You may have had questions or have had others ask you:
Does a WSP deployment (Install-SPSolution) cause a farm outage?
Does a WSP deploy perform an IISRESET?
We are upgrading (Update-SPSolution) a solution, will it cause an outage?
What causes an outage when working with SharePoint WSP files?
Over the years, I have had many discussions / meetings with clients & colleagues about this and I would like to publish my findings and the links that I have had with me for a while. Let’s clarify this ambiguity:
Here are answers straight up in three scenarios:
1. Deploying a solution
Do you need to prepare for a SharePoint WFE & Farm outage / IISRESET?: Yes, it stops all Web Application app pools and then starts them all at the end.
2. Retracting a solution
Do you need to prepare for a SharePoint WFE & Farm outage / IISRESET?: Yes
Upgrading a solution
Do you need to prepare for a SharePoint WFE & Farm outage / IISRESET?: Yes, it stops all Web Application app pools and then starts them all at the end.