How to sort a composite aggregation on the basis of a sub aggregation ? Below is the query

GET myIndex/_search
{
  "from": 0,
  "size": 0,
  "query": {
    "bool": {
      "must": [
        {
          "term": {
            "user_id": {
              "value": "a88604b0",
              "boost": 1
            }
          }
        },
        {
          "term": {
            "entity_status.keyword": {
              "value": "ACTIVE",
              "boost": 1
            }
          }
        }
      ],
      "adjust_pure_negative": true,
      "boost": 1
    }
  },
  "sort": [
    {
      "txn_date": {
        "order": "desc"
      }
    }
  ], 
  "aggs": {
    "my_buckets": {
      "composite": {
        "sources": [
          {
            "group_by": {
              "terms": {
                "field": "category"
              }
            }
          }
        ]
      },
      "aggs": {
        "total_amount": {
          "sum": {
            "field": "amount"
          }
        }
      }
    }
  }
}

I am executing above query but I want the aggregations to be sorted by sub-aggregation
total_amount in descending order. Any modification or other ways to achieve this ?

Here is the result of the above query.

{
  "took" : 3,
  "timed_out" : false,
  "_shards" : {
    "total" : 4,
    "successful" : 4,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 22,
      "relation" : "eq"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "my_buckets" : {
      "after_key" : {
        "group_by" : "Travel"
      },
      "buckets" : [
        {
          "key" : {
            "group_by" : "Bills"
          },
          "doc_count" : 2,
          "total_amount" : {
            "value" : 86710.44
          }
        },
        {
          "key" : {
            "group_by" : "Grocery"
          },
          "doc_count" : 1,
          "total_amount" : {
            "value" : 43355.22
          }
        },
        {
          "key" : {
            "group_by" : "Fashion"
          },
          "doc_count" : 5,
          "total_amount" : {
            "value" : 216776.1
          }
        },
        {
          "key" : {
            "group_by" : "Recharge"
          },
          "doc_count" : 7,
          "total_amount" : {
            "value" : 303486.54
          }
        },
        {
          "key" : {
            "group_by" : "Shopping"
          },
          "doc_count" : 2,
          "total_amount" : {
            "value" : 86710.44
          }
        },
        {
          "key" : {
            "group_by" : "Travel"
          },
          "doc_count" : 5,
          "total_amount" : {
            "value" : 216776.1
          }
        }
      ]
    }
  }
}

I want to have the aggregations to be in sorted manner according to total_amount.

Unfortunately, this is not possible right now. Each source can be ordered in ascending or descending order, but that's pretty much it.

Ordering by a sub-aggregation would require gathering all compound keys and computing the total amount for each bucket, which would be very costly in terms of memory and exactly the opposite of what the composite aggregation is trying to achieve, i.e. a way to paginate through buckets with a very low memory footprint

Also note that if you have a low cardinality of categories (<1000), you don't really need the composite aggregation, you can achieve what you need with the terms aggregation, like this:

{
  ...
  "aggs": {
    "group_by": {
      "terms": {
        "field": "category",
        "size": 100,
        "order": {
          "total_amount": "desc"
        }
      },
      "aggs": {
        "total_amount": {
          "sum": {
            "field": "amount"
          }
        }
      }
    }
  }
}