Proper use of HTTP status codes in a "validation" server

Question

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.

Answer 1

Status code 422 ("Unprocessable Entity") sounds close enough:

"The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions."