SOW and Subscribe

const reportVanPosition = async client => {
  /**
  * sowAndSubscribe() method to begin receiving information about
  * all of the active delivery vans in the system. All of the vans
  * in the system now are returned as Message objects whose
  * `message.header.command()` method returns `sow`. New messages
  * coming in are returned as Message objects whose
  * `message.header.command()` method returns `p` (publish).
  */
  return client.sowAndSubscribe(

    // Message handler
    message => {
      const cmdName = message.header.command();
      if (cmdName === 'sow' || cmdName === 'p') {
        /**
        * For each of these messages we call addOrUpdateVan(),
        * that presumably adds the van to our application’s
        * display. As vans send updates to the AMPS server,
        * those are also received by the client because of the
        * subscription placed by sowAndSubscribe(). Our
        * application does not need to distinguish between
        * updates and the original set of vans we found via the
        * SOW query, so we use addOrUpdateVan() to display
        * the new position of vans as well.
        */
        addOrUpdateVan(message);
      }
      else if (cmdName === 'oof') {
        removeVan(message);
      }
    },

    'van_location',         // SOW Topic

    '/status = "ACTIVE"',   // Filter

    // Additional parameters
    {
      batchSize: 100,
      options: 'oof'
    }
  );
};

// Message Handler
const onVanPositionUpdate = message => {
  const cmdName = message.header.command();
  if (cmdName === 'sow' || cmdName === 'p') {
    addOrUpdateVan(message);
  }
  else if (cmdName === 'oof') {
    removeVan(message);
  }
};

const reportVanPosition = async client => {
  // Command object to execute
  const cmd = new Command('sow_and_subscribe');
  cmd.topic('van_location');
  cmd.filter('/status = "ACTIVE"');
  cmd.batchSize(100);
  cmd.options('oof');

  // Execute the command with the above message handler
  return client.execute(cmd, onVanPositionUpdate);
};