How external business rules processing capability can be built with Xojo? Is there is any supportive rule engine available in the market that Xojo supports with?
Nothing built in
There are some that you could integrate with - Decisions (decisions.com)
You can call their API as a restful service
There may be others but I haven’t kept up to date in that field since its not really required in the role I’m in now.
Its been about 10+ years since I last did anything with Jess but you might be able to run it in a shell & interact with it that way.
One thing to consider is whether a business rue engine is really suitable
Some good guidelines to keep in mind http://www.jessrules.com/guidelines.shtml
EDIT : quick google suggest there are others that can be accessed as REST api’s
http://wso2.com/products/business-rules-server/
http://blogs.technet.com/b/wikininjas/archive/2013/01/27/microsoft-offers-rest-support-in-biztalk.aspx