There are certain tasks that only the master can perform, such as creating a new index or moving shards around the cluster. Since a cluster can have only one master, only one node can ever process cluster-level metadata changes. For 99.9999% of the time, this is never a problem. The queue of metadata changes remains essentially zero.

In some rare clusters, the number of metadata changes occurs faster than the master can process them. This leads to a buildup of pending actions that are queued.

The pending-tasks API will show you what (if any) cluster-level metadata changes are pending in the queue:

GET _cluster/pending_tasks

Usually, the response will look like this:

{
   "tasks": []
}

This means there are no pending tasks. If you have one of the rare clusters that bottlenecks on the master node, your pending task list may look like this:

{
   "tasks": [
      {
         "insert_order": 101,
         "priority": "URGENT",
         "source": "create-index [foo_9], cause [api]",
         "time_in_queue_millis": 86,
         "time_in_queue": "86ms"
      },
      {
         "insert_order": 46,
         "priority": "HIGH",
         "source": "shard-started ([foo_2][1], node[tMTocMvQQgGCkj7QDHl3OA], [P],
         s[INITIALIZING]), reason [after recovery from gateway]",
         "time_in_queue_millis": 842,
         "time_in_queue": "842ms"
      },
      {
         "insert_order": 45,
         "priority": "HIGH",
         "source": "shard-started ([foo_2][0], node[tMTocMvQQgGCkj7QDHl3OA], [P],
         s[INITIALIZING]), reason [after recovery from gateway]",
         "time_in_queue_millis": 858,
         "time_in_queue": "858ms"
      }
  ]
}

You can see that tasks are assigned a priority (URGENT is processed before HIGH, for example), the order it was inserted, how long the action has been queued and what the action is trying to perform. In the preceding list, there is a create-index action and two shard-started actions pending.