SharePoint 2016 upgrades without down time

Steps for upgrading zero downtime upgrade
1.         Remove half of the web servers (WFE#1) from rotation in the load balancer, or pause the load balancer to stop incoming requests to the servers.
2.         Run the executable file to install the update on each web server that is out of the load-balancing rotation (WFE#1).
3.         Add the updated web server (WFE#1) back into the load-balancing rotation.
4.         Remove the remaining web server (WEB#2 from rotation in the load balancer.
5.         Run the executable file to install the update on each web server that is still out of the load-balancing rotation (WFE#2).
6.         Add the updated web server (WFE#2) back to the load-balancing rotation.
7.         Run the executable file to install the update on the Central Administration server (APP#1).
8.         Run the executable file to install the update on the second application server (APP#2).
9.         Run the executable file to install the update on the first Search server (SRC#1).
10.    Run the executable file to install the update on the second Search server (SRC#2).
11.    Run the executable file to install the update on the first distributed cache server (DCH#1).
12.    Run the executable file to install the update on the second distributed cache server (DCH#2).
At this point in the process, the databases and other components such as settings, features, and site-level data must still be upgraded because the SharePoint Products Configuration Wizard was not run on any of the farm servers. However, the farm should be capable of running in backward compatibility mode for a limited time.

Upgrade Servers

The following illustration shows the steps that upgrade the farm servers to finish the patching process.

1.                    (Optional) Use the Windows PowerShell Upgrade-SPContentDatabase cmdlet to upgrade each content database. This is an optional step, but it will help ensure that all content databases are upgraded first. It has the advantage of enabling some parallelism to reduce upgrade time. If it is not performed, all remaining non-upgraded content databases will be upgraded serially when you run the SharePoint Products Configuration Wizard to upgrade the farm servers.
2.           Run the SharePoint Products Configuration Wizard on the Central Administration server (APP#1).
3.                  The SharePoint Products Configuration Wizard also starts an immediate upgrade of the configuration database and all other databases that are not already upgraded. Because it is likely that the content databases are the only databases that have already been upgraded, as described in the previous step, all the service application databases are also upgraded in this step.
4.           Run the SharePoint Products Configuration Wizard on the remaining application server (APP#2).
5.           Remove the first web server (WFE#1) from rotation in the load balancer, or pause the load balancer to stop incoming requests to the servers.
6.           Run the SharePoint Products Configuration Wizard on the web servers (WFE#1)
7.           Add the upgraded web server (WFE#1) back into rotation in the load balancer
8.           Remove the first web server (WFE#2) from rotation in the load balancer, or pause the load balancer to stop incoming requests to the servers.
9.           Run the SharePoint Products Configuration Wizard on the web servers (WFE#2)
10.       Add the upgraded web server (WFE#2) back into rotation in the load balancer
11.       Run the SharePoint Products Configuration Wizard on all remaining servers, one at a time (DCH#1, DCH#2, SRC#1, SRC#2)


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