Explain Magento 2 stock index

Question

Can somebody explain how stock index works in Magento 2 and what is it used for and how?

If I set it "index on schedule" what exactly happens and how does it affect the shop if stock index is in "reindex needed" status?

Answer 1

"index on schedule" means you need to trigger a reindexing (normaly you do this via crontab) in order to get your indexes up-to-date. I highly reccommend reading https://devdocs.magento.com/guides/v2.3/config-guide/cli/config-cli-subcommands-cron.html

"reindex needed" means your index tables are out of sync. which means that your shop is not displaying accurate data.

If reindexing is set to "index on schedule" and order comes, is the stock index really out of sync for a while (until next cron runs)...? Is there a possibility that somebody sees the product in stock even if it is out of stock?

Yes.