Solr server instance reindex in parallel whille other solr responding searches

I can think of a lot of approaches that might work, but unfortunately none which would work without some custom code. So, I hope that’s OK for you.
Probably the simplest of those would be to implement an event listener for \Solarium\Core\Event\PreExecuteRequest events and change the request URL according to the type of request: the old core for searches, the new one for all other operations. Then just queue all items for re-indexing and re-index them. Once that is finished, you can remove/disable your event listener and then (for instance) use the Solr SWAP command to swap the place of the two cores – or just edit the Search API server settings to point the server to the new core. Then, delete the old core.

Hi, and thanks for your reply, that already show me the limittions on the issue, that I had no clear idea.

Unfortunately, Implementing custom code is not an option, at least for the moment.
So in that case, avoid zero index lost, is not an option for the moment in that case.

But about reducing the lmpact as much as possible, how would impact having the second core being indexed to the site?
Would the searches retrieve content from both cores? and having the risk of showing duplicated content ?

In the other hand, what it´s the safest approach, or the recomendation on drupal 9 to handle the reindexation of a site after the update of the solr core to the new version generated.
Just clear all the index and reindex ?
Is there a way to minize the impact or update only content affected by the new configuration version of solr core, so there is no need to clear the whole indexed content ?

Thanks.
regards.

Would the searches retrieve content from both cores? and having the risk of showing duplicated content ?

No, that will never happen. A search will always only go to a single core, unless you do some very complicated setup to achieve the opposite.
So, if you manage to index to a new core while still searching from the old one, there’s no risk of duplicates from that.

For the rest, I think I’m gonna move this issue to the Search API Solr issue queue, as the experts are sitting there.