How to update Craft CMS/plugins without downtime? On our production server, we have strict SLAs concerning when our site can be down.  I've noticed that when I update Craft CMS 3 (using "./craft update all" on the server), the site will go down for anywhere from 1-30 seconds.  Once the update is completed, everything returns to working.  Has anyone seen this before?  How can I update Craft without downtime?
I think the best solution would be to integrate a CI with "Zero downtime deployment". 

You may have a look at this article:
https://craftquest.io/articles/what-are-zero-downtime-atomic-deployments

Or have a look at this deploy plugin:
https://github.com/elfacht/craft-deploy

Or just google for "zero downtime deployment".