RPC: Remote procedure call

 RPC allows programs to call procedures located on
other machines.
 When a process on machine A calls' a procedure on
machine 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 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.

 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.

 Implementation of RPC involves the five elements of
program:
1. Client
2. Client Stub
3. RPC Runtime
4. Server stub
5. Server
 The client, the client stub, and one instance of RPCRuntime
execute on the client machine.
 The server, the server stub, and one instance of RPCRuntime
execute on the server machine.
 As far as the client is concerned, remote services are
accessed by the user by making ordinary LPC

 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.

 It is responsible for the following two tasks:
◦ On receipt of a call request from the client,
 it packs a specifications of the target procedure and the arguments
into a message and
 asks the local RPC Runtime to send it to the server stub.
◦ On receipt of the result of procedure execution, it unpacks the
result and passes it to the client.

 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.

 The arguments and results in RPC are language-
level data structures, which are transferred in the
form of message data.
 Transfer of data between two computers requires
encoding and decoding of the message data.
 Encoding and decoding of messages in RPC is
known as marshaling & Unmarshaling.

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.

 In the call-by-value method, all parameters are
copied into a message that is transmitted from the
client to the server through the network.
 It is time consuming for passing large data types
such as trees, arrays, etc.

 Most RPC mechanisms does not use the call-by-
reference semantics for parameter passing when
client and the server exist in different address
space.
 Distributed systems having distributed shared-
memory mechanisms can allow passing of
parameters by reference.

 Callback RPC
 Broadcast RPC
 Batch-mode RPC

 It facilitates a peer-to-Peer paradigm among
participating processes.
 It allows a process to be both a client and a
server.
 Remotely processed interactive Application
 Issues
◦ Providing server with clients handle
◦ Making the client process wait for the callback RPC
◦ Handling callback deadlocks

Client Server
Process callback
request and
send reply Reply (result of callback)
Call (parameter list)
Start Procedure execution
Callback (parameter list)
Reply (result of call)
Stop Procedure execution
temporarily
Procedure execution ends
resume Procedure
execution

 A client’s request is broadcast on the network and
is processed by all servers that have the
procedure for processing that request.
 Methods for broadcasting a client’s request:
1. Use of broadcast primitive to indicate that the client’s
request message has to be broadcasted.
2. Declare broadcast ports.
 Back-off algorithm can be used to increase the
time between retransmissions.
◦ It helps in reducing the load on the physical network and
computers involved.

 Batch-mode RPC is used to queue separate RPC
requests in a transmission buffer on the client side
and then send them over the network in one batch
to the server.
◦ It reduces the overhead involved in sending requests and
waiting for a response for each request.
◦ It is efficient for applications requiring lower call rates and
client doesn’t need a reply.
◦ It requires reliable transmission protocol.

RPC: Remote procedure call

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RPC: Remote procedure call