Drupal - Do I need a cron task for processing a queue?

When Drupal runs cron tasks, it automatically handles any cron queue defined from modules, in drupal_cron_run(); first hook_cron() implementations are invoked, and then the cron queues are emptied.

Inmplementing hook_cronapi(), you can add an entry for another function handling the cron queue of your module.

function mymodule_cronapi($op, $job = NULL) {
  $items = array();

  $items['queue_users_for_synch'] = array(
    'description' => 'Queue all user accounts for synching.',
    'rule' => '0 3 * * *', // Run this job every day at 3am.
    'callback' => 'mymodule_queue_all_users_for_synching',
  );

  $items['clean_queue'] = array(
    'description' => 'Clean the queue for the user synching.',
    'rule' => '0 4 * * *', // Run this job every day at 4 AM.
    'callback' => 'mymodule_clean_queue',
  );

  return $items;
}

function mymodule_clean_queue() {
  $queues = module_invoke('mymodule', 'cron_queue_info');
  drupal_alter('cron_queue_info', $queues);

  // Make sure every queue exists. There is no harm in trying to recreate an
  // existing queue.
  foreach ($queues as $queue_name => $info) {
    DrupalQueue::get($queue_name)->createQueue();
  }

  foreach ($queues as $queue_name => $info) {
    $function = $info['worker callback'];
    $end = time() + (isset($info['time']) ? $info['time'] : 15);
    $queue = DrupalQueue::get($queue_name);
    while (time() < $end && ($item = $queue->claimItem())) {
      $function($item->data);
      $queue->deleteItem($item);
    }
  }
}

The alternative is letting Drupal handle the cron queue for you, but that happens when Drupal cron tasks are executed. If you want to empty the cron queue of your module more frequently, you can only add a new cron task handled by the Elysia Cron module.

The Elysia Cron module handles the cron queues in elysia_cron_run(); this function is being invoked from elysia_cron_cron() (an implementation of hook_cron()), drush_elysia_cron_run_wrapper() (a Drush command callback), and from its own cron.php. If you followed the instructions in the INSTALL.txt file (in particular in "STEP B: CHANGE SYSTEM CRONTAB (OPTIONAL)"), and replaced any invocation of http://example.com/cron.php with http://example.com/sites/all/modules/elysia_cron/cron.php, the Elysia Cron module should be already handling the cron queues. The code I suggested could be used to speed up handling the cron queues used from your module, if there is effectively the need of doing so.

// This code is part of the code executed from modules/elysia_cron/cron.php.
define('DRUPAL_ROOT', getcwd());

include_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_override_server_variables(array(
  'SCRIPT_NAME' => '/cron.php',
));
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

if (!isset($_GET['cron_key']) || variable_get('cron_key', 'drupal') != $_GET['cron_key']) {
  watchdog('cron', 'Cron could not run because an invalid key was used.', array(), WATCHDOG_NOTICE);
  drupal_access_denied();
}
elseif (variable_get('maintenance_mode', 0)) {
  watchdog('cron', 'Cron could not run because the site is in maintenance mode.', array(), WATCHDOG_NOTICE);
  drupal_access_denied();
}
else {
  if (function_exists('elysia_cron_run')) {
    elysia_cron_run();
  }
  else {
    drupal_cron_run();
  }
}

The queue will be populated via the Elysia cronapi hook at the set time.

However, the queue will be processed whenever the standard Drupal cron run occurs.

See this worker callback processing snippet at the end of core: drupal_cron_run

 foreach ($queues as $queue_name => $info) {
    $function = $info['worker callback'];
    $end = time() + (isset($info['time']) ? $info['time'] : 15);
    $queue = DrupalQueue::get($queue_name);
    while (time() < $end && ($item = $queue->claimItem())) {
      $function($item->data);
      $queue->deleteItem($item);
    }
  }