Composite Aggregation and Sorting with a non source

Elastic Stack Elasticsearch
2

The composite aggregation is a way to paginate over all buckets of a complex aggregation. It doesn't allow custom sort. If you want a list of messages collapsed by user and sorted by most recent you can use field collapsing:
https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-collapse.html

GET my_index/_search
{
    "collapse" : {
        "field" : "user", 
    },
    "sort": [ {"timestamp": "desc" } ]
}

The query will return the most recent message by user and you can paginate over the results using from.