# Using command services

To use services, add services as third parameter to your command or middleware function.

E.g., if you want to use services from within the issue command, use the following code:

const commands = {
  issue (invoice, command, services, mark) {
    // ...
    mark.asDone();
  }
};

# Reading other aggregates

Sometimes you need to read another aggregate, e.g. to base your decision upon its state. For that use the app service, access the desired context and aggregate, and call the read function.

E.g., if you want to read another invoice from the issue command, use the following code:

const commands = {
  issue (invoice, command, services, mark) {
    const app = services.get('app');

    const otherInvoiceId = // ...

    app.accounting.invoice(otherInvoiceId).read((err, otherInvoice) => {
      // ...
      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 message from within the issue command, use the following code:

const commands = {
  issue (invoice, command, services, mark) {
    const logger = services.get('logger');

    logger.info('Issuing an invoice...');

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