I'm writing an API in WCF 4.6.1. The client(s) will not be written by me, and will not necessarily be in .NET (they could be in any language/platform).
There is a web method which does something that can take a long time, so I want to encourage the client to call it asynchronously. I know that the client can be written to treat the web method as async (threading, etc), but is there a way of "enforcing" the actual web service as an async operation? i.e. Does WSDL have a way to saying "this is an async method"?
Does WSDL have a way to saying "this is an async method"?
No it doesn't. The communication between the client and the service is synchronous even if the client thread does not block while that call is taking place. This is to say the invocation is asynchronous not that the web service method is asynchronous.
If you provide good documentation to say that for a particular operation it's advisable to use a separate thread because the response is slow to be generated you should be OK. Clients need to be built and the integration with the web service tested. The developers will notice the slow response and they will decide if they need to make the call in a non blocking way. Even blocking might be a solution for them, you never know, what you consider slow other might have no issue with.
If you want to "force" clients to not block for the response you could use for example WS-Addressing (I'm assuming here that you are using WCF for a SOAP web service) where your client provides a callback endpoint that you can invoke when the response is ready. This complicates a bit the client since it needs to have a receiving endpoint now. But a client developer might prefer to chose how she invokes the service (in a blocking/non blocking way) as opposed to having to implement the WS-Addressing spec.