The pending cluster tasks API returns a list of any cluster-level changes (e.g. create index, update mapping, allocate or fail shard) which have not yet been executed.
This API returns a list of any pending updates to the cluster state. These are distinct from the tasks reported by the Task Management API which include periodic tasks and tasks initiated by the user, such as node stats, search queries, or create index requests. However, if a user-initiated task such as a create index command causes a cluster state update, the activity of this task might be reported by both task api and pending cluster tasks API.
GET /_cluster/pending_tasks
Usually this will return an empty list as cluster-level changes are usually fast. However if there are tasks queued up, the output will look something 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 shard_store]",
"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 shard_store]",
"time_in_queue_millis": 858,
"time_in_queue": "858ms"
}
]
}