1. When initiating a search index re-indexing, we should actually initiate the creation of a new, temporary index in the background (on cron runs).
  2. While the (re)indexing happens in the background, the old index will still be used so that there is no "downtime".
  3. Once the new index is done, we replace the old one with it.
  4. Once the old index is replaced with the new and no errors have occurred, we delete the old index.
GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/1337
GitHub Issue #: 
1337