Upgrading Tomcat version without breaking existing applications

Question

I have Tomcat 8.5.15 installed and I need to upgrade it to 8.5.16 to match all the other environments. From what I read, in place upgrade can be performed with monor version updates as long as the config files are updated with the correct parameters.

Tomcat documentation states:

When upgrading instances of Apache Tomcat from one version of Tomcat 8 to 
another, particularly when using separate locations for $CATALINA_HOME and 
$CATALINA_BASE, it is necessary to ensure that any changes in the 
configuration files such as new attributes and changes to defaults are 
applied as part of the upgrade. To assist with the identification of these 
changes, the form below may be used to view the differences between the 
configuration files in different versions of Tomcat 8.

I ran the tool provided by tomcat to look at the differences and does not look like it would be a problem, but my concern is will the web apps currently hosted on Tomcat break during the upgrade process?

In particular I am concerned about APEX ORDS application

Answer 1

will the web apps currently hosted on Tomcat break during the upgrade process?

There's always a chance for an application to have an unknown dependency on behavior of a specific version of the platform. This will be unknown until that behavior changes - be it through a regression bug or because this behavior was unspecified and the choice/implementation changed. It's still unspecified, but different.

That being said, you shouldn't rely on an internet forum post that tells you your application (whichever it is) is compatible with a random version of your platform. The odds are that it will be compatible, but in the end you'll have to validate that for yourself.

For sure, such an update must be done while Tomcat is down and not during its uptime. If you really rely on the availability of your app, I suggest you restore your latest backup to a new machine, update it and test if it's still working. This has the nice side effect that you're also proving that you are able to restore your backup. The testing, however, is inevitable. I'm not sure how to pronounce this. It's either "You must do it" or "You must do it".