[TSDB] The routing path is not detected automatically when using a time series index.

[gmarouli]

Problem Description

When a user creates a time series data stream, they do not have to specify the index.routing_path because it can be inferred from the mapping, given the mapping is specifying dimensions. However, the logic that infers this is located in a DataStreamIndexSettingsProvider which entails that it's only applied when we create a time series data streams and not when we create a stand alone time series index.

Steps to Reproduce

PUT my-ts-index
{
 "settings": {
      "index.mode": "time_series"
    },
    "mappings": {
              "properties": {
                "@timestamp": {
                  "type": "date"
                },
                "dimension": {
                  "type": "keyword",
                  "time_series_dimension": true
                },
                "counter": {
                  "type": "double",
                  "time_series_metric": "counter"
                }
              }
            }
    }
}

The above should work but now it throws an illegal argument exception, [index.mode=time_series] requires a non-empty [index.routing_path].

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[martijnvg]

If I recall correctly, when this logic was added, the idea was that automatically detecting routing path for data streams was good enough. Given that using tsdb without data streams is not easy (mostly because users would need to manage the start and end time setting manually). Hence the routing path generation logic was never added for plain index creation.