1. A system to support session management in a distributed transactional service system, comprising: a first distributed transactional server and a second distributed transactional server, wherein the first distributed transactional server provides a first distributed transactional service, and wherein the second distributed transactional server provides a second distributed transactional service; and an affinity client that operates to receive a service request from a user application and invoke the first distributed transactional service provided by the first distributed transactional server; wherein the first distributed transactional server operates to: create and maintain a server-side session, including a first session context, for use by the user application when the first distributed transactional service is invoked by the affinity client as part of a session; determine a first affinity scope associated with the server-side session, wherein the first affinity scope limits the level of access to the first service across the first distributed transactional server, and wherein the first affinity scope defines, and includes, a first routing scope within the distributed transactional service system into which subsequent service requests within the server-side session, and from the user application to the affinity client, are routed; communicate the first affinity scope to the affinity client; and forward the service request to the second distributed transactional service provided by the second distributed transactional server; wherein, in response to receiving the forwarded service request from the first distributed transactional server, the second distributed transactional server operates to: create and maintain a second session context for the server-side session associated with the second distributed transactional service; determine a second affinity scope, wherein the second affinity scope limits the level of access to the second distributed transactional service at least within the limits of the first affinity scope; and communicate the second affinity scope to the affinity client; and wherein the affinity client operates to maintain the first affinity scope associated with the session, for use by the user application in dispatching the subsequent requests into the routing scope, and maintain the second affinity scope associated with the session.