README

While working with .Net Core and MassTransit, it became frustrating that Laravel lacked a ready-made state machine engine. Inspired by MassTransit, I wrote a library that functions similarly to MassTransit sagas.

Installation

1. composer require smskin/laravel-saga
2. php artisan vendor:publish --provider=SMSkin\LaravelSaga\Providers\ServiceProvider

Configuration

In the configuration file config/saga.php, you will find the engine settings and descriptions of state machines.

• logger - a class responsible for logging the state machine's operation process. Can be changed to another class implementing the ISagaLogger interface.
• state-machines - an array of registered state machines.
• repositories
  • default - the repository for storing the state machine's state (database).
  • database
    • class - the repository class. Can be changed to another class implementing the ISagaRepository interface.
    • table - the name of the table where the state machine's state will be stored.

Saga Structure

Let's examine a saga from an example in this library (SMSkin\LaravelSaga\Example\SagaExample).

Property $context

This property describes the type (cast) of the stored object of the state machine. It can be any class inheriting from SagaContext. This object allows storing intermediate values obtained during interaction with other services during operation.

Method setup()

This method describes the state machine's operation algorithm.

Three key blocks of a saga:

• correlation
• initialization event\command
• state machine transition logic

Correlation

This block describes the algorithm for obtaining the identifier of the state machine's context in the repository. It's described using two methods:

• correlatedById - getting the object by ID.
• correlatedBy - getting it by any storage field.

$this->builder()
     ->correlatedById(EUserCreated::class, static function (EUserCreated $event) {
            return $event->corrId;
     })

This block can be read as: Upon receiving the EUserCreated event, take the state machine's ID from the corrId property.

$this->builder()
     ->correlatedBy(EUserBlocked::class, 'userId', static function (EUserBlocked $event) {
            return $event->userId;
     });

This block can be read as: Upon receiving the EUserBlocked event, find the state machine by the userId field, taking the value from the userId event. Thus, the engine can search for the state machine's context both by UUID and by any context field.

Initialization Event\Command

This block describes the events\commands that will initialize the state machine.

The onInitEvent method takes two arguments:

• The event class to be registered in Laravel.
• A Closure for transforming the event into the state machine's context. With this method, you can save some initialization data in the context object.

 $this->builder()
      ->onInitEvent(CreateUserCommand::class, static function (CreateUserCommand $command) {
            return (new SagaExampleContext($command->correlationId))
                ->setEmail($command->email);
      });

This block can be read as:

• Upon receiving the CreateUserCommand command, initialize the state machine.
• Take the state machine's ID from the correlationId command.
• Save the email from the command in the state machine's context.

State Machine Transition Logic

This block describes the state machine's algorithm. Key phrases:

• duringState - while in the state machine's state.
• on - upon receiving an event.
• then - do (closure).
• activity - perform a subroutine (class implementing the IActivity interface).
• transitionTo - switch the state machine's status.
• publish - publish an event.
• initial - sugar for initialization (first stage).
• finalize - sugar for finalization.

$this->builder()
     ->initial()
     ->transitionTo(SagaExampleStates::USER_CREATING)
     ->activity(UserCreatingActivity::class)
     ->then(function () {
            (new UserCommandService())->create(
                $this->context->getId(),
                $this->context->getEmail()
            );
    });

This block can be read as:

• Upon initialization.
• Switch the state to USER_CREATING.
• Perform the UserCreatingActivity subprogram.
• Execute the Closure - call UserCommandService->create, passing the context ID and email (which we stored in the context during initialization).

 $this->builder()
      ->duringState(SagaExampleStates::USER_CREATING)
      ->on(EUserCreated::class)
      ->then(function () {
            $event = $this->getHandledEvent();
            $this->context->setUserId($event->userId);
      })
      ->transitionTo(SagaExampleStates::USER_BLOCKING)
      ->then(function () {
            (new UserCommandService())->block(
                $this->context->getUserId()
            );
      });

This block can be read as:

• While in the state USER_CREATING.
• Upon receiving the EUserCreated event.
• Execute the Closure, which will write the userId (from the event) into the state machine's context.
• Switch the state to USER_BLOCKING.
• Execute the Closure - call UserCommandService->block, passing the userId from the context.

$this->builder()
     ->duringState(SagaExampleStates::USER_BLOCKING)
     ->on(EUserBlocked::class)
     ->finalize()
     ->publish(function () {
            return new ESagaExampleFinalized($this->context->getId());
     });

This block can be read as:

• While in the state USER_BLOCKING.
• Upon receiving the EUserBlocked event.
• Finalize the state machine.
• Publish the event ESagaExampleFinalized, passing the saga ID.

Basic Operation Principle

The engine operates based on the Laravel Events. The events described in the setup() method are registered in the EventServiceProvider. The saga acts as a Listener.

When an event enters the bus, the Laravel broker executes the handle method of the sagas registered for that event.

Execution Optimization

Since the events to which the saga registers are described within the saga itself, Laravel will require time to compute these events from all sagas. To optimize this, an artisan command is written that saves a pre-computed cache of event=saga mapping ready for registration.

Caching

php artisan saga:cache

Cache Clearing

php artisan saga:cache:clear

Configuration Options

Changing the Saga Data Storage Repository

1. Create a class implementing the ISagaRepository interface.
2. Add it to the saga.repositories configuration.
3. Specify it in the saga.repositories.default configuration variable.

Changing the Logger

1. Create a class implementing the ISagaLogger interface.
2. Specify it in the saga.logger configuration.

Creating Custom Sagas

1. Create a class inheriting from BaseSaga.
2. Describe the logic of the state machine in the setup() method.
3. Specify the class in the saga.state-machines configuration.