So for your first question you cannot aggregate on the message field, it’s a special field, and due to that fact that it’s optimized for searching it’s not possible to aggregate. Specifically it’s analyzed, which lets it function for things you would expect with full text search, you can read the details here if you really want to know all the details Analysis and Analyzers | Elasticsearch: The Definitive Guide [2.x] | Elastic
For your purpose, if you really need to be able to do a show top values on these, you would need to copy the values to another field when it’s ingested, and then aggregate that field.