT: testResult.generalResultEventHandler}}

sub, _ := sm.Subscribe(subscriberId, entityEventTypeMap, false)
// check for errors first

Event FQN-Type registration

Event types registration:

sm.RegisterEventType(FOOBAR_FOO_EVENT, FooEvent{})
sm.RegisterEventType(FOOBAR_BAR_EVENT, BarEvent{})

This is obviously best done prior to .Subscribe(..).

Event handler

Event handler must implement the type eventuate.EventResultHandler, which looks like:

type EventResultHandler func(*DeserializedEvent) *Settler

The eventuate.DeserializedEvent contains these useful fields: Id, EntityId, EntityType, EventType, and EventData.

The eventuate.Settler interface is a Javascripts's rough equivalent of a Promise, and Java's equivalent of a CompletableFuture<T>:

type Settler interface {
	IsSettled() bool
	Settle(interface{}, error)
	GetValue() (interface{}, error)
}

You can use the implementation supplied by the Library: eventuate.FutureResult and the instantiation helper functions: eventuate.NewPassedFutureResult(val interface{}) and eventuate.NewFailedFutureResult(err error). (Important! Please be careful with calling .GetValue() without checking for settled-ness (.IsSettled()) on instances since this may easily block your code in async scenarios.)

Event handler sample (referred to from the subscription snippet above):

type result struct {
	count int
}

func (rslt *result) generalResultEventHandler(evt *eventuate.DeserializedEvent) *eventuate.Settler {
	log.Printf("Event handler for: %#v\n", evt)
	switch evtData := evt.EventData.(type) {
	case *FooEvent:
		{
			rslt.count += 1
			log.Printf("Data is of type `*FooEvent`, value: %v\n", evtData.Foo)
		}
	case *BarEvent:
		{
			rslt.count += 8
			log.Printf("Data is of type `*BarEvent`, value: %v\n", evtData.Bar)
		}
	}
	return eventuate.NewPassedFutureResult(true)
}

Expect async work before exiting.

Run tests

To check tests you need to run go test in the project root. Before you run the tests export your API token id and API token secret, for example:

export EVENTUATE_API_KEY_ID=key_id
export EVENTUATE_API_KEY_SECRET=key_secret