Pipelined bucket aggregation

[HansPeterSloot]

Hello
I am ingesting logs from different servers into elastic and want to be alerted when a server suddenly stops sending data.

For this I have come up with this aggregation:

GET .xxxt*/_search?size=0
{
  "query": {
    "bool": {
      "filter": [
        {
          "range": {
            "@timestamp": {
              "gte": "now-31d"
            }
          }
        }
      ]
    }
  },
  "aggs": {
    "hostnames": {
      "terms": {
        "field": "server",
        "size": 10
      },
      "aggs": {
        "docs_per_5m": {
          "date_range": {
            "field": "@timestamp",
            "ranges": [
              {
                "to": "now"
              },
              {
                "from": "now-5m"
              }
            ]
          },
          "aggs": {
            "last_bucket": {
              "bucket_sort": {
                "sort": [
                  {
                    "_key": {
                      "order": "desc"
                    }
                  }
                ],
                "size": 1,
                "gap_policy": "insert_zeros"
              }
            }
          }
        }
      }
    }
  }
}

This yields the following output:

  "aggregations": {
    "hostnames": {
...................
      "buckets": [
        {
          "key": "server1",
          "doc_count": 18684883,
          "docs_per_5m": {
            "buckets": [
              {
                "key": "2023-04-14T09:28:04.900Z-*",
                "from": 1681464484900,
                "from_as_string": "2023-04-14T09:28:04.900Z",
                "doc_count": 2128
              }
            ]
          }
        },
        {
          "key": "server2",
          "doc_count": 761066,
          "docs_per_5m": {
            "buckets": [
              {
                ............
                "doc_count": 47
              }
            ]
          }
        },
        {
          "key": "server3",
          "doc_count": 66066,
          "docs_per_5m": {
            "buckets": [
              {
................................
                "doc_count": 7
              }
            ]
          }
        },
        {
          "key": "server4",
          "doc_count": 14,
          "docs_per_5m": {
            "buckets": [
              {
.....................",
                "doc_count": 0
              }
            ]
          }
        }
      ]
    }
  }

But now I only want to have the buckets with "doc_count" : 0
Since I want to use it for a watcher that alerts when there is a server with 0 documents for the last 5 minutes.

I am getting frustrated