Remote Procedure Call

(RPC)

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Complexity of the distributed applications
 The programming of distributed applications is
difficult. In addition to the usual tasks, programmers
who build clients and servers must deal with the
complex issues of communication.
 Although many of the needed functions are supplied
by a standard API such as the socket interface, the
socket calls require the programmer to specify many
low level details as names ,addresses,protocols and
ports.
 Moreover, asinchronous communications models are
complex to implement.
 Distributed implementations tend to use the same
standard API (e.g.,the socket interface). As a
consequence most of the detailed code found in one
program is replicated in others
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 For example, all client programs that use a connection
oriented transport must create a socket, specify the
server’s endpoint address, open a connection to the
server, send requests,receive responses and close
the connection when interaction is complete.
 Tools (software that generates all or part of a
computer program) have been created to construct
clients and servers.
 Tools cannot eliminate all programming: a
programmer must supply the code that perform the
computation for the particular service. However, a
tool can handle the communication details.
 As a result the code contains fever bugs.

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 RPC = Remote Procedure Call
 The basic model has been proposed by Birrell e Nelson in 1984.
 The essence of this technique is to allow programs on different
machines to interact using simple procedure call/return
semantics, just as if the two programs were in the same
computer
 The popularity of this approach is due to:
 The procedure call is a widely accepted, used and
understood abstraction.
 The use of RPC enables remote interfaces to be specified
as a set of named operations with designed types
 Because a standardized and precisely defined interface is
specified, the communication code for a an application can
be generated automatically

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Local Procedure Call
 Semantic of a procedure call. When a program calls a procedure:
- the parameters of the call are made available to the
procedure -
- the control is transfered to the procedure;
- when the procedure completes its execution, the results
are made available to the program and the control of the CPU is
returned to it.

 When programmers build a program, they begin by dividing the

program into major pieces . A procedure is associated with each
of them.
 If the resulting tasks require a large amount of code, the
programmer subdivides the task, and uses a subprocedure to
perform each of the subtasks.
 The overall form of a program consist of a hierarchy of procedure
calls.

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 Because conventional programming languages do not
allow procedure calls to pass from a program on one
computer across a network to a program on another,
tools are used to help programmers build client-server
software using the procedure call abstraction.
 After deciding which procedures to place in the server
(remote procedures) and which to place in the client
(local procedures), a RPC tool can be used.
 If a programmer follows the same procedure call
paradigm used to build conventional programs when
building client and server software, the programmer
will find the task easier and will make fewer mistakes

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RPC model

 As in conventional procedure calls, when a client calls

a remote procedure, the client will block until a reply is
returned

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Communications Stubs
 Extra software (RPC Tools) must be added to each
part of the program to implement the interaction.
 In the client side the extra software handles the details
of sending a message across the network and waiting
for a response. On the server side, receiving the
message, calling the specified procedure and sending
a response.
 The extra software is called as a communication stub
or proxy.
 If a call is made to a procedure that is not local, a
communication stub intercepts the procedure
call,gathers values for the arguments (marshaling)
and send a message across the network to the
communication stub on the server

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 The communication stub on the server site uses the
conventional procedure call mechanism to invoke the
specified procedure and sends the result back to the
client stub.
 When the client stub receives the response it returns
the result to its caller exactly as if a local procedure
was returning.
 The following figure (a) shows the procedure call in
the original program before RPC stubs are added.
 When the main program call procedure B, the
arguments it passes must agree exactly with the
formal parameters of B.
 Figure (b) shows the communication stubs that must
be added to the program when it is divided into client
and server pieces

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 client server

server
main main
stub
prog prog
for B

proc client
proc
B stub
b
for B

(a) (b)

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 It is important to note that the procedural interfaces in
part b) use the same number and type of arguments
as the original interface in a).
 The call from main to the client stub and the call from
the server stub to procedure B use exactly the same
interface as the conventional call from main to
procedure B.
 Each client stub acts as an exact replacement for one
of the procedures in the original program.
 Client stubs can be given the same name as the
procedure they replace
 As a result , code from the original program need not
be changed

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A remote procedure call occurs in the following steps:
The client procedure calls the client stub in the normal
way (local call).
The client stub builds a message and traps to the local
O.S.
The local O.S. sends the message to the remote O.S.
The remote O.S. gives the message to the server stub.
The server stub unpacks the parameters and calls the
server.
The server does the work and returns the result to the
stub.
The server stub packs it in a message and traps to the
O.S.
The remote O.S. sends the message to the client O.S.
The client’ O.S. gives the message to the client stub.
The stub unpacks the result and returns to the client

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I problemi di RPC
 As long the client and the server machines are identical and all
the parameters and results are scalar types (integer, characters
and boolean) this model works fine.
 However, in a large distributed system, it is common that multiple
machine type are present. Each machine has its own
representation for numbers, characters and others data items.
 Problems can occur with the representation of integers (1s
complent versus 2s complement)and especially with floating
points numbers.
 Some machines, such as the INTEL, number their bytes from rigth
to left(little endian format), whereas others, such as the Sun
SPARC, number them the other way(big endian format).
 A widely accepted method defines a standard external
representation and requires each side to translate between the
external and local representations.

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Stubs creation
 The programmer specifies a set of procedures that
will be remote by giving the interface details(i.e.,
number and type of arguments).
 To do so, the programmer uses the tool’s IDL
(Interface Definition Languages).
 A IDL compiler is used to generate the client and
server stubs in a high level language. (for example, C
language)
 The programmer then compiles and links two separate
programs.The server stubs are combined with the
remote procedures to form the server program. The
client stubs are combined with the main program and
local procewdures to form the client program.

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IDL (DCE)

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Binding a client to a server
 To allow a client to call a server, it is necessary that
the server be registered and prepared to accept
incoming calls.

 Registration of a server makes it possible for a client

to actually locate the server and bind to it. Server
location is made in two steps:
 Locate the server machine
 Locate the server(i.e.,the correct process) on
that machine.

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 To communicate with a server, the client needs to know
an endpoint on the server machine to which to send
messages.
 An endpoint (port) is used by the server O.S. to
distinguish incoming messages for different messages.
 A table of (server-endpoint)-pairs is maintained on each
server machine by a process called the DCE-daemon.
 Before it becomes available for incoming requests, the
server must ask the O.S. for an endpoint. It then
registers this endpoint with the DCE daemon.
 The server also registers with the directory service by
providing it the network address of the server machine
and a name under which the server can be looked up

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 Directory machine

directory
2.Register service
server
3. Look up server
Server machine
Client machine 1.Register
endpoint
5.Do RPC
client server

DCE
4.Ask for endpoint DCE
daemon
daemon
Endpoint
table

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Semantic options
 DCE provides several semantic options.The default is
at-most-once operation. No call is ever carried out
more than once, even in face of system crashes.
 In practice, if a server crashes during an RPC and then
recovery quickly, the client does not repeat the
operation, for fear that it might already have been
carried out once.
 Alternatively, it is possible to mark a remote
procedure as idempotent ( in the IDL file), in which
case it can be repeated multiple times without harm.
For example, reading a specified block from a file can
be tried over and over until it succeeds.

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RMI
 RMI = Remote Method Invocation
 The distributed object systems use RMI that may be
considered as an object oriented extension of the RPC
tool.
 A fundamental characteristic of the OO model is the
separation among the method interface and its
implementation.
 The execution of the methods causes object state
modifications following the interface properties.
 In a distributed system the object interface belong to
one machine and its implementation on the other.

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 Proxy (o stub) = client stub
 Skeleton = server stub
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RMI
 On the client is create only a local proxy of the object
that it is active on a server.
 The client calls the object methods by using the proxy.
 The widely-used object oriented technologies are:
 CORBA (Common Object Request Broker
Architecture): standard aperto, multilinguaggio
 DCOM (Distributed Component Object Model)
 Java RMI:

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Java RMI and OSI

application

presentation

sessione

transport

 Stub and skeleton implement the nertwork

representation level
datalink
 The Remote Reference Layer (RRL)
implements the session level phisic

 The application protocol, based on

TCP/IP, is named IIOP (Internet Inter Orb
Protocol) and derive from CORBA

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RMI and IDL
 Interface language description and authomatic
creation of stub e skeleton are utilized in RMI
 IDL have been extended with object oriented
concepts (interfaces, subtyping, polimorphirsm ecc.)
 IDL example: an object that provides date and hour

module DateTimeApp
{
interface DateTime
{
string getDate();
string getTime();
long getMSecs();
};
};
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