20.4 Mapping for Interfaces

The server-side mapping for interfaces provides an up-call API for the Ice run time: by implementing methods in a servant class, you provide the hook that gets the thread of control from the Ice server-side run time into your application code.

20.4.1 Skeleton Classes

On the client side, interfaces map to proxy classes (see Section 5.12). On the server side, interfaces map to skeleton classes. A skeleton is an abstract base class from which you derive your servant class and define a method for each operation on the corresponding interface. For example, consider the Slice definition for the Node interface we defined in Chapter 5 once more:

module Filesystem {
    interface Node {
        idempotent string name();
    };
    // ...
};

The Python mapping generates the following definition for this interface:

class Node(Ice.Object):
    def __init__(self):
        # ...

    #
    # Operation signatures.
    #
    # def name(self, current=None):

The important points to note here are:

• As for the client side, Slice modules are mapped to Python modules with the same name, so the skeleton class definitions are part of the Filesystem module.

• The name of the skeleton class is the same as the name of the Slice interface (Node).

• The skeleton class contains a comment summarizing the method signature of each operation in the Slice interface.

• The skeleton class is an abstract base class because its constructor prevents direct instantiation of the class.

• The skeleton class inherits from Ice.Object (which forms the root of the Ice object hierarchy).

20.4.2 Servant Classes

In order to provide an implementation for an Ice object, you must create a servant class that inherits from the corresponding skeleton class. For example, to create a servant for the Node interface, you could write:

import Filesystem

class NodeI(Filesystem.Node):
    def __init__(self, name):
        self._name = name

    def name(self, current=None):
        return self._name

By convention, servant classes have the name of their interface with an I‑suffix, so the servant class for the Node interface is called NodeI. (This is a convention only: as far as the Ice run time is concerned, you can choose any name you prefer for your servant classes.) Note that NodeI extends Filesystem.Node, that is, it derives from its skeleton class.

As far as Ice is concerned, the NodeI class must implement only a single method: the name method that is defined in the Node interface. This makes the servant class a concrete class that can be instantiated. You can add other member functions and data members as you see fit to support your implementation. For example, in the preceding definition, we added a _name member and a constructor. (Obviously, the constructor initializes the _name member and the name function returns its value.)

Normal and idempotent Operations

Whether an operation is an ordinary operation or an idempotent operation has no influence on the way the operation is mapped. To illustrate this, consider the following interface:

interface Example {
               void normalOp();
    idempotent void idempotentOp();
};

The mapping for this interface is shown below:

class Example(Ice.Object):
    # ...

    #
    # Operation signatures.
    #
    # def normalOp(self, current=None):
    # def idempotentOp(self, current=None):

Note that the signatures of the methods are unaffected by the idempotent qualifier.