# Using services

To use services, add services as second parameter to your event handler.

E.g., if you want to use services from within the accounting.invoice.issued event handler, use the following code:

const when = {
  'accounting.invoice.issued' (event, services, mark) {
    // ...
    mark.asDone();
  }
};

# Sending commands

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

E.g., if you want to send a check command from the accounting.invoice.issued event handler to the invoice that caused the event, use the following code:

const when = {
  'accounting.invoice.issued' (event, services, mark) {
    const app = services.get('app');

    app.accounting.invoice(event.aggregate.id).check();

    mark.asDone();
  }
};

# 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 from within the accounting.invoice.issued event handler, use the following code:

const when = {
  'accounting.invoice.issued' (event, services, mark) {
    const logger = services.get('logger');

    logger.info('Handling an issued invoice...');

    // ...
    mark.asDone();
  }
};