Synthetic _source is Generally Available only for TSDB indices (indices that have index.mode set to time_series ). For other indices synthetic _source is in technical preview. Features in technical preview may be changed or removed in a future release. Elastic will apply best effort to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.
I have a question in... on expected Discover behavior with _source disabled but feel free to open a ticket.
The fields option returns values in the way that matches how Elasticsearch indexes them. For standard fields, this means that the fields option looks in _source to find the values, then parses and formats them using the mappings. Selected fields that can’t be found in _source are skipped.
Huh.... so that is totally not what I expected in Discover... When I see the word fields I always think about the actual fields, not the _source... I (and I suspect others) did not read the docs closely I always thought the values were actually pulled from the doc_values etc..So I guess I am hearing that Discover is working as designed, but it is pretty much not of much value if _source is disabled? I guess I was basically expecting Discover to work like it does with Synthetic _source
Right, I don't think Discover supports _source being disabled. We should definitely improve the experience to at least let you know something is wrong
Read fields from _source
When enabled will load documents directly from _source. This is soon going to be deprecated. When disabled, will retrieve fields via the new Fields API in the high-level search service.
Reading this I assumed that the Fields API is unrelated to the presence of the _source field, which seems not to be the case from what you shared.