Starting from Version 6.1 Portal Supports Web Services for Remote Portlets(WSRP 2.0). By using this one Portal can provide a portlet as WSRP service and other portal can integrate the WSRP service as remote portlets for their users.
Take a look at the diagram that defines the basic WSRP architecture.
The WSRP architecture can be divided into two basic parts one is producer portal, this the portal which will display your portlet to user. And consumer portal, this the portal where the portal code is deployed and executed.
The way it works is in the consumer portlet you mark particular portlet as WSRP provider, once you do that the WebSphere portal will make that portlet available as web service in addition to the normal way of adding portlet to page. Now on your consumer portal first you create a webservice connection to producer Portal server and once you create the connection, consumer portlet will display what all portlets are available as web service on producer, you choose one of them and portal will create a proxy for that portlet on the consumer. Once this proxy is created you can add it to any portal page as normal portlet.
Best part about this implementation is that you dont have to do any coding to either produce or consume portlet, portal takes care of converting your regular portlet into web service and also has generic stub portlet that can consume the web service.
Important NOteDepending on the API version of the provided portlet, the WSRP Producer provides the portlet in different WSRP protocol versions. JSR 168 portlets and IBM API portlets (also called IBM legacy portlets) are provided via both WSRP V 1.0 and V 2.0. JSR 286 portlets can be provided via WSRP V 2.0 only. The following table shows which provided portlets on a WSRP