Remote Method invocation, Lecture notes of Distributed Programming and Computing

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OSI Layered Architecture

Remote

Procedure

Calls

Introduction

 It can be said as the special case of message-passing

model.

 It has become widely accepted because of the

following features:

 (^) Simple call syntax and similarity to local procedure calls.  (^) Its ease of use, efficiency and generality.  (^) It can be used as an IPC mechanism between  (^) processes on different machines and  (^) also between different processes on the same machine.

RPC Model

 It is similar to commonly used procedure call model.

It works in the following manner:

1. For making a procedure call, the caller places arguments to the procedure in some well specified location.
2. Control is then transferred to the sequence of instructions that constitutes the body of the procedure.
3. The procedure body is executed in a newly created execution environment that includes copies of the arguments given in the calling instruction.
4. After the procedure execution is over, control returns to the calling point, returning a result.

Cont…

 (^) The RPC enables a call to be made to a procedure that does not reside in the address space of the calling process.  (^) Since the caller and the callee processes have disjoint address space, the remote procedure has no access to data and variables of the callers environment.  (^) RPC facility uses a message-passing scheme for information exchange between the caller and the callee processes.  (^) On arrival of request message, the server process  (^) extracts the procedure’s parameters,  (^) computes the result,  (^) sends a reply message, and  (^) then awaits the next call message.

Cont…

 Only one of the two processes is active at any given

time.

 It is not always necessary that the caller gets blocked.

 There can be RPC implementations depending on the

parallelism of the caller and the callee’s environment.

 The RPC could be asynchronous, so that the client

may do useful work while waiting for the reply from

the server.

 Server could create a thread to process an incoming

request so that server is free to receive other requests.

Cont…

 (^) Difference between remote procedure calls and local procedure calls:

1. Unlike local procedure calls, with remote procedure calls,  (^) Disjoint Address Space  (^) Absence of shared memory.  (^) Meaningless making call by reference, using addresses in arguments and pointers.
2. RPC’s are more vulnerable to failure because of:  (^) Possibility of processor crashes or  (^) communication problems of a network.
3. RPC’s are much more time consuming than LPC’s due to the involvement of communication network.

 (^) Stubs  (^) Provide a normal / local procedure call abstraction by concealing the underlying RPC mechanism.  (^) A separate stub procedure is associated with both the client and server processes.  (^) To hide the underlying communication network, RPC

communication package known as RPC Runtime is used

on both the sides.

RPCRuntime

 It handles transmission of messages across the

network between Client and the server machine.

 It is responsible for

 (^) Retransmission,  (^) Acknowledgement,  (^) Routing and  (^) Encryption.

Server Stub

 It is responsible for the following two tasks:

 (^) On receipt of a call request message from the local RPCRuntime, it unpacks it and makes a perfectly normal call to invoke the appropriate procedure in the server.  (^) On receipt of the result of procedure execution from the server, it unpacks the result into a message and then asks the local RPCRuntime to send it to the client stub.

Working

To summarize, a remote procedure call occurs in the following steps:  (^) 1. The client procedure calls the client stub in the normal way.  (^) 2. The client stub builds a message and calls the local operating system.  (^) 3. The client's as sends the message to the remote as.  (^) 4. The remote as gives the message to the server stub.  (^) 5. The server stub unpacks the parameters and calls the server.

 6. The server does the work and returns the

result to the stub.

 7. The server stub packs it in a message and

calls its local as.

 8. The server's as sends the message to the

client's as.

 9. The client's as gives the message to the

client stub.

 10. The stub unpacks the result and returns

to the client.