Documentation for sagas and compensation for failures about cqrs

[Insidexa]

I think and making my custom prototype for compensation activity but not with nestjs saga
With current implementation in nestjs compensation its not impossible.
Because you need manager for start rollback.

Maybe add to current implementation SagaManager, and add as process - Command, compensaction - Command but for rollback.

For example I see very useful in Camunda software. Example Repo

ProcessBuilder saga = Bpmn.createExecutableProcess("trip");
    
    // - flow of activities and compensating actions
    saga.startEvent()
        .serviceTask("car").name("Reserve car").camundaClass(ReserveCarAdapter.class)
          .boundaryEvent().compensateEventDefinition().compensateEventDefinitionDone()
          .compensationStart().serviceTask("car-compensate").name("Cancel car").camundaClass(CancelCarAdapter.class).compensationDone()
        .serviceTask("hotel").name("Book hotel").camundaClass(BookHotelAdapter.class)
          .boundaryEvent().compensateEventDefinition().compensateEventDefinitionDone()
          .compensationStart().serviceTask("hotel-compensate").name("Hotel car").camundaClass(CancelCarAdapter.class).compensationDone()
        .serviceTask("flight").name("Book flight").camundaClass(BookFlightAdapter.class)
          .boundaryEvent().compensateEventDefinition().compensateEventDefinitionDone()
          .compensationStart().serviceTask("flight-compensate").name("Cancel flight").camundaClass(CancelCarAdapter.class).compensationDone()
        .endEvent();
    
    // - trigger compensation in case of any exception (other triggers are possible)
    saga.eventSubProcess()
        .startEvent().error("java.lang.Throwable")
        .intermediateThrowEvent().compensateEventDefinition().compensateEventDefinitionDone()
        .endEvent();

[Insidexa]

I offer my own version of pseudocode to implement command compensation

    const travelProcess = this.processBuilder
        .registerHandler(ReserveCarHandler, CancelReserveCarHandler)
        .registerHandler(BookHotelHandler, CancelBookHotelHandler)
        .registerHandler(BookFlightHandler, CancelBookFlightHandler)
        .build('travel process');

    // result is Observable
    const result = this.processRunner.run(payload, travelProcess);

How it works:

Call first command ReserveCarHandler and pass payload.
After call BookHotelHandler with payload and pass as 2 argument previous command result.

Result can return info about successful and failed activities and general status of process.

Maybe as feature you can register process in some module, and call processes from current module.

@kamilmysliwiec what you think about it ?

CQRS Commands:

• ReserveCarHandler
• BookHotelHandler
• BookFlightHandler
• CancelReserveCarHandler
• CancelBookHotelHandler
• CancelBookFlightHandler

[kamilmysliwiec]

We're tracking this here nestjs/docs.nestjs.com#957