If so, what are the parameters for the fail-over? Timeout or 50X/40X response?
Yes, this can be done with a message proxy.
You would create a message proxy and configure a Secondary Connection:
The fail over triggers when the proxy receives a Error status code 4XX or 5XX.
Clip of the Virtualize user guide in regards to this feature:
Can you be more specific on the following statement
"if a virtual asset does not handle a use case" Does this mean if a match is not found on the primary endpoint the proxy would then attempt to use the secondary option? I understand that if the primary option is offline the proxy would fail over. But, if it is simply a matter of not finding an exact matching response (IOW The book ID was not found in the data source) would the proxy try the secondary?
I did some research and based on what I encountered, the Virtual Asset will return an HTTP 400 when it can't find a match. That, as mentioned above, will cause a request to the secondary endpoint.
It will fail over for any http status code that is 400 or greater. So a 404 would fail over to the virtual asset and so would a 500.