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Concept
Distributed object communication
Summary
In a distributed computing environment, distributed object communication realizes communication between distributed objects. The main role is to allow objects to access data and invoke methods on remote objects (objects residing in non-local memory space). Invoking a method on a remote object is known as remote method invocation (RMI) or remote invocation, and is the object-oriented programming analog of a remote procedure call (RPC).
The widely used approach on how to implement the communication channel is realized by using stubs and skeletons. They are generated objects whose structure and behavior depends on chosen communication protocol, but in general provide additional functionality that ensures reliable communication over the network.
In RMI, a stub (which is the bit on the client) is defined by the programmer as an interface. The rmic (rmi compiler) uses this to create the class stub. The stub performs type checking. The skeleton is defined in a class which implements the interface stub.
When a caller wants to perform remote call on the called object, it delegates requests to its stub which initiates communication with the remote skeleton. Consequently, the stub passes caller arguments over the network to the server skeleton. The skeleton then passes received data to the called object, waits for a response and returns the result to the client stub. Note that there is no direct communication between the caller and the called object.
In more details, the communication consists of several steps:
caller calls a local procedure implemented by the stub
stub marshalls call type and the input arguments into a request message
client stub sends the message over the network to the server and blocks the current execution thread
server skeleton receives the request message from the network
skeleton unpacks call type from the request message and looks up the procedure on the called object
skeleton unmarshalls procedure arguments
skeleton executes the procedure on the called object
called object performs a computation and returns the result
skeleton packs the output arguments into a response message
skeleton sends the message over the network back to the client
client stub receives the response message from the network
stub unpacks output arguments from the message
stub passes output arguments to the caller, releases execution thread and caller then continues in execution
The advantage of this architecture is that neither the caller nor the called object has to implement network related logic.
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