Among the data my application sends to a third-party SOA server are complex XMLs. The server owner does provide the XML schemas (.xsd) and, since the server rejects invalid XMLs with a meaningless message, I need to validate them locally before sending. I could use a stand-alone XML schema validator but they are slow, mainly because of the time required to parse the schema files. So I wrote my own schema validator (in Java, if that matters) in the form of an HTTP Server which caches the already parsed schemas. The problem is: many things can go wrong in the course of the validation process. Other than unexpected exceptions and successful validation: the server may not find the schema file specified the file specified may not be a valid schema file the XML is invalid against the schema file Since it's an HTTP Server I'd like to provide the client with meaningful status codes. Should the server answer with a 400 error (Bad request) for all the above cases? Or they have nothing to do with HTTP and it should answer 200 with a message in the body? Any other suggestion? Update: the main application is written in Ruby, which doesn't have a good xml schema validation library, so a separate validation server is not over-engineering.