$Id: README,v 1.3 1996/01/11 15:52:43 omfadmin Exp $

This is version 2 of RemoteCall (11 Jan 1996).

In this directory:

README                  This file
NEWS                    Information on updates
RemoteCall.py           Sending calls from a client to a server
RemoteCallServer.py     Class for implementing python servers
RemoteCalldefs.py       Common definitions
Agent.py                Representative in client process for a remote server object

test/exec_client.py     Client using RemoteCall to do some maths.
test/agent_client.py    Client using Agents to do some simple interactions with
                       a pretty dumb server.
test/agent_server.py    The aforementioned server
test/agent_serverdefs.py Definitions for the server


DESCRIPTION
The RemoteCall package is a number of classes for writing client/server
applications in Python. With RemoteCall you can send

       *  arbitrary expressions
       *  function calls (i.e. anything you can 'apply' arguments on)
       *  attribute assignments (setattr(obj, attr, value))

over a socket connection. Values transferred across the wire are marshalled
using the 'pickle' module, which means not only builtin data types (integers,
lists, tuples, etc) can be transferred, but also user defined classes.

After an incomming value is unpickled, but before it is further processed
(e.g. sent as arguments to a function) one can optionally process it with
a 'before' method. Likewise, before pickling the results and sending them back
an after method may massage it. The main reason for this facility is to
implement 'Agents'.

AGENTS
Agents are objects that acts as local representatives of remote objects.
Agents are also known in other systems as proxys, ambassadors, and other
names. Agents are useful for objects that use facilities local to the server
that might not be available at the client (e.g. they might interface to
an application with python embedded).

By installing agent support, and telling the RemoteCallServer object what
objects and/or classes not to send over the wire, all such objects appearing
in results from a RemoteCall will automatically be converted to agents.
An agent sent as an argument is likewise translated automatically to the
corresponding server object.

WHAT ABOUT EXCEPTIONS?
Server side exceptions are reraised on the client side with the original
value. There are some problems with exceptions though. Currently, the client
searches for an exception with the same name as the string value of the
exception. If this fails, it will be impossible to catch the exception.

PROTOCOL
A remote request is a 2-tuple where the first element indicates whether
the second element is to be considered a statement, an expression, a call,
or an assignment.

If the request is an expression or a call, it is evaluated and if the evaluation
succeeds the tuple '(None, result)' is returned. In the case an exception is
raised the tuple '(exc_type, exc_value)' is returned instead.

If the request is a statement or an assignment it is executed and if
successfull the tuple '(None, None)' is returned. In the case an exception is
raised the tuple '(exc_type, exc_value)' is returned instead.

THE SIMPLE EXAMPLE
The simple example included demonstrates a client and server using agents.
By commenting out the code

       server.SendAsAgent(agent_serverdefs.StupidServer)

in the main() function in agent_server.py, the server objects will be
transferred to the client process instead of staying in the server.

To run the example program start the server with python, e.g.

       python agent_server.py unix  # Unix sockets used
       python agent_server.py tcp   # TCP sockets used

Note that the python interpreter needs Tkinter support (or X, but in this
case you have to do some changes to the code).

The server will print something like "OK, waiting on 7979" (TCP) or
"OK, waiting on /tmp/remcall" (Unix). Then start the client with the
address as argument, or if the server is running on a remote node, with
the host name and port address, e.g.

       python agent_client.py unix /tmp/remcall
       python agent_client.py tcp 1376, or
       python agent_client.py pollux 1376

BUGS
If you have any problems, questions, suggestions, or if you improved this code
yourself, please don't hesitate to contact me at <[email protected]>.
I am often busy, but I'll do my best to be of help ;^)

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