Communication in Distributed Systems:
RPC/RMI
Motivation
❒ Sockets API ≡ send & recv calls ≡ I/O
❒ Remote Procedure Calls (RPC)
❍ Goal: to provide a procedural interface for
distributed (i.e., remote) services
❍ To make distributed nature of service
transparent to the programmer
• No longer considered a good thing
❒ Remote Method Invocation (RMI)
❍ RPC + Object Orientation
❍ Allows objects living in one process to invoke
methods of an object living in another process
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Middleware layers
Applications, services
RMI and RPC
request-reply protocol Middleware
layers
marshalling and external data representation
UDP and TCP
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Conventional Procedure Call
a) Parameter passing in a local procedure call: the stack
before the call to read(fd,buf,bytes)
b) The stack while the called procedure is active
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Remote Procedure Call
❒ Principle of RPC between a client and server program.
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Remote Procedure Calls
❒ Remote procedure call (RPC) abstracts
procedure calls between processes on
networked systems.
❒ Stubs – client-side proxy for the actual
procedure on the server.
❒ The client-side stub locates the server and
marshalls the parameters.
❒ The server-side stub receives this
message, unpacks the marshalled
parameters, and peforms the procedure on
the server.
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Steps of a Remote Procedure Call
1. Client procedure calls client stub in normal way
2. Client stub builds message, calls local OS
3. Client's OS sends message to remote OS
4. Remote OS gives message to server stub
5. Server stub unpacks parameters, calls server
6. Server does work, returns result to the stub
7. Server stub packs it in message, calls local OS
8. Server's OS sends message to client's OS
9. Client's OS gives message to client stub
10. Stub unpacks result, returns to client
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Passing Value Parameters (1)
2-8
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Passing Value Parameters (2)
a) Original message on the Pentium
b) The message after receipt on the SPARC
c) The message after being inverted. The little numbers in boxes
indicate the address of each byte
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Parameter Specification and Stub Generation
a) A procedure
b) The corresponding message.
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Request-reply communication
Client Server
Request
doOperation
message getRequest
select procedure
(wait) Execute procedure
Reply
message sendReply
(continuation)
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Writing a Client and a Server
The steps in writing a client and a server in DCE RPC
(SUN RPC is similar)
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Files interface in Sun XDR
const MAX = 1000; struct readargs {
typedef int FileIdentifier; FileIdentifier f;
typedef int FilePointer; FilePointer position;
typedef int Length; Length length;
struct Data { };
int length;
char buffer[MAX]; program FILEREADWRITE {
}; version VERSION {
struct writeargs { void WRITE(writeargs)=1; 1
FileIdentifier f; Data READ(readargs)=2; 2
FilePointer position; }=2;
Data data; } = 9999;
};
See additional slides on client and server programs
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Binding a Client to a Server
❒ Client-to-server binding in DCE.
2-15
NOTE: In SunRPC, you only have to register the service
(not both service and endpoint). Each host runs its own
binder process called portmapper
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RMI
❒ RMI = RPC + Object-orientation
❍ Java RMI
❍ CORBA
• Middleware that is language-independent
❍ Microsoft DCOM/COM+
❍ SOAP
• RMI on top of HTTP
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Interfaces in distributed systems
❒ Programs organized as a set of modules that
communicate with one another via procedure
calls/method invocations
❒ Explicit interfaces defined for each module in
order to control interactions between modules
❒ In distributed systems, modules can be in
different processes
❒ A remote interface specifies the methods of an
object that are available for invocation by objects
in other processes defining the types of the input
and output arguments of each of them
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Remote and local method invocations
remote local C
E
invocation invocation local
remote
invocation invocation F
B local
A
invocation D
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A remote object and its remote interface
remoteobject
Data
remote
interface
m1 m4
{
implementation m5
m2
of methods m6
m3
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The role of proxy and skeleton in remote
method invocation
client server
remote
object A proxy for B skeleton
object B
Request & dispatcher
for B’s class
Reply
Remote Communication Communication Remote reference
reference module module module module
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Request-reply communication for RPC/RMI
Client Server
Request
doOperation
message getRequest
select object
(wait) execute
Reply method
message sendReply
(continuation)
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Operations of the request-reply protocol
public byte[] doOperation (RemoteObjectRef o, int methodId, byte[] arguments)
sends a request message to the remote object and returns the reply.
The arguments specify the remote object, the method to be invoked and the
arguments of that method.
public byte[] getRequest ();
acquires a client request via the server port.
public void sendReply (byte[] reply, InetAddress clientHost, int clientPort);
sends the reply message reply to the client at its Internet address and port.
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Request-reply message structure
messageType int (0=Request, 1= Reply)
requestId int
objectReference RemoteObjectRef
methodId int or Method
arguments array of bytes
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Request-Reply protocol
❒ Issues in marshalling of parameters and
results
❍ Input, output, Inout parameters
❍ Data representation
❍ Passing pointers? (e.g., call by reference in C)
❒ Distributed object references
❒ Handling failures in request-reply protocol
❍ Partial failure
• Client, Server, Network
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CORBA CDR message
index in notes
sequence of bytes 4 bytes on representation
0–3 5 length of string
4–7 "Smit" ‘Smith’
8–11 "h___"
12–15 6 length of string
16–19 "Lond" ‘London’
20-23 "on__"
24–27 1934 unsigned long
The flattened form represents a Person struct with value: {‘Smith’, ‘London’, 1934}
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Java serialization
Serialized values Explanation
Person 8-byte version number h0 class name, version number
java.lang.String java.lang.String number, type and name of
3 int year
name: place: instance variables
1934 5 Smith 6 London h1 values of instance variables
The true serialized form contains additional type markers; h0 and h1 are handles
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RMI Programming
❒ RMI software
❍ Generated by IDL compiler
❍ Proxy
• Behaves like remote object to clients (invoker)
• Marshals arguments, forwards message to remote object,
unmarshals results, returns results to client
❍ Skeleton
• Server side stub;
• Unmarshals arguments, invokes method, marshals results
and sends to sending proxy’s method
❍ Dispatcher
• Receives the request message from communication module,
passes on the message to the appropriate method in the
skeleton
❒ Server and Client programs
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RMI Programming
❒ Binder
❍ Client programs need a means of obtaining a remote
object reference
❍ Binder is a service that maintains a mapping from textual
names to remote object references
❍ Servers need to register the services they are exporting
with the binder
❍ Java RMIregistry, CORBA Naming service
❒ Server threads
❍ Several choices: thread per object, thread per invocation
❍ Remote method invocations must allow for concurrent
execution
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RPC/RMI systems
❒ RPC systems
❍ SUN RPC
❍ DCE RPC
❒ RMI systems
❍ CORBA
❍ DCOM
❍ Java RMI
❍ SOAP (Simple Object Access Protocol)
• HTTP is request-reply protocol
• XML for data representation
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Java RMI
❒ Features
❍ Integrated with Java language + libraries
• Security, write once run anywhere, multithreaded
• Object orientation
❍ Can pass “behavior”
• Mobile code
• Not possible in CORBA, traditional RPC systems
❍ Distributed Garbage Collection
❍ Remoteness of objects intentionally not
transparent
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Remote Interfaces, Objects, and Methods
❒ Objects become remote by implementing a
remote interface
❍ A remote interface extends the interface
java.rmi.Remote
❍ Each method of the interface declares
java.rmi.RemoteException in its throws clause
in addition to any application-specific clauses
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Creating distributed applications using RMI
1. Define the remote interfaces
2. Implement the remote objects
3. Implement the client (can be done anytime after
remote interfaces have been defined)
4. Register the remote object in the name server
registry
5. Generate the stub and client using rmic
6. Start the registry
7. Start the server
8. Run the client
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Java Remote interfaces Shape and ShapeList
import java.rmi.*;
import java.util.Vector;
public interface Shape extends Remote {
int getVersion() throws RemoteException;
GraphicalObject getAllState() throws RemoteException; 1
}
public interface ShapeList extends Remote {
Shape newShape(GraphicalObject g) throws RemoteException; 2
Vector allShapes() throws RemoteException;
int getVersion() throws RemoteException;
}
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The Naming class of Java RMIregistry
void rebind (String name, Remote obj)
This method is used by a server to register the identifier of a remote
object by name, as shown in Figure 15.13, line 3.
void bind (String name, Remote obj)
This method can alternatively be used by a server to register a remote
object by name, but if the name is already bound to a remote object
reference an exception is thrown.
void unbind (String name, Remote obj)
This method removes a binding.
Remote lookup(String name)
This method is used by clients to look up a remote object by name, as
shown in Figure 15.15 line 1. A remote object reference is returned.
String [] list()
This method returns an array of Strings containing the names bound in
the registry.
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Java class ShapeListServer with main method
import java.rmi.*;
public class ShapeListServer{
public static void main(String args[]){
System.setSecurityManager(new RMISecurityManager());
try{
ShapeList aShapeList = new ShapeListServant(); 1
Naming.rebind("Shape List", aShapeList ); 2
System.out.println("ShapeList server ready");
}catch(Exception e) {
System.out.println("ShapeList server main " + e.getMessage());}
}
}
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Java class ShapeListServant implements interface
ShapeList
import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
import java.util.Vector;
public class ShapeListServant extends UnicastRemoteObject implements ShapeList {
private Vector theList; // contains the list of Shapes 1
private int version;
public ShapeListServant()throws RemoteException{...}
public Shape newShape(GraphicalObject g) throws RemoteException { 2
version++;
Shape s = new ShapeServant( g, version); 3
theList.addElement(s);
return s;
}
public Vector allShapes()throws RemoteException{...}
public int getVersion() throws RemoteException { ... }
}
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Java client of ShapeList
import java.rmi.*;
import java.rmi.server.*;
import java.util.Vector;
public class ShapeListClient{
public static void main(String args[]){
System.setSecurityManager(new RMISecurityManager());
ShapeList aShapeList = null;
try{
aShapeList = (ShapeList) Naming.lookup("//bruno.ShapeList") ; 1
Vector sList = aShapeList.allShapes(); 2
} catch(RemoteException e) {System.out.println(e.getMessage());
}catch(Exception e) {System.out.println("Client: " + e.getMessage());}
}
}
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