We have this Magento 2.1 project running Production mode with "Update on save" mode for all indexes
Our customer manages catalog himself, and reported us some stock issues. Problem was changing quantity for a new product was not working. Process apparently ends fine, regarding the admin message ("Product was saved ok") but stock was not changed. We look in Magento log files, but we don't see any related report there
Our solution, finally, was to change indexes to "Update by schedule" and then force a reindex by console command. This solution, as far as I understand, means there is something wrong with that partial reindex fired when saving a product with "Update on save" index mode. What I am asking is for a confirmation about this... should not this problem be reported in Magento log files?
Assuming there is something wrong with our catalog data, I'd like to trace the problem, so I'd like to know which processes (the complete code trace) are fired after a product save in admin, with "Update on save" index mode. In this case, I suppose catalog_inventory ones are the most relevant
Apart from that, and checking the catalog, we've noticed some configurable products were sharing simple products, which is rare & we are not sure if could be the origin of the problem (although we assume system wouldn't let to do that if that's not allowed). Maybe this could have registered some wrong data in catalog database tables, and causing the problems with partial reindex? If that's the case... Where can I find in database which data is pending to be processed in the next full reindex process?
Finally, it was not an index related issue, but a cache issue https://github.com/magento/magento2/issues/7876
I am going to update question title, to clarify that for future user requests for this bug. About the bounty, I'll give the points for the most complete answer regarding the original question