The mapping of Slice interfaces revolves around the idea that, to invoke a remote operation, you call a method on a local class instance that represents the remote object. This makes the mapping easy and intuitive to use because, for all intents and purposes (apart from error semantics), making a remote procedure call is no different from making a local procedure call.
On the client side, Slice interfaces map to Python classes with methods that correspond to the operations on those interfaces. Consider the following simple interface:
class SimplePrx(Ice.ObjectPrx):
def op(self, _ctx=None):
# ...
# ...
In the client’s address space, an instance of SimplePrx is the local ambassador for a remote instance of the
Simple interface in a server and is known as a
proxy instance. All the details about the server-side object, such as its address, what protocol to use, and its object identity are encapsulated in that instance.
Note that SimplePrx inherits from
Ice.ObjectPrx. This reflects the fact that all Ice interfaces implicitly inherit from
Ice::Object.
For each operation in the interface, the proxy class has a method of the same name. In the preceding example, we find that the operation
op has been mapped to the method
op. Note that
op accepts an optional trailing parameter
_ctx representing the operation context. This parameter is a Python dictionary for use by the Ice run time to store information about how to deliver a request. You normally do not need to use it. (We examine the context parameter in detail in
Chapter 28. The parameter is also used by IceStorm—see
Chapter 41.)
A value of None denotes the null proxy. The null proxy is a dedicated value that indicates that a proxy points "nowhere" (denotes no object).
18.11.2 The Ice.ObjectPrx Class
All Ice objects have Object as the ultimate ancestor type, so all proxies inherit from
Ice.ObjectPrx.
ObjectPrx provides a number of methods:
class ObjectPrx(object):
def equals(self, other):
def ice_getIdentity(self):
def ice_hash(self):
def ice_isA(self, id):
def ice_id(self):
def ice_ping(self):
# ...
This operation compares two proxies for equality. Note that all aspects of proxies are compared by this operation, such as the communication endpoints for the proxy. This means that, in general, if two proxies compare unequal, that does
not imply that they denote different objects. For example, if two proxies denote the same Ice object via different transport endpoints,
equals returns
false even though the proxies denote the same object.
module Ice {
struct Identity {
string name;
string category;
};
};
proxy1 = ...
proxy2 = ...
id1 = proxy1.ice_getIdentity()
id2 = proxy2.ice_getIdentity()
if id1 == id2:
# proxy1 and proxy2 denote the same object
else:
# proxy1 and proxy2 denote different objects
proxy = ...
if proxy != None and proxy.ice_isA("::Printer"):
# proxy denotes a Printer object
else:
# proxy denotes some other type of object
Note that there are other methods in ObjectPrx, not shown here. These methods provide different ways to dispatch a call. (We discuss these methods in
Chapter 28.)
class SimplePrx(Ice.ObjectPrx):
# ...
def checkedCast(proxy, facet=''):
# ...
checkedCast = staticmethod(checkedCast)
def uncheckedCast(proxy, facet=''):
# ...
uncheckedCast = staticmethod(uncheckedCast)
Both the checkedCast and
uncheckedCast methods implement a down-cast: if the passed proxy is a proxy for an object of type
Simple, or a proxy for an object with a type derived from
Simple, the cast returns a reference to a proxy of type
SimplePrx; otherwise, if the passed proxy denotes an object of a different type (or if the passed proxy is
None), the cast returns
None.
The method names checkedCast and
uncheckedCast are reserved for use in proxies. If a Slice interface defines an operation with either of those names, the mapping escapes the name in the generated proxy by prepending an underscore. For example, an interface that defines an operation named
checkedCast is mapped to a proxy with a method named
_checkedCast.
Given a proxy of any type, you can use a checkedCast to determine whether the corresponding object supports a given type, for example:
obj = ... # Get a proxy from somewhere...
simple = SimplePrx.checkedCast(obj)
if simple != None:
# Object supports the Simple interface...
else:
# Object is not of type Simple...
Note that a checkedCast contacts the server. This is necessary because only the implementation of a proxy in the server has definite knowledge of the type of an object. As a result, a
checkedCast may throw a
ConnectTimeoutException or an
ObjectNotExistException.
In contrast, an uncheckedCast does not contact the server and unconditionally returns a proxy of the requested type. However, if you do use an
uncheckedCast, you must be certain that the proxy really does support the type you are casting to; otherwise, if you get it wrong, you will most likely get a run-time exception when you invoke an operation on the proxy. The most likely error for such a type mismatch is
OperationNotExistException. However, other exceptions, such as a marshaling exception are possible as well. And, if the object happens to have an operation with the correct name, but different parameter types, no exception may be reported at all and you simply end up sending the invocation to an object of the wrong type; that object may do rather non-sensical things. To illustrate this, consider the following two interfaces:
interface Process {
void launch(int stackSize, int dataSize);
};
// ...
interface Rocket {
void launch(float xCoord, float yCoord);
};
Suppose you expect to receive a proxy for a Process object and use an
uncheckedCast to down-cast the proxy:
obj = ... # Get proxy...
process = ProcessPrx.uncheckedCast(obj) # No worries...
process.launch(40, 60) # Oops...
If the proxy you received actually denotes a Rocket object, the error will go undetected by the Ice run time: because
int and
float have the same size and because the Ice protocol does not tag data with its type on the wire, the implementation of
Rocket::launch will simply misinterpret the passed integers as floating-point numbers.
In fairness, this example is somewhat contrived. For such a mistake to go unnoticed at run time, both objects must have an operation with the same name and, in addition, the run-time arguments passed to the operation must have a total marshaled size that matches the number of bytes that are expected by the unmarshaling code on the server side. In practice, this is extremely rare and an incorrect
uncheckedCast typically results in a run-time exception.
The base proxy class ObjectPrx supports a variety of methods for customizing a proxy (see
Section 28.10). Since proxies are immutable, each of these "factory methods" returns a copy of the original proxy that contains the desired modification. For example, you can obtain a proxy configured with a ten second timeout as shown below:
A factory method returns a new proxy object if the requested modification differs from the current proxy, otherwise it returns the current proxy. With few exceptions, factory methods return a proxy of the same type as the current proxy, therefore it is generally not necessary to repeat a down-cast after using a factory method. The example below demonstrates these semantics:
The only exceptions are the factory methods ice_facet and
ice_identity. Calls to either of these methods may produce a proxy for an object of an unrelated type, therefore they return a base proxy that you must subsequently down-cast to an appropriate type.
Proxy objects support comparison using the built-in relational operators as well as the
cmp function. Note that proxy comparison uses
all of the information in a proxy for the comparison. This means that not only the object identity must match for a comparison to succeed, but other details inside the proxy, such as the protocol and endpoint information, must be the same. In other words, comparison tests for
proxy identity,
not object identity. A common mistake is to write code along the following lines:
p1 = ... # Get a proxy...
p2 = ... # Get another proxy...
if p1 != p2:
# p1 and p2 denote different objects # WRONG!
else:
# p1 and p2 denote the same object # Correct
Even though p1 and
p2 differ, they may denote the same Ice object. This can happen because, for example, both
p1 and
p2 embed the same object identity, but each uses a different protocol to contact the target object. Similarly, the protocols may be the same, but denote different endpoints (because a single Ice object can be contacted via several different transport endpoints). In other words, if two proxies compare equal, we know that the two proxies denote the same object (because they are identical in all respects); however, if two proxies compare unequal, we know absolutely nothing: the proxies may or may not denote the same object.
def proxyIdentityCompare(lhs, rhs)
def proxyIdentityAndFacetCompare(lhs, rhs)
proxyIdentityCompare allows you to correctly compare proxies for identity:
p1 = ... # Get a proxy...
p2 = ... # Get another proxy...
if Ice.proxyIdentityCompare(p1, p2) != 0:
# p1 and p2 denote different objects # Correct
else:
# p1 and p2 denote the same object # Correct
The function returns 0 if the identities are equal, −1 if
p1 is less than
p2, and 1 if
p1 is greater than
p2. (The comparison uses
name as the major sort key and
category as the minor sort key.)
The proxyIdentityAndFacetCompare function behaves similarly, but compares both the identity and the facet name (see
Chapter 30).
