Windchill System Administrator's Guide

communication transport used for the call (HTTP) is not sufficient to handle a call
in the reverse direction.
The Windchill architecture addresses these concerns by implementing a
lightweight feedback mechanism into the remote method-invoking protocol. This
is done by allowing feedback objects to be sent from the server to the client as part
of the RMI reply marshaling stream. They are received and processed within the
thread performing the call, and they share the same communication connection as
the call, thus remaining logically coupled to the call itself.
When processing a method invocation from a client, the server-side method is
invoked from within the RMI reply marshaling code, allowing the server-side
method to flush feedback objects onto the reply stream at will. The client reply
unmarshaling code recognizes these objects as feedback and calls their init
methods, then continues to wait for the real reply. When starting a long operation,
the server methods can send a GUI component such as a progress bar and cancel
button. The server can periodically flush additional feedback objects that update
this component. The cancel button is programmed to invoke an operation
canceling method in a second thread capable of interrupting the first thread in the
method server.
User Authorization
To authorize access to a given object or operation, the method server must be able
to reliably identify the user performing the action. Various aspects of user
authentication (securely establishing session credentials) have already been
discussed. These things come together in the method server to allow a method to
inquire about the user associated with the current execution thread. This capability
allows applications to implement access control policies, which are described in
detail in the Administering Access Control chapter.
Java RMI does not provide an inherent means of reliably identifying the calling
user. However, the Windchill runtime architecture satisfies this need within the
method server's remote method-invoking interface. Client credentials are
implicitly included with RMI method arguments, and digital signatures are used
to securely associate the RMI thread with an authenticated user name. This
association is established before the target method is called, so method signatures
do not need to contain an extra context or user argument. The information is
retrieved if and when it is needed.
Additionally, the association can be dynamically modified in the course of
executing an operation. For example, it may be necessary to carry out certain steps
of a transaction as a principal other than the user initiating the transaction. To
implement arbitrary authorization delegation schemes, methods are allowed to
push and pop the principal currently associated with the execution thread.
Background Processing
Windchill provides for background processing through the use of background
method queues stored in the database. The queues are tables of method invocation
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