eGo

eGo is a minimal library that help build event-sourcing and CQRS application through a simple interface, and it allows developers to describe their commands, events and states **are defined using google protocol buffers **. Under the hood, ego leverages Go-Akt to scale out and guarantee performant, reliable persistence.

Table of Content

• Installation
• Versioning
• Features
  • Event Sourced Behavior
    • Howto
    • Events Stream
    • Projection
    • Events Store
    • Offset Store
  • Durable State Behavior
    • State Store
    • Howto
    • State Stream
• Cluster
• Testkit
• Mocks
• Examples
• Sample
• Contribution

Installation

go get github.com/tochemey/ego

Versioning

The version system adopted in eGo deviates a bit from the standard semantic versioning system. The version format is as follows:

• The MAJOR part of the version will stay at v3 for the meantime.
• The MINOR part of the version will cater for any new features, breaking changes with a note on the breaking changes.
• The PATCH part of the version will cater for dependencies upgrades, bug fixes, security patches and co.

The versioning will remain like v3.x.x until further notice.

Features

Event Sourced Behavior

The EventSourcedBehavior is crucial for maintaining data consistency, especially in distributed systems. It defines how to handle the various commands (requests to perform actions) that are always directed at the event sourced entity. In eGo commands sent to the EventSourcedBehavior are processed in order. When a command is processed, it may result in the generation of events, which are then stored in an event store. Every event persisted has a revision number and timestamp that can help track it. The EventSourcedBehavior in eGo is responsible for defining how to handle events that are the result of command handlers. The end result of events handling is to build the new state of the event sourced entity. When running in cluster mode, aggregate root are sharded.

• Commands handler: The command handlers define how to handle each incoming command, which validations must be applied, and finally, which events will be persisted if any. When there is no event to be persisted a nil can be returned as a no-op. Command handlers are the meat of the event sourced actor. They encode the business rules of your event sourced actor and act as a guardian of the event sourced entity consistency. The command handler must first validate that the incoming command can be applied to the current model state. Any decision should be solely based on the data passed in the commands and the state of the Behavior. In case of successful validation, one or more events expressing the mutations are persisted. Once the events are persisted, they are applied to the state producing a new valid state.
• Events handler: The event handlers are used to mutate the state of the event sourced entity by applying the events to it. Event handlers must be pure functions as they will be used when instantiating the event sourced entity and replaying the event store.

Howto

To define an event sourced entity, one needs to:

1. define the state of the event sourced entity using google protocol buffers message
2. define the various commands that will be handled by the event sourced entity
3. define the various events that are result of the command handlers and that will be handled by the event sourced entity to return the new state of the event sourced entity
4. define and make sure the events store is properly implemented.
5. implement the EventSourcedBehavior interface.
6. call the Entity method of eGo engine

Events Stream

Every event handled by event sourced entity are pushed to an events stream. That enables real-time processing of events without having to interact with the events store. Just use Subscribe method of Engine and start iterating through the messages and cast every message to the Event.

Projection

One can add a projection to the eGo engine to help build a read model. Projections in eGo rely on an offset store to track how far they have consumed events persisted by the write model. The offset used in eGo is a timestamp-based offset. One can also:

• remove a given projection: this will stop the projection and remove it from the system
• check the status of a given projection

Events Store

One can implement a custom events store. See EventsStore. There are some pre-built events stores one can use out of the box. See Contrib

Offsets Store

One can implement a custom offsets store. See OffsetStore. There are some pre-built offsets stores one can use out of the box. See Contrib

Durable State Behavior

The DurableStateBehavior represents a type of Actor that persists its full state after processing each command instead of using event sourcing. This type of Actor keeps its current state in memory during command handling and based upon the command response persists its full state into a durable store. The store can be a SQL or NoSQL database. The whole concept is given the current state of the actor and a command produce a new state with a higher version as shown in this diagram: (State, Command) => State DurableStateBehavior reacts to commands which result in a new version of the actor state. Only the latest version of the actor state is persisted to the durable store. There is no concept of history regarding the actor state since this is not an event sourced actor. However, one can rely on the version number of the actor state and exactly know how the actor state has evolved overtime. DurableStateBehavior version number are numerically incremented by the command handler which means it is imperative that the newer version of the state is greater than the current version by one. DurableStateBehavior will attempt to recover its state whenever available from the durable state. During a normal shutdown process, it will persist its current state to the durable store prior to shutting down. This behavior help maintain some consistency across the actor state evolution.

Durable Store

One can implement a custom state store. See Durable Store. There are some pre-built durable stores one can use out of the box. See Contrib

Howto

To define a durable state entity, one needs to:

1. define the state of the entity using google protocol buffers message
2. define the various commands that will be handled by the entity
3. define and make sure the durable state store is properly implemented.
4. implements the DurableStateBehavior interface
5. start eGo engine with the option durable store using WithStateStore
6. call the DurableStateEntity method of eGo engine

Events Stream

DurableStateBehavior full state is pushed to an events stream. That enables real-time processing of state without having to interact with the state store. Just use Subscribe method of Engine and start iterating through the messages and cast every message to the DurableState.

Cluster

The cluster mode heavily relies on Go-Akt clustering. To enable clustering one need to use WithCluster option when creating the eGo engine.

Testkit

eGo comes bundle with in-memory datastore that can be found in the testkit package. This can help play with eGo.

Mocks

eGo ships in some mocks that can help mock the data stores for unit tests purpose.

Examples

Check the examples

Sample

package main

import (
	"context"
	"errors"
	"log"
	"os"
	"os/signal"
	"syscall"
	"time"

	"github.com/google/uuid"
	"google.golang.org/protobuf/proto"

	"github.com/tochemey/ego/v3"
	samplepb "github.com/tochemey/ego/v3/example/pbs/sample/pb/v1"
	"github.com/tochemey/ego/v3/testkit"
)

func main() {
	// create the go context
	ctx := context.Background()
	// create the event store
	eventStore := testkit.NewEventsStore()
	// connect the event store
	_ = eventStore.Connect(ctx)
	// create the ego engine
	engine := ego.NewEngine("Sample", eventStore)
	// start ego engine
	_ = engine.Start(ctx)
	// create a persistence id
	entityID := uuid.NewString()
	// create an entity behavior with a given id
	behavior := NewAccountBehavior(entityID)
	// create an entity
	_ = engine.Entity(ctx, behavior)

	// send some commands to the pid
	var command proto.Message
	// create an account
	command = &samplepb.CreateAccount{
		AccountId:      entityID,
		AccountBalance: 500.00,
	}
	// send the command to the actor. Please don't ignore the error in production grid code
	reply, _, _ := engine.SendCommand(ctx, entityID, command, time.Minute)
	account := reply.(*samplepb.Account)
	log.Printf("current balance on opening: %v", account.GetAccountBalance())

	// send another command to credit the balance
	command = &samplepb.CreditAccount{
		AccountId: entityID,
		Balance:   250,
	}

	reply, _, _ = engine.SendCommand(ctx, entityID, command, time.Minute)
	account = reply.(*samplepb.Account)
	log.Printf("current balance after a credit of 250: %v", account.GetAccountBalance())

	// capture ctrl+c
	interruptSignal := make(chan os.Signal, 1)
	signal.Notify(interruptSignal, os.Interrupt, syscall.SIGINT, syscall.SIGTERM)
	<-interruptSignal

	// disconnect the event store
	_ = eventStore.Disconnect(ctx)
	// stop the actor system
	_ = engine.Stop(ctx)
	os.Exit(0)
}

// AccountBehavior implements EventSourcedBehavior
type AccountBehavior struct {
	id string
}

// make sure that AccountBehavior is a true persistence behavior
var _ ego.EventSourcedBehavior = &AccountBehavior{}

// NewAccountBehavior creates an instance of AccountBehavior
func NewAccountBehavior(id string) *AccountBehavior {
	return &AccountBehavior{id: id}
}

// ID returns the id
func (a *AccountBehavior) ID() string {
	return a.id
}

// InitialState returns the initial state
func (a *AccountBehavior) InitialState() ego.State {
	return ego.State(new(samplepb.Account))
}

// HandleCommand handles every command that is sent to the persistent behavior
func (a *AccountBehavior) HandleCommand(_ context.Context, command ego.Command, _ ego.State) (events []ego.Event, err error) {
	switch cmd := command.(type) {
	case *samplepb.CreateAccount:
		// TODO in production grid app validate the command using the prior state
		return []ego.Event{
			&samplepb.AccountCreated{
				AccountId:      cmd.GetAccountId(),
				AccountBalance: cmd.GetAccountBalance(),
			},
		}, nil

	case *samplepb.CreditAccount:
		// TODO in production grid app validate the command using the prior state
		return []ego.Event{
			&samplepb.AccountCredited{
				AccountId:      cmd.GetAccountId(),
				AccountBalance: cmd.GetBalance(),
			},
		}, nil

	default:
		return nil, errors.New("unhandled command")
	}
}

// HandleEvent handles every event emitted
func (a *AccountBehavior) HandleEvent(_ context.Context, event ego.Event, priorState ego.State) (state ego.State, err error) {
	switch evt := event.(type) {
	case *samplepb.AccountCreated:
		return &samplepb.Account{
			AccountId:      evt.GetAccountId(),
			AccountBalance: evt.GetAccountBalance(),
		}, nil

	case *samplepb.AccountCredited:
		account := priorState.(*samplepb.Account)
		bal := account.GetAccountBalance() + evt.GetAccountBalance()
		return &samplepb.Account{
			AccountId:      evt.GetAccountId(),
			AccountBalance: bal,
		}, nil

	default:
		return nil, errors.New("unhandled event")
	}
}

Contribution

kindly follow the instructions in the contribution doc