SEC. 9.1 CORBA 501
simple and soon additional communication facilities were added. In the following, we take a closer look at invocation facilities in CORBA, and also consider its alternatives to these object invocations. As we will see, the extensions to the basic object invocation model are motivated by the need for asynchronous communication. This motivation also led to the alternative message-passing models that we discussed in Chap. 2.
Object Invocation Models
By default, when a client invokes an object, it sends a request to the corresponding object server and blocks until it receives a response. In the absence of failures, these semantics correspond exactly to a normal method invocation when the caller and callee reside in the same address space.
However, matters become somewhat more complicated in the presence of failures. In the case of a synchronous invocation, as just described, a client will eventually receive an exception indicating that the invocation did not fully complete. CORBA specifies that in this case, the invocation should follow at-mostonce semantics, implying that the invoked method may have been invoked once, or not at all. Note that it is up to an implementation to provide these semantics.
Synchronous invocation is therefore useful when the client expects an answer. If a proper response is returned, CORBA guarantees that the method has been invoked exactly once. However, in those cases where no response is needed, it would be better for the client to simply invoke the method and continue with its own execution as soon as possible. This type of invocation is similar to the asynchronous RPCs we discussed in Chap. 2.
In CORBA, this form of asynchronous invocation is called a one-way request. A method can be specified as being one-way only if it is specified to return no results. However, unlike the guaranteed delivery of asynchronous RPCs, one-way requests in CORBA offer only a best-effort delivery service. In other words, no guarantees are given to the caller that the invocation request is delivered to the object’ s server.
Besides one-way requests, CORBA also supports what is known as a deferred synchronous request. Such a request is, in fact, a combination of a one-way request and letting the server asynchronously send the result back to the client. As soon as the client sends its request to the server, it continues without waiting for any response from the server. In other words, the client can never know for sure whether its request is actually delivered to the server.
These three different invocation models are summarized in Fig. 9-4.