A quick but interesting question came out this week: “if I see an error in the Event Viewer of the Application Tier, is it going to break a TFS upgrade?”
Generally speaking – no. The errors you see in the Event Viewer are client-side logs reported to the server. You might see an error on a client not able to connect, another one because of a non-existent user or value in the Work Item query, or more critically an error because you cannot contact your data tier.
It is the data tier which contains all the meat. All the data is stored over there and, if you look at the Configuration logs after an upgrade, the big show is there.
Of course there is a correlation between the data tier’s schema and the application tier version – everything is handled by the same binaries.