DWR

Although it is not impossible to invoke DWR services from two different domains, it is quite cumbersome.
There are two ways to do it AFAIK:
1. if you have one remote server that is different from the domain that the page is running on, then you can set an init-param in your web.xml, specifying an overridePath
    This will be picked up by DefaultRemoter. But you can only specify it once, so you cannot invoke DWR services from two different servers.

2. by changing the _path of your service after loading the DWR interface stub in your html page
   For example, consider a service "MooService" running on host-2 used on a page hosted on host-1 (the context root of the MooService is the server root)

<script src="http://host-2/dwr/interface/MooService.js"></script>
MooService._path="http://host-2/dwr"; // was "/dwr"

That is cumbersome, since you have to do that for every service. It is also quite easy to improve.

I've attached a new version of InterfaceHandler based on DWR-2.0.5 (I've checked that DWR-3.0.M1.RC2 has the same issue)
By default, it does nothing special (don't want to break an existing installations)

But when your web.xml specifies the following init-param:

<init-param>
    <param-name>absoluteUrlPaths</param-name>
    <param-value>true</param-value>
</init-param>

.. then it will output full urls in the generated interface scripts, so they will always be correct.

N.B. this doesn't violate cross-site scripting constraints since the interface stub that invokes the service is loaded from the same server as the DWR services that it invokes.