# Collecting IoT events

If you are building an IoT application, you need to collect events that happen in the real-world, e.g. sensor data. It may feel difficult to map them to commands, since the events have already happened, and hence do not serve well as tasks.

# Defining physical events

In order to handle physical events you need to define them in the same way as you define other events, too. If you need to, you may also update the aggregate state from the events. This allows you to access the data from previously recorded physical events when handling a command.

E.g., when you want to set the sentAsLetterPost property of the aggregate state to true once an invoice was sent as letter post, use the following code:

const events = {
  sentAsLetterPost (invoice, event) {
    invoice.setState({
      sentAsLetterPost: true
    });
  };
};

# Recording physical events

To record physical events register a generic event-recording command such as recordEvent and use the handle.physicalEvents middleware from the wolkenkit-command-tools module.

E.g., if you want to collect the information that an invoice was sent as letter post, use the following code:

const formats = require('formats'),
      { handle, only } = require('wolkenkit-command-tools');

// ...

const commands = {
  recordEvent: [
    only.ifExists(),
    handle.physicalEvents({
      sentAsLetterPost: {
        recipient: formats.string({ minLength: 1 })
      }
    })
  ]
};