SharePoint Solution Deploy, retract, upgrade – What causes your SharePoint farm to go offline IISRESET (web server outage).

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.

Ok, lets take a closer look:
Deploying a solution

Deploying a new solution or in other words install-spsolution stops all Web Application app pools and then starts them all at the end. All your sites will display a Service Unavailable HTTP Error 503. The service is unavailable.

Initially, the package manifest.xml is checked for its content & destination locations. The files specified in the manifest.xml are copied to the locations. After solution files are copied to the target servers, a configuration reset is scheduled for all front-end Web servers; the reset then deploys the files and restarts Microsoft Internet Information Services (IIS). Farm administrators can specify when this occurs (a timer job).
Reference: http://msdn.microsoft.com/en-us/library/aa544500.aspx

Retracting a solution

On each front-end Web server, the following occurs:

– Internet Information Services (IIS) is disabled.
– Files are removed from the system.
– IIS is re-enabled and Microsoft SharePoint Foundation is reloaded when a user browses to a page.

– Farm administrators can specify the time when the retraction takes place.

Reference: http://msdn.microsoft.com/en-us/library/aa543958.aspx

Upgrading a solution

There are two ways to do an upgrade of a solution. i.e. Replacement and Update. Both inolve an IISRESET.

Replacements are essentially a retract and optionally removal of a WSP. Then the new version is added and deployed. Causing an outage (twice!).
Updates are essentially a deployment with the same file name but different GUID. SharePoint detects this and retracts the old solution and deploys the new one..causing an outage.

Reference: http://msdn.microsoft.com/en-us/library/aa543659.aspx

Please note that there is a -local option when deploying a solution, but according to MSDN, this is to be used for troubleshooting purposes.
Reference: http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.administration.spsolution.deploylocal.aspx

Updating WSP solutions with the same file name and same GUID causes an outage. It stops all Web Application App Pools and starts them again after it updates the files. During that time, all your sites will display a Service Unavailable HTTP Error 503. The service is unavailable.