Formatted 14 January 1998.
Copyright (C) 1995 Xerox Corporation
All Rights Reserved.
This tutorial will show how to use the ILU system with the programming language Python, both as a way of developing software libraries, and as a way of building distributed systems. In an extended example, we'll build an ILU module that implements a simple four-function calculator, capable of addition, subtraction, multiplication, and division. It will signal an error if the user attempts to divide by zero. The example demonstrates how to specify the interface for the module; how to implement the module in Python; how to use that implementation as a simple library; how to provide the module as a remote service; how to write a client of that remote service; and how to use subtyping to extend an object type and provide different versions of a module. We'll also demonstrate how to use OMG IDL with ILU, and discuss the notion of network garbage collection.
Each of the programs and files referenced in this tutorial is available as a complete program in a separate appendix to this document; parts of programs are quoted in the text of the tutorial.
Our first task is to specify more exactly what it is we're trying to provide. A typical four-function calculator lets a user enter a value, then press an operation key, either +, -, /, or *, then enter another number, then press = to actually have the operation happen. There's usually a CLEAR button to press to reset the state of the calculator. We want to provide something like that.
We'll recast this a bit more formally as the interface of our module; that is, the way the module will appear to clients of its functionality. The interface typically describes a number of function calls which can be made into the module, listing their arguments and return types, and describing their effects. ILU uses object-oriented interfaces, in which the functions in the interface are grouped into sets, each of which applies to an object type. These functions are called methods.
For example, we can think of the calculator as an object type, with several methods: Add, Subtract, Multiply, Divide, Clear, etc. ILU provides a standard notation to write this down with, called ISL (which stands for "Interface Specification Language"). ISL is a declarative language which can be processed by computer programs. It allows you to define object types (with methods), other non-object types, exceptions, and constants.
The interface for our calculator would be written in ISL as:
INTERFACE Tutorial;
EXCEPTION DivideByZero;
TYPE Calculator = OBJECT
METHODS
SetValue (v : REAL),
GetValue () : REAL,
Add (v : REAL),
Subtract (v : REAL),
Multiply (v : REAL),
Divide (v : REAL) RAISES DivideByZero END
END;
This defines an interface Tutorial, an exception DivideByZero,
and an object type Calculator. Let's consider these one by one.
The interface, Tutorial, is a way of grouping a number of type
and exception definitions. This is important to prevent collisions
between names defined by one group and names defined by another group.
For example, suppose two different people had defined two different
object types, with different methods, but both called Calculator!
It would be impossible to tell which calculator was meant. By
defining the Calculator object type within the scope of the
Tutorial interface, this confusion can be avoided.
The exception, DivideByZero, is a formal name for a particular
kind of error, division by zero. Exceptions in ILU can specify
an exception-value type, as well, which means that real errors
of that kind have a value of the exception-value type associated with them.
This allows the error to contain useful information about why it might
have come about. However, DivideByZero is a simple exception,
and has no exception-value type defined. We should note that the full
name of this exception is Tutorial.DivideByZero, but for this
tutorial we'll simply call our exceptions and types by their short name.
The object type, Calculator (again, really Tutorial.Calculator),
is a set of six methods. Two of those methods, SetValue and
GetValue, allow us to enter a number into the calculator object,
and "read" the number. Note that SetValue takes a single
argument, v, of type REAL. REAL is a
built-in ISL type, denoting a 64-bit floating point number.
Built-in ISL types are things like INTEGER (32-bit
signed integer), BYTE (8-bit unsigned byte), and CHARACTER
(16-bit Unicode character). Other more complicated types are
built up from these simple types using ISL type constructors,
such as SEQUENCE OF, RECORD, or ARRAY OF.
Note also that SetValue does not return a value,
and neither do Add, Subtract, Multiply,
or Divide. Rather,
when you want to see what the current value of the calculator
is, you must call GetValue, a method which has no arguments,
but which returns a REAL value, which is the value of the
calculator object. This is an arbitrary decision on our part;
we could have written the interface differently, say as
TYPE NotOurCalculator = OBJECT
METHODS
SetValue () : REAL,
Add (v : REAL) : REAL,
Subtract (v : REAL) : REAL,
Multiply (v : REAL) : REAL,
Divide (v : REAL) : REAL RAISES DivideByZero END
END;
-- but we didn't.
Our list of methods on Calculator is bracketed by the two
keywords METHODS and END, and the elements are separated
from each other by commas. This is pretty standard in ISL:
elements of a list are separated by commas; the keyword END
is used when an explicit list-end marker is needed (but not when it's
not necessary, as in the list of arguments to a method); the list often
begins with some keyword, like METHODS.
The raises clause (the list of exceptions which a method
might raise) of the method Divide provides another example
of a list, this time with only one member, introduced by the keyword
RAISES.
Another standard
feature of ISL is separating a name, like v,
from a type, like REAL, with a colon character. For example,
constants are defined with syntax like
CONSTANT Zero : INTEGER = 0;Definitions, of interface, types, constants, and exceptions, are terminated with a semicolon.
We should expand our interface a bit by adding more documentation on what our methods actually do. We can do this with the docstring feature of ISL, which allows the user to add arbitrary text to object type definitions and method definitions. Using this, we can write
INTERFACE Tutorial;
EXCEPTION DivideByZero
"this error is signalled if the client of the Calculator calls
the Divide method with a value of 0";
TYPE Calculator = OBJECT
COLLECTIBLE
DOCUMENTATION "4-function calculator"
METHODS
SetValue (v : REAL) "Set the value of the calculator to `v'",
GetValue () : REAL "Return the value of the calculator",
Add (v : REAL) "Adds `v' to the calculator's value",
Subtract (v : REAL) "Subtracts `v' from the calculator's value",
Multiply (v : REAL) "Multiplies the calculator's value by `v'",
Divide (v : REAL) RAISES DivideByZero END
"Divides the calculator's value by `v'"
END;
Note that we can use the DOCUMENTATION keyword on object types
to add documentation about the object type, and can simply add documentation
strings to the end of exception and method definitions. These docstrings
are passed on to the Python docstring system, so that they are available
at runtime from Python. Documentation
strings cannot currently be used for non-object types.
ILU provides a program, islscan, which can be used
to check the syntax of an ISL specification. islscan
parses the specification and summarizes it to standard output:
% islscan Tutorial.isl
Interface "Tutorial", imports "ilu"
{defined on line 1
of file /tmp/tutorial/Tutorial.isl (Fri Jan 27 09:41:12 1995)}
Types:
real {<built-in>, referenced on 10 11 12 13 14 15}
Classes:
Calculator {defined on line 17}
methods:
SetValue (v : real); {defined 10, id 1}
"Set the value of the calculator to `v'"
GetValue () : real; {defined 11, id 2}
"Return the value of the calculator"
Add (v : real); {defined 12, id 3}
"Adds `v' to the calculator's value"
Subtract (v : real); {defined 13, id 4}
"Subtracts `v' from the calculator's value"
Multiply (v : real); {defined 14, id 5}
"Multiplies the calculator's value by `v'"
Divide (v : real) {DivideByZero}; {defined 16, id 6}
"Divides the calculator's value by `v'"
documentation:
"4-function calculator"
unique id: ilu:cigqcW09P1FF98gYVOhf5XxGf15
Exceptions:
DivideByZero {defined on line 5, refs 15}
%
islscan simply lists the types defined in the interface, separating
out object types (which it calls "classes"), the exceptions, and
the constants. Note that for the Calculator object type,
it also lists something called its unique id. This is a 160-bit
number (expressed in base 64) that ILU assigns automatically
to every type, as a way of distinguishing them. While
it might interesting to know that it exists (:-),
the ILU user never has know what it is; islscan
supplies it for the convenience of the ILU implementors, who
sometimes do have to know it.
After we've defined an interface, we then need to supply an implementation of our module. Implementations can be done in any language supported by ILU. Which language you choose often depends on what sort of operations have to be performed in implementing the specific functions of the module. Different languages have specific advantages and disadvantages in different areas. Another consideration is whether you wish to use the implementation mainly as a library, in which case it should probably be done in the same language as the rest of your applications, or mainly as a remote service, in which case the specific implementation language is less important.
We'll demonstrate an implementation of the Calculator
object type in Python, which is one of the most capable
of all the ILU-supported languages. This is just a matter
of defining a Python class, corresponding to the Tutorial.Calculator type. Before we do that,
though, we'll explain how the names and signatures of the Python functions
are arrived at.
For every programming language supported by ILU, there is a standard mapping defined from ISL to that programming language. This mapping defines what ISL type names, exception names, method names, and so on look like in that programming language.
The mapping for Python is straightforward. For type names,
such as Tutorial.Calculator, the Python name
of the ISL type Interface.Name
is Interface.Name, with any hyphens replaced by underscores. That is, the name of the interface in ISL
becomes the name of the module in Python.
So the name of our Calculator type in Python
would be Tutorial.Calculator, which is really the name of a Python class.
The Python mapping for a method name such as SetValue
is the method name, with any hyphens replaced by underscores.
The return type of this Python method is whatever is specified
in the ISL specification for the method, or None if
no type is specified. The arguments for the Python method are the
same as specified in the ISL; their types are the
Python types corresponding to the ISL types, except
that one extra argument is added to the beginning of each Python
version of an ISL method; it is an instance of the object type
on which the method is defined. An instance is simply a value of that
type. Thus the Python method corresponding
to our ISL SetValue would have the prototype signature
def SetValue (self, v):
Similarly, the signatures for the other methods, in Python, are
def GetValue (self): def Add (self, v): def Subtract (self, v): def Multiply (self, v): def Divide (self, v):Note that even though the
Divide method can raise an exception,
the signature looks like those of the other methods. This is because
the normal Python exception signalling mechanism is used to
signal exceptions back to the caller.
The mapping of exception names is similar to the mapping used for types.
So the exception Tutorial.DivideByZero
would also have the name Tutorial.DivideByZero, in Python.
One way to see what all the Python names for an interface
look like is to run the program python-stubber. This program
reads an ISL file, and generates the necessary Python
code to support that interface in Python. One of the files
generated is `Interface.py', which contains the definitions
of all the Python types for that interface.
% python-stubber Tutorial.isl client stubs for interface "Tutorial" to Tutorial.py ... server stubs for interface "Tutorial" to Tutorial__skel.py ... %
To provide an implementation of our interface, we subclass the
generated Python class for our Calculator class:
# CalculatorImpl.py
import Tutorial, Tutorial__skel
class Calculator (Tutorial__skel.Calculator):
def __init__ (self):
self.the_value = 0.0
def SetValue (self, v):
self.the_value = v
def GetValue (self):
return self.the_value
def Add (self, v):
self.the_value = self.the_value + v
def Subtract (self, v):
self.the_value = self.the_value - v
def Multiply (self, v):
self.the_value = self.the_value * v
def Divide (self, v):
try:
self.the_value = self.the_value / v
except ZeroDivisionError:
raise Tutorial.DivideByZero
Each instance of a CalculatorImpl.Calculator object
inherits from Tutorial__skel.Calculator, which in turn
inherits from Tutorial.Calculator. Each has an instance
variable called the_value, which maintains a running total
of the `accumulator' for that instance. We can create an instance
of a Tutorial.Calculator object by simply calling CalculatorImpl.Calculator().
So, a very simple program to use the Tutorial module might be
the following:
# simple1.py, a simple program that demonstrates the use of the
# Tutorial true module as a library.
#
# run this with the command "python simple1.py NUMBER [NUMBER...]"
#
import Tutorial, CalculatorImpl, string, sys
# A simple program:
# 1) make an instance of Tutorial.Calculator
# 2) add all the arguments by invoking the Add method
# 3) print the resultant value.
def main (argv):
c = CalculatorImpl.Calculator()
if not c:
error("Couldn't create calculator")
# clear the calculator before using it
c.SetValue (0.0)
# now loop over the arguments, adding each in turn */
for arg in argv[1:]:
v = string.atof(arg)
c.Add (v)
# and print the result
print "the sum is", c.GetValue()
sys.exit(0)
main(sys.argv)
This program would be compiled and run as follows:
% python simple1.py 34.9 45.23111 12 the sum is 92.13111 %
This is a completely self-contained use of the Tutorial
implementation; when a method is called, it is the true method
that is invoked. The use of ILU in this program adds
some overhead in terms of included code, but has almost
the same performance as a version of this program that does not
use ILU.
Suppose, instead of the Add method, we'd called the Divide
method. In that case, we might have had to handle a DivideByZero
exception; that is, notice the exception and do something sensible.
We do this by establishing a handler for the exception:
...
# now loop over the arguments, Dividing by each in turn */
try:
for arg in argv[2:]:
v = string.atof(arg)
c.Divide (v)
except:
print 'exception signalled: ' + str(sys.exc_type)
sys.exit(1)
...
And here's an example of what we get when it runs:
% python simple2.py 12345 6 7 8 9 the sum is 4.08234126984 % python simple2.py 12345 6 0 8 9 exception signalled: Tutorial: DivideByZeroActually, every method may return an exception, as there are a number of standard system exceptions which may be signalled even by methods which have no declared exceptions. So we should check every method to see if it succeeded, even simple ones like
GetValue.
Now let's see what's involved in providing the calculator functionality as a network service. Basically, there are three things to look at:
When one program uses code from another address space, it has to get its hands on an instance of an ILU object, to be able to call methods. In our library application, we simply made a call into the true module, to create an instance of the calculator object. In the networked world, we need to do the same kind of thing, but this time the call into the true module has to be a method on an object type. In short, we need to have some object type which exports a method something like
CreateCalculator () : Calculator
There are several ways to provide this. The standard
way of doing it is to add an object type to our Tutorial
interface, which contains this method. This kind of object type
is sometimes called a factory, because it exists only in order
to build instances of other object types. We'll add the following
type definition to our `Tutorial.isl':
TYPE Factory = OBJECT
METHODS
CreateCalculator () : Calculator
END;
Then we need to provide an implementation of the Factory
object type, just as we did with the Calculator type:
import Tutorial, Tutorial__skel, CalculatorImpl
class Factory (Tutorial__skel.Factory):
# have the __init__ method take handle and server args
# so that we can control which ILU kernel server is used,
# and what the instance handle of the Factory object on
# that server is. This allows us to control the object ID
# of the new Factory instance.
def __init__(self, handle=None, server=None):
self.IluInstHandle = handle
self.IluServer = server
def CreateCalculator (self):
return (CalculatorImpl.Calculator())
Now, to provide other programs a way of creating calculator objects,
we'll just create just one instance of Tutorial.Factory, and let
programs call the CreateCalculator method on that at will, to
obtain new calculator objects.
The question then arises, how does a program that wants to use
the Factory object get its hands on that one well-known instance?
The answer is to use the simple binding system built into ILU.
Simple binding allows a program acting as a "server" to publish
the location of a well-known object, and allows programs acting as "clients"
of that server to look up the location, given the object's name.
The name of an ILU object instance has two parts, which are the instance handle of the object, and the name of its kernel server, called the server ID. (The kernel server is a data structure maintained by the ILU kernel which takes care of all communication between different address spaces.) These two combined must form a universally unique ID for the object. Usually you can simply let the ILU system choose names for your objects automatically, in which case it takes care to choose names which will never conflict with names in use by others. However, for objects which we wish to publish, we need to specify what the name of an object will be, so that users of the well-known object can find it.
When working with the Python programming language, this act of explicitly specifying an object name is divided into two steps. First, we create a kernel server with a specified server ID. Secondly, we create an instance of an object on this new server, with a specified instance handle. Together, the server ID and the instance handle form the name of the instance.
For instance, we might use a server ID of
Tutorial.domain, where domain is your
Internet domain (typically something like
department.company.com, or
department.univerity.edu). This serves to
distinguish your server from other servers on the net. Then we can use
a simple instance handle, like theFactory. The name, or object ID,
of this object would then be
theFactory@Tutorial.domain, where domain
would vary from place to place. Note that this implies that only one
instance of this object is going to exist in the whole domain. If you
have many people using different versions of this object in your domain,
you should introduce more qualifiers in the server ID so that your
kernel server can be distinguished from that run by others.
Given this information, we can now write a complete program that will
serve as a provider of calculator objects to other programs. It will
create a single Factory instance with a well-known name, publish
that instance, then hang out servicing methods invoked on its objects.
Here's what it looks like:
# server.py -- a program that runs a Tutorial.Calculator server
#
import ilu, FactoryImpl, sys
def main(argv):
if (len(argv) < 2):
print "Usage: python server.py SERVER-ID"
sys.exit(1)
# Create a kernel server with appropriate server ID, which
# is passed in as the first argument
theServer = ilu.CreateServer (argv[1])
# Now create an instance of a Factory object on that server,
# with the instance handle "theFactory"
theFactory = FactoryImpl.Factory ("theFactory", theServer)
# Now make the Factory object "well-known" by publishing it.
theFactory.IluPublish()
# Now we print the string binding handle (the object's name plus
# its location) of the new instance.
print "Factory instance published."
print "Its SBH is '" + theFactory.IluSBH() + "'."
handle = ilu.CreateLoopHandle()
ilu.RunMainLoop (handle)
main(sys.argv)
When we run this program, we'll see something like:
% python server.py Tutorial.dept.company.com & Factory instance published. Its SBH is 'theFactory@Tutorial.dept.company.com@somegibberish'. %This indicates that the object known as
theFactory@Tutorial.dept.company.com is being exported in a particular way, which is encoded in the somegibberish part of the string binding handle. Your specific numbers will
vary, but it should look similar.
ilu.LookupObject(), which takes the name
and type of an instance, and attempts to find that instance on the
net. The name of the object is specified as a pair of strings,
the server ID of the object's kernel server, and the instance handle
of the object on that kernel server.
So, in our first example, we could replace the call to
Create_Tutorial_Calculator with a routine that calls
ilu.LookupObject() to find the factory, then creates an instance of
a Calculator. The full code of the revised example, `simple3.py',
is available as section simple3.py, but here's what the new code for obtaining
an instance of a Calculator looks like:
def Get_Tutorial_Calculator (factoryObjectSID, factoryObjectIH):
# We have to call ilu.LookupObject() with the object ID of
# the factory object, and the "type" of the object we're looking
# for, which is always available as MODULE.TYPENAME
f = ilu.LookupObject (factoryObjectSID, factoryObjectIH, Tutorial.Factory)
if not f:
print "Can't find Tutorial.Factory instance " + factoryObjectSID + factoryObjectIH
sys.exit(1)
c = f.CreateCalculator()
return (c)
We then can use the simple3 program:
% python simple3.py Tutorial.dept.company.com theFactory 1 2 3 4 5 6 the sum is 2.10000 %
ILU ISL contains support for a number of types other than
object types and REAL. The primitive ISL types include 16, 32, and 64 bit signed and unsigned
integers, bytes, 8 and 16 bit characters, a boolean type, and 32, 64, and 128 bit
floating point types. A number of type constructors allow specification of
arrays, sequences, records, unions, and enumerations, as well as object types.
The ISL OPTIONAL type constructor provides an
implicit union of some type with NULL, which is useful for building
recursive data structures such as linked lists or binary trees.
To illustrate some of these types, we'll extend the Tutorial.Calculator
type. Many real-world desktop calculators include a register tape,
a printed listing of all the operations that have been performed, with
a display of what the value of the calculator was after each operation.
We'll add a register tape to Tutorial.Calculator.
We could do it by adding a new method to Tutorial.Calculator, called
GetTape. Unfortunately, this would break our existing code, because
it would change the Tutorial.Calculator object type, and existing
compiled clients wouldn't be able to recognize the new object type. Instead,
we'll extend the object type by subtyping; that is, by creating a new
object type which uses Tutorial.Calculator as a supertype, but
adds new methods of its own. This subtype will actually have two types;
both its own new type, and Tutorial.Calculator. We'll also define
a subtype of the Tutorial.Factory type, to allow us to create new
instances of the new Calculator subtype. Finally, we'll define a new
module interface for the new types, so that we don't have to modify the
Tutorial interface.
First, let's define the necessary type to represent the operations performed on the calculator:
INTERFACE Tutorial2 IMPORTS Tutorial END;
TYPE OpType = ENUMERATION
SetValue, Add, Subtract, Multiply, Divide END;
TYPE Operation = RECORD
op : OpType,
value : REAL,
accumulator : REAL
END;
TYPE RegisterTape = SEQUENCE OF Operation;
The enumerated type OpType defines an abstract type with five possible
values. The type Operation defines a record type (in Python,
a dictionary) with 3 fields: the op field, which tells us which
of the five possible calculator operations was performed, the value
field, which tells us the value of the operand for the operation, and
the accumulator field, which tells us what the value of the
calculator was after the operation had been performed. Finally, the
Operation type is a simple sequence, or list, of Operation.
Note that Tutorial2 imports Tutorial; that is,
it allows the use of the Tutorial types, exceptions, and constants,
in the specifications in Tutorial2.
Now we define the new object types (in the same file):
TYPE TapeCalculator = OBJECT COLLECTIBLE
SUPERTYPES Tutorial.Calculator END
DOCUMENTATION "4 function calculator with register tape"
METHODS
GetTape () : RegisterTape
END;
TYPE Factory = OBJECT SUPERTYPES Tutorial.Factory END
METHODS
CreateTapeCalculator () : TapeCalculator
END;
The SUPERTYPES attribute of an object type may take multiple
object type names, so ISL supports multiple inheritance.
The Tutorial2.TapeCalculator type will now support the six methods of
Tutorial.Calculator, as well as its own method, GetTape.
We then need to provide an implementation for Tutorial2.TapeCalculator
(see section TapeCalculatorImpl.py for the actual code). We
modify each method on the TapeCalculator object to record
its invocation, and add a slot to hold the contents of the `tape'.
We also provide an implementation for Tutorial2.Factory:
import Tutorial2, Tutorial2__skel, TapeCalculatorImpl
class Factory (Tutorial2__skel.Factory):
# have the __init__ method take handle and server args
# so that we can control which ILU kernel server is used,
# and what the instance handle of the Factory object on
# that server is. This allows us to control the object ID
# of the new Factory instance.
def __init__(self, handle=None, server=None):
self.IluInstHandle = handle
self.IluServer = server
def CreateCalculator (self):
return (TapeCalculatorImpl.TapeCalculator())
CreateTapeCalculator = CreateCalculator
Note that both the Tutorial2.Factory.CreateCalculator and
Tutorial2.Factory.CreateTapeCalculator methods create and return
instances of Tutorial2.TapeCalculator. This is valid, because
instances of Tutorial2.TapeCalculator are also instances of
Tutorial.Calculator.
Now we modify `server.py' to create an instance of Tutorial2.Factory,
instead of Tutorial.Factory, and to initialize the Tutorial2
true-side code (See section server2.py for the actual code).
Finally, see section simple4.py, for an example of a client of the
TapeCalculator type.
Note that one nice result of this approach to versioning is that old
clients, which know nothing about the new TapeCalculator class, or
about the whole Tutorial2 interface in general, will continue
to function, since every instance of Tutorial2.TapeCalculator is also
an instance of Tutorial.Calculator, and every instance of
Tutorial2.Factory is also an instance of Tutorial.Factory.
OPTIONAL types, so
not every ILU interface can be expressed in OMG IDL, but
many of them can. For example, here is the OMG IDL version
of the Tutorial interface:
module Tutorial {
exception DivideByZero {};
interface Calculator {
// Set the value of the calculator to `v'
void SetValue (in double v);
// Return the value of the calculator
double GetValue ();
// Adds `v' to the calculator's value
void Add (in double v);
// Subtracts `v' from the calculator's value
void Subtract (in double v);
// Multiplies the calculator's value by `v'
void Multiply (in double v);
// Divides the calculator's value by `v'
void Divide (in double v) raises (DivideByZero);
};
interface Factory {
// Create and return an instance of a Calculator object
Calculator CreateCalculator();
};
};
This can be used with the python-stubber:
% python-stubber Tutorial.idl client stubs for interface "Tutorial" to Tutorial.py ... server stubs for interface "Tutorial" to Tutorial__skel.py ... %
This will be a bit slower than running the python-stubber on the equivalent ISL file, as the program works by converting the OMG IDL into ISL, then compiling from the ISL description. OMG IDL interfaces can be checked by running the OMG IDL-to-ILU ISL converter, idl2isl, directly:
% idl2isl Tutorial.idl
INTERFACE Tutorial;
EXCEPTION DivideByZero;
TYPE Calculator = OBJECT OPTIONAL
METHODS
SetValue (v : REAL),
GetValue () : REAL,
Add (v : REAL),
Subtract (v : REAL),
Multiply (v : REAL),
Divide (v : REAL)
RAISES DivideByZero END
END;
TYPE Factory = OBJECT OPTIONAL
METHODS
CreateCalculator () : Calculator
END;
%
You will notice that the ISL interface generated by idl2isl is a bit different, in that the object type modifier OPTIONAL is used in the description of the Calculator and Factory types. This is because
CORBA has the notion that any object type instance passed as a parameter or return value (or field in an array, or element of a sequence, etc.) may be None, instead of being a valid instance pointer. Thus, when working with OMG IDL descriptions of your interfaces, it is necessary to check the return type of methods like Tutorial.Factory.CreateCalculator to see that a valid object reference has been returned, before using the object. ISL allows you to have these CORBA-style objects, by using the OPTIONAL modifier in the declaration of an object type, but it also allows object pointers which can't be None. By default ILU object instances may not be None.
The OMG IDL version of Tutorial2 can be found in section Tutorial2.idl.
Calculator type in `Tutorial.isl'
is marked with the modifier COLLECTIBLE, while the Factory type is not.
ILU differentiates between two kinds of objects, which we call transient
and permanent.
Transient objects are typically created for a specific use, and typically have a fixed lifetime. They often have state associated with them. They are frequently used to represent some ongoing computation or operation.
Permanent objects are usually expected to be around forever (or until some activity not captured by a programming concept terminates). Permanent objects are often gateways to some sort of service, or represent some external piece of data, such as a cell in a spreadsheet, or a file on a disk somewhere. They are often stateless; when they do have state, it is typically backed by some form of stable storage, as they have to be available across crashes of their server program.
ILU provides an automatic system for distributed garbage collection of
transient objects; that is, of removing them from everyone's address space
when everyone is done with them. This uses the underlying garbage collection techniques
of the particular programming language being used. With Python, the ILU
garbage collector works in conjunction with the Python collector; with ANSI C,
we use the extremely primitive collector provided for C; that is, when the
process terminates, the objects are collected. To indicate to ILU that a particular
object type is to be treated as transient, you mark the object type specification, in the ISL,
with the keyword COLLECTIBLE.
For permanent objects, like the Tutorial.Factory object type, the ILU
Python support will maintain a reference to the instance, preventing the
Python collector from collecting it.
Python comes with a module called Tkinter, which is an interface
to John Ousterhout's Tk user interface system. When you use both ILU
and Tkinter together in a program, you should call the ilu_tk
main loop to block and dispatch requests, instead of the ilu main loop.
To illustrate this, here is a version of the server2.py program which
puts up a Tk button. When the user presses the button, the server exits.
Notice that instead of calling ilu.RunMainLoop() to block indefinitely,
the program calls ilu_tk.RunMainLoop(). This call allows both Tk
and ILU events to be handled when neither a Tk callback nor an ILU
method call is active.
# server3.py -- a program that runs a Tutorial.Calculator server
# Puts up a Tk button to kill it with.
# import the ilu_tk module first, so that
# initialization happens in the right order:
import ilu_tk
# now import the other modules...
import ilu, FactoryImpl2, sys, Tkinter
def main(argv):
def quit():
sys.exit(0)
if (len(argv) < 2):
print "Usage: python server3.py SERVER-ID"
sys.exit(1)
theServer = ilu.CreateServer (argv[1])
theFactory = FactoryImpl2.Factory ("theFactory", theServer)
theFactory.IluPublish()
# Now we put up a Tk button so that the user can kill the
# server by pressing the button
f = Tkinter.Frame() ; Tkinter.Pack.config(f)
b = Tkinter.Button (f, {'text' : theFactory.IluObjectID(),\
'command': quit})
b.pack ({'side': 'left', 'fill': 'both'})
# Then we wait in the ilu_tk mainloop, instead of either
# the ILU mainloop or the Tkinter mainloop
ilu_tk.RunMainLoop()
main(sys.argv)
The 2.0 release of ILU contains support for the programming languages ANSI C, C++, Modula-3, Python, and Common Lisp. It has been installed on many flavors of UNIX, including SPARC machines running SunOS 4.1.3 and Solaris 2, SGI MIPS machines running IRIX 5.2, Intel 486 machines running Linux 1.1.78, DEC Alpha machines with OSF/1, IBM RS/6000 machines running AIX, and HP machines running HP/UX. It runs on Microsoft Windows 3.1, Windows 95, and Windows NT environments. It supports both threaded and non-threaded operation. Since one of the implementation goals of ILU is to maximize compatibility with existing open standards, ILU provides support for use of the OMG CORBA IDL interface description language, and can be thought of as a CORBA ORB system (though with omissions from and extensions to the CORBA spec). As another result, ILU includes a self-contained implementation of ONC RPC.
ILU
is available free from ftp://ftp.parc.xerox.com/pub/ilu/ilu.html.
INTERFACE Tutorial;
EXCEPTION DivideByZero
"this error is signalled if the client of the Calculator calls
the Divide method with a value of 0";
TYPE Calculator = OBJECT COLLECTIBLE
DOCUMENTATION "4-function calculator"
METHODS
SetValue (v : REAL) "Set the value of the calculator to `v'",
GetValue () : REAL "Return the value of the calculator",
Add (v : REAL) "Adds `v' to the calculator's value",
Subtract (v : REAL) "Subtracts `v' from the calculator's value",
Multiply (v : REAL) "Multiplies the calculator's value by `v'",
Divide (v : REAL) RAISES DivideByZero END
"Divides the calculator's value by `v'"
END;
TYPE Factory = OBJECT
METHODS
CreateCalculator () : Calculator
END;
import Tutorial, Tutorial__skel
class Calculator (Tutorial__skel.Calculator):
def __init__ (self):
self.the_value = 0.0
def SetValue (self, v):
self.the_value = v
def GetValue (self):
return self.the_value
def Add (self, v):
self.the_value = self.the_value + v
def Subtract (self, v):
self.the_value = self.the_value - v
def Multiply (self, v):
self.the_value = self.the_value * v
def Divide (self, v):
try:
self.the_value = self.the_value / v
except ZeroDivisionError:
raise Tutorial.DivideByZero
import Tutorial, Tutorial__skel, CalculatorImpl
class Factory (Tutorial__skel.Factory):
# have the __init__ method take handle and server args
# so that we can control which ILU kernel server is used,
# and what the instance handle of the Factory object on
# that server is. This allows us to control the object ID
# of the new Factory instance.
def __init__(self, handle=None, server=None):
self.IluInstHandle = handle
self.IluServer = server
def CreateCalculator (self):
return (CalculatorImpl.Calculator())
# simple1.py, a simple program that demonstrates the use of the
# Tutorial true module as a library.
#
# run this with the command "python simple1.py NUMBER [NUMBER...]"
#
import Tutorial, CalculatorImpl, string, sys
# A simple program:
# 1) make an instance of Tutorial.Calculator
# 2) add all the arguments by invoking the Add method
# 3) print the resultant value.
def main (argv):
c = CalculatorImpl.Calculator()
if not c:
error("Couldn't create calculator")
# clear the calculator before using it
c.SetValue (0.0)
# now loop over the arguments, adding each in turn */
for arg in argv[1:]:
v = string.atof(arg)
c.Add (v)
# and print the result
print "the sum is", c.GetValue()
sys.exit(0)
main(sys.argv)
# simple2.py, a simple program that demonstrates the use of the
# Tutorial true module as a library.
#
# run this with the command "python simple1.py NUMBER [NUMBER...]"
#
import Tutorial, CalculatorImpl, string, sys
# A simple program:
# 1) make an instance of Tutorial.Calculator
# 2) add all the arguments by invoking the Add method
# 3) print the resultant value.
def main (argv):
c = CalculatorImpl.Calculator()
if not c:
error("Couldn't create calculator")
# clear the calculator before using it
if (len(sys.argv) < 2):
c.SetValue (0.0)
else:
c.SetValue (string.atof(argv[1]))
# now loop over the arguments, Dividing by each in turn */
try:
for arg in argv[2:]:
v = string.atof(arg)
c.Divide (v)
except:
print 'exception signalled: ' + str(sys.exc_type)
sys.exit(1)
# and print the result
print "the sum is", c.GetValue()
sys.exit(0)
main(sys.argv)
# server.py -- a program that runs a Tutorial.Calculator server
#
import ilu, FactoryImpl, sys
def main(argv):
if (len(argv) < 2):
print "Usage: python server.py SERVER-ID"
sys.exit(1)
# Create a kernel server with appropriate server ID, which
# is passed in as the first argument
theServer = ilu.CreateServer (argv[1])
# Now create an instance of a Factory object on that server,
# with the instance handle "theFactory"
theFactory = FactoryImpl.Factory ("theFactory", theServer)
# Now make the Factory object "well-known" by publishing it.
theFactory.IluPublish()
# Now we print the string binding handle (the object's name plus
# its location) of the new instance.
print "Factory instance published."
print "Its SBH is '" + theFactory.IluSBH() + "'"
handle = ilu.CreateLoopHandle()
ilu.RunMainLoop (handle)
main(sys.argv)
# simple3.py -- a simple client program that finds the Calculator Factory,
# creates a calculator, and adds up its arguments as before
#
# to run: python simple4.py ARG [ARG...]
import Tutorial, ilu, sys, string
# We define a new routine, "Get_Tutorial_Calculator", which
# finds the tutorial factory, then creates a new Calculator
# object for us.
def Get_Tutorial_Calculator (factoryObjectID):
# We have to call ilu.LookupObject() with the object ID of
# the factory object, and the "type" of the object we're looking
# for, which is always available as MODULE.TYPENAME
f = ilu.LookupObject (factoryObjectID, Tutorial.Factory)
if not f:
print "Can't find Tutorial.Factory instance " + factoryObjectID
sys.exit(1)
c = f.CreateCalculator()
return (c)
def main (argv):
# A simple program:
# 1) make an instance of Tutorial.Calculator
# 2) add all the arguments by invoking the Add method
# 3) print the resultant value.
if (len(argv) < 2):
print "Usage: python simple3.py FACTORY-OBJECT-ID NUMBER [NUMBER...]\n",
sys.exit(1)
c = Get_Tutorial_Calculator(argv[1])
if not c:
print "Couldn't create calculator"
sys.exit(1)
# clear the calculator before using it
c.SetValue (0.0)
# now loop over the arguments, adding each in turn
for arg in argv[2:]:
v = string.atof (arg)
c.Add (v)
# and print the result
print "the sum is " + str(c.GetValue())
sys.exit (0);
main(sys.argv)
INTERFACE Tutorial2 IMPORTS Tutorial END;
TYPE OpType = ENUMERATION
SetValue, Add, Subtract, Multiply, Divide END;
TYPE Operation = RECORD
op : OpType,
value : REAL,
accumulator : REAL
END;
TYPE RegisterTape = SEQUENCE OF Operation;
TYPE TapeCalculator = OBJECT COLLECTIBLE
SUPERTYPES Tutorial.Calculator END
DOCUMENTATION "4 function calculator with register tape"
METHODS
GetTape () : RegisterTape
END;
TYPE Factory = OBJECT SUPERTYPES Tutorial.Factory END
METHODS
CreateTapeCalculator () : TapeCalculator
END;
import Tutorial, Tutorial2, Tutorial2__skel
class TapeCalculator (Tutorial2__skel.TapeCalculator):
def __init__ (self):
self.value = 0.0
self.tape = []
def SetValue (self, v):
global value
self.value = v
self.tape.append({'op' : Tutorial2.OpType__SetValue, 'value' : v, 'accumulator' : self.value})
def GetValue (self):
return self.value
def Add (self, v):
self.value = self.value + v
self.tape.append({'op' : Tutorial2.OpType__Add, 'value' : v, 'accumulator' : self.value})
def Subtract (self, v):
self.value = self.value - v
self.tape.append({'op' : Tutorial2.OpType__Subtract, 'value' : v, 'accumulator' : self.value})
def Multiply (self, v):
self.value = self.value * v
self.tape.append({'op' : Tutorial2.OpType__Multiply, 'value' : v, 'accumulator' : self.value})
def Divide (self, v):
try:
self.value = self.value / v
except ZeroDivisionError:
raise Tutorial.DivideByZero
self.tape.append({'op' : Tutorial2.OpType__Divide, 'value' : v, 'accumulator' : self.value})
def GetTape (self):
return (self.tape)
import Tutorial2, Tutorial2__skel, TapeCalculatorImpl
class Factory (Tutorial2__skel.Factory):
# have the __init__ method take handle and server args
# so that we can control which ILU kernel server is used,
# and what the instance handle of the Factory object on
# that server is. This allows us to control the object ID
# of the new Factory instance.
def __init__(self, handle=None, server=None):
self.IluInstHandle = handle
self.IluServer = server
def CreateCalculator (self):
return (TapeCalculatorImpl.TapeCalculator())
CreateTapeCalculator = CreateCalculator
# server2.py -- a program that runs a Tutorial2.TapeCalculator factory
#
import ilu, FactoryImpl2, sys
def main(argv):
if (len(argv) < 2):
print "Usage: python server2.py SERVER-ID"
sys.exit(1)
theServer = ilu.CreateServer (argv[1])
theFactory = FactoryImpl2.Factory ("theFactory", theServer)
# Now make the Factory object "well-known" by publishing it.
theFactory.IluPublish()
# Now we print the string binding handle (the object's name plus
# its location) of the new instance.
print "Factory2 instance published."
print "Its SBH is '" + theFactory.IluSBH() + "'"
handle = ilu.CreateLoopHandle ();
ilu.RunMainLoop (handle)
main(sys.argv)
# simple4.py -- a simple client program that finds the TapeCalculator Factory,
# creates a calculator, and provides a simple interactive calculator
#
# to run: python simple4.py ARG [ARG...]
import Tutorial2, ilu, sys, string
# We define a new routine, "Get_Tutorial_Calculator", which
# finds the tutorial factory, then creates a new Calculator
# object for us.
def Get_Tutorial_Calculator (factoryObjectID):
# We have to call ilu.LookupObject() with the object ID of
# the factory object, and the "type" of the object we're looking
# for, which is always available as MODULE.TYPENAME
f = ilu.LookupObject (factoryObjectID, Tutorial2.Factory)
if not f:
print "Can't find Tutorial.Factory instance " + factoryObjectID
sys.exit(1)
c = f.CreateTapeCalculator()
return (c)
opname = ['SetValue', 'Add', 'Subtract', 'Divide', 'Multiply']
def Print_Tape (tape):
# print out the Calculator tape nicely
for op in tape:
print " %s(%f) => %f" % (opname[op['op']], op['value'], op['accumulator'])
def main (argv):
if (len(argv) < 2):
print "Usage: python simple3.py FACTORY-OBJECT-ID NUMBER [NUMBER...]\n",
sys.exit(1)
c = Get_Tutorial_Calculator(argv[1])
if not c:
print "Couldn't create calculator"
sys.exit(1)
# clear the calculator before using it
newval = 0.0
c.SetValue (newval)
quitflag = 0
while not quitflag:
sys.stdout.write("%.5f\n> " % newval)
sys.stdout.flush()
line = sys.stdin.readline()
if (not line):
sys.exit(0)
try:
if (line[0] == '\n'):
pass
elif (line[0] == '+'):
val = string.atof(line[1:-1])
c.Add(val)
elif (line[0] == '-'):
val = string.atof(line[1:-1])
c.Subtract(val)
elif (line[0] == '*'):
val = string.atof(line[1:-1])
c.Multiply(val)
elif (line[0] == '/'):
val = string.atof(line[1:-1])
c.Divide(val)
elif (line[0] == 'q'):
quitflag = 1
elif (line[0] == 'c'):
c.SetValue(0.0)
elif (line[0] == 't'):
Print_Tape ( c.GetTape() )
else:
print "Invalid operation <" + line[:-1] + ">"
print "Valid ops are +, -, *, /, tape, clear, quit"
newval = c.GetValue()
except:
print "Operation <%s> signals error <%s>." % (line[:-1], sys.exc_type)
sys.exit(0)
main(sys.argv)
module Tutorial {
exception DivideByZero {};
interface Calculator {
// Set the value of the calculator to `v'
void SetValue (in double v);
// Return the value of the calculator
double GetValue ();
// Adds `v' to the calculator's value
void Add (in double v);
// Subtracts `v' from the calculator's value
void Subtract (in double v);
// Multiplies the calculator's value by `v'
void Multiply (in double v);
// Divides the calculator's value by `v'
void Divide (in double v) raises (DivideByZero);
};
interface Factory {
// Create and return an instance of a Calculator object
Calculator CreateCalculator();
};
};
#include "Tutorial.idl"
module Tutorial2 {
enum OpType { SetValue, Add, Subtract, Multiply, Divide };
struct Operation {
OpType op;
double value;
double accumulator;
};
typedef sequence<Operation> RegisterTape;
// A four function calculator with a register tape
interface TapeCalculator : Tutorial::Calculator {
RegisterTape GetTape ();
};
// A factory that produces TapeCalculators
interface Factory : Tutorial::Factory {
TapeCalculator CreateTapeCalculator ();
};
};
# server3.py -- a program that runs a Tutorial.Calculator server
# Puts up a Tk button to kill it with.
import ilu, FactoryImpl2, sys, Tkinter, ilu_tk
def main(argv):
def quit():
sys.exit(0)
if (len(argv) < 2):
print "Usage: python server3.py SERVER-ID"
sys.exit(1)
theServer = ilu.CreateServer (argv[1])
theFactory = FactoryImpl2.Factory ("theFactory", theServer)
theFactory.IluPublish()
# Now we put up a Tk button so that the user can kill the
# server by pressing the button
f = Tkinter.Frame() ; Tkinter.Pack.config(f)
b = Tkinter.Button (f, {'text' : theFactory.IluObjectID(),\
'command': quit})
b.pack ({'side': 'left', 'fill': 'both'})
# Then we wait in the ilu_tk mainloop, instead of either
# the ILU mainloop or the Tkinter mainloop
ilu_tk.RunMainLoop()
main(sys.argv)