Reindex while docs are added / updated in source index? Online reindex?

Elastic Search
1

From the docs
I don't see any guarantees about what happens with new and updated docs in the source index. Might newly-added docs get left out of the reindex process? Might an updated doc in the rebuilt index not show the latest updates?

Is there a concept of online index rebuild?

If not, is there a prescribed way of simulating an online index rebuild?

I think the problem for me is that with the index compatibility, I always have to make sure indexes stay somewhat up-to-date by rebuilding them. But if the index is being updated, then my application needs to block startup until it's rebuilt. The rebuilding process actually slows down customer upgrades quite a bit.

Note: I am using the terms rebuild and reindex a bit interchangeably

2

Every reindex operation is based on a snapshot of the data when the reindex operation is started, so any additions or updates/deletions made after the operation started will not be included. There is no automatic way to do what you are requesting so you need to design the process yourself and some downtime is likely required at some point.