# Using services

To use services, add services as third parameter to your reaction.

E.g., if you want to use services when a flow transitions from the pristine state to the awaiting-payment state, use the following code:

const reactions = {
  pristine: {
    'awaiting-payment' (flow, event, services) {
      // ...
    }
  }  
};

Since you do not usually need all services at the same time, it will make sense to request only the services you need. To do this, use destructuring to specify the services you need, e.g.:

const reactions = {
  pristine: {
    'awaiting-payment' (flow, event, { app }) {
      // ...
    }
  }  
};

# Sending commands

Sometimes you may need to send commands to the application from a reaction. For that use the app service, access the context and the aggregate, and call the command function.

E.g., if you want to send an awaitPayment command to the invoice that caused a transition from the pristine state to the awaiting-payment state, use the following code:

const reactions = {
  pristine: {
    'awaiting-payment' (flow, event, { app }) {
      app.accounting.invoice(event.aggregate.id).awaitPayment();
    }
  }  
};

# Writing log output

To write JSON-formatted log output use the logger service. Internally this service uses flaschenpost. For details on how to use flaschenpost see its documentation.

E.g., to log messages when a flow transitions from the pristine state to the awaiting-payment state, use the following code:

const reactions = {
  pristine: {
    'awaiting-payment' (flow, event, { logger }) {
      logger.info('Transitioning from pristine to awaiting payment...');
      // ...
    }
  }  
};