Beginner question: Skipping the filtering of id into [@metadata][_id]

[pyrovoice]

I followed the following tutorial: https://www.elastic.co/blog/how-to-keep-elasticsearch-synchronized-with-a-relational-database-using-logstash

The configuration file for the pipeline contains the following:

    input {
      jdbc {
        jdbc_driver_library => "<path>/mysql-connector-java-8.0.16.jar"
        jdbc_driver_class => "com.mysql.jdbc.Driver"
        jdbc_connection_string => "jdbc:mysql://<MySQL host>:3306/es_db"
        jdbc_user => <my username>
        jdbc_password => <my password>
        jdbc_paging_enabled => true
        tracking_column => "unix_ts_in_secs"
        use_column_value => true
        tracking_column_type => "numeric"
        schedule => "*/5 * * * * *"
        statement => "SELECT *, UNIX_TIMESTAMP(modification_time) AS unix_ts_in_secs FROM es_table WHERE (UNIX_TIMESTAMP(modification_time) > :sql_last_value AND modification_time < NOW()) ORDER BY modification_time ASC"
      }
    }
    filter {
      mutate {
        copy => { "id" => "[@metadata][_id]"}
        remove_field => ["id", "@version", "unix_ts_in_secs"]
      }
    }
    output {
      # stdout { codec =>  "rubydebug"}
      elasticsearch {
          index => "rdbms_sync_idx"
          document_id => "%{[@metadata][_id]}"
      }
    }

Specifically for the filter. If I got this right, we want each database entry to have a unique document_id so that the data will be correctly updated.

• Does passing the data to metadata._id achieves anything? We simply pass this data again to document_id in the output, so why not passing the ID directly?
• In tables with multiple primary keys, it is not possible to have a single ID table. My solution was to add all columns forming the primary key into document_id (document_id => "%{col1}%{col2}...") Is this the correct approach? The data seems to get updated correctly

Thank you for your help.