Remote Procedure Call (RPC) Mechanism, Slides of Computer Networks

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 RPC allows programs to call procedures located on other machines.  When a process on machine A calls' a procedure onmachine^ B,^ the^ calling^ process^ on^ A^ is^ suspended,^ and^ execution^ of the called procedure takes place on B. Information can be transported from the caller to the callee in the parameters and can come back in the procedure result.  No message passing at all is visible to the programmer. This method is known as Remote Procedure Call, or often just RPC.

 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.

 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.

 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.

 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.

Stub ◦ (^) 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.

 It handles transmission of messages across the network

between Client and the server machine.

 It is responsible for

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

 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.

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.