Transcript Distributed Objects and Remote Invocation

CSS434 Distributed Objects and
Remote Invocation
Textbook Ch5
Professor: Munehiro Fukuda
CSS434 RMI
1
Outline


RPC/RMI Model
Implementation Issues






Parameter-passing semantics
Invocation semantics
Stateless/stateful server
Server creation semantics
SunRPC
Java RMI
CSS434 RMI
2
Why RPC/RMI
Calling a function/method at a remote server:
 Advanced form of communication



Transparency in function calls



Sockets and MPI: data transfer
RPC/RMI: control transfer
No distinguish between local and remote calls (in
theoretical)
Also applied to IPC on the same machine
Ease of use


Compiled-time argument type checking
Automatic stub generation
CSS434 RMI
3
Remote and Local
Method/Function Invocation
local
remote
invocation
A
B
C
local E
invocation
invocation
local
invocation
D
CSS434 RMI
remote
invocation
F
4
Service Interface and Remote
Interface
RPC: an service interface defined with an XDR file
struct doubles { double a; double b; };
program EXAMPLE_PROG {
version EXAMPLE_VERS {
int
FACT( int )
= 1;
double POWER( doubles )
= 2;
} = 1;
} = 0x31234567;
/*
/*
/*
/*
procedure number
procedure number
version number =
program number =
= 1 */
= 2 */
1 */
0x31234567 */
remoteobject
remote
interface
{
Data
m
m2
1
m3
implementation
of methods
m4
m5
m6
RMI: a remote interface defined by extending the “Remote” interface
Interface MyRmoteInterface extends
ReturnClass1 m1( ArgClass1 arg )
ReturnClass2 m2( ArgClass2 arg )
ReturnClass3 m3( ArgClass3 arg )
}
Remote
throws
throws
throws
{
RemoteException;
RemoteException;
RemoteException;
CSS434 RMI
5
RPC/RMI Model
Caller (Client)
RPC and wait
Callee (Server)
Request message
including arguments
Suspended
Reply message
Including a return value
Request message accepted
Execution environment created
Execution of function body
Send reply and wait for the next
request
Resume execution
CSS434 RMI
6
Implementation Issues

Transparency property



Analogy in semantics b/w local and remote procedure calls




Syntactic transparency
Semantic transparency
Caller capable of passing arguments (Automatic marshalling)
Caller suspended till a return from a function
Callee capable of returning a value to caller
Difference in semantics b/w local and remote procedure calls



No call by reference and no pointer-involved arguments
Error handling required for communication (Ex. RemoteException in
Java)
Performance much slower than local calls.
CSS434 RMI
7
Parameter-Passing Semantics

Client
Server

class objA {
objB b;
objC c;
}

class objB {
}
class objC {
}
Call by Value

Most PRCs take this semantics.

Voluminous data incurs copying overhead.
Call by Reference

Passing pointers and references are
meaningless.

Then, how about object-based systems?
 The value of a variable is a reference
to an object
Call by object reference

Additional remote object invocations

Call by visit: all related objects moved
to a server every RPC.

Call by move: all related objects moved
and left to a server upon the first RPC.
CSS434 RMI
8
Invocation Semantics
Fault tolerance measures
Retransmit request
message
Duplicate
filtering
Invocation
semantics
Re-execute procedure
or retransmit reply
No
Not applicable
Not applicable
Yes
No
Re-execute procedure
At-least-once
Yes
Yes
Retransmit reply
At-most-once
CSS434 RMI
Maybe
9
Stateful/Stateless Servers

Stateful servers:






Servers keep track of client information.
RPC/RMI reply depends on that client information.
Pros: Simplify client design
Cons: A server crash loses client information. A client crash leaves
old client information at server.
At-most-once invocation semantics (Java RMI takes this design.)
Stateless servers:



Clients must maintain Inter-call information.
RPC/RMI reply is always the same.
At-least-once invocation semantics (Some RPC implementations
take this design.)
CSS434 RMI
10
Server Creation Semantics

Instance-per-Call Servers




Instance-per-Session Servers




A server process maintained for the entire session with a client
Stateful servers: inter-call state maintained for a single client
OS resource allocation/de-allocation involved every session
Persistent Servers




A new server process launched every call
Statelss servers only
OS resource allocation/de-allocation involved every call
A server process remains in existence indefinitely.
Stateful and shared servers: concurrency control required
OS resource allocation/de-allocation involved only once.
SunRPC/Java RMI take instance-per-call servers, while a thread but not
a process is created per a call. (The server process must remain active
to accept a new call and thus can still be stateful.)
CSS434 RMI
11
RPC Mechanism
Client Program
Return
Call
Interface Definition Language
File
Define arguments
Register remote functions
Server Program
Return
IDL Compiler
Server Stub
Client Stub
(5) Exception?
Message
(4) Invalid arguments?
Decoding Encoding
(3) Invalid procedure?
Message
marshaling
Decoding Encoding
RPC Runtime
Receive
Send
Retransmission
acknowledgments
Routing
encryption
args
PRC id client id
Call
RPC Runtime (Dispatcher)
(2) Unauthorized client?
Receive
Send (1) Intelligible messages?
type
(call)
msg id
msg id
CSS434 RMI
reply
type
(reply) status
fesults
failur
12
Client-Server Binding
(SunRPC)
Server Machine
Client Machine
(2) Locating server
Client
(3) Server port (xxx)
(4) RPC with port xxx
(1) register
Transport level or below
port: 111
port: xxx
Server
write(), read(),
Sendto(), recvfrom()
TCP or UDP
Portmap
Daemon
write(), read(),
Sendto(), recvfrom()
TCP or UDP
Network (LAN/WAN)



(1) pmap_set(prognum, versnum, protocol, port)
(2) and (3) pmap_getport(addr, prognum, versnum, protocol)
To check the status of portmap, rpcinfo
CSS434 RMI
13
SunRPC
Modify by yourself
example_client.c
rpcgen –a example.x
example_client.o
example_client
Client program
Your client
example_clnt.o
example_clnt.c
Client stub
example_h.c
example.x
Interface
descriptions
gcc –c -o
Header
ld -o
example_xdr.c
example_xdr.o
Marshalling
example_svc.o
example_svc.c
Server program
Server stub
example_server.o
example_server.c
Your server
example_server
Modify by yourself
CSS434 RMI
14
Sun RPC (Interface definition)
/*
* example.x - Speicification of some arithmetic/string service.
* Define 2 procedures:
*
fact( int n ) returns n!.
*
power( double x, double y ) returns x^^y.
*
strconc( struct strings ) concatenates src to dest.
*/
const BUFFER_SIZE = 1024;
struct doubles {
double a;
double b;
};
struct strings {
char src[BUFFER_SIZE];
char dst[BUFFER_SIZE];
};
program EXAMPLE_PROG {
version EXAMPLE_VERS {
int
FACT( int )
= 1;
double POWER( doubles )
= 2;
string STRCONC( strings ) = 3;
} = 1;
} = 0x31234567;
/* procedure number = 1 */
/* procedure number = 2 */
/* procedure number = 3 */
/* version number = 1 */
/* program number = 0x31234567 */
CSS434 RMI
15
Sun RPC (Client)
#include "example.h"
void example_prog_1(char *host)
{
CLIENT *clnt;
int *result_1;
int fact_1_arg;
double *result_2;
doubles power_1_arg;
char * *result_3;
strings strconc_1_arg;
clnt = clnt_create (host, EXAMPLE_PROG, EXAMPLE_VERS, "udp");
if (clnt == NULL) {
clnt_pcreateerror (host);
exit (1);
}
fact_1_arg = 10;
result_1 = fact_1(&fact_1_arg, clnt);
if (result_1 == (int *) NULL) {
clnt_perror (clnt, "call failed");
}
printf( "fact( 10 ) = %d\n", *result_1 );
}
int
main (int argc, char *argv[])
{
char *host;
exit (0);
}
if (argc < 2) {
exit (1);
host = argv[1];
example_prog_1 (host);
power_1_arg.a = 2.0;
power_1_arg.b = 6.0;
result_2 = power_1(&power_1_arg, clnt);
if (result_2 == (double *) NULL) {
clnt_perror (clnt, "call failed");
}
printf( "power( 2.0, 6.0 ) = %f\n", *result_2 );
strncpy( strconc_1_arg.dst, "xyz\0", BUFFER_SIZE );
strncpy( strconc_1_arg.src, "abc\0", BUFFER_SIZE );
result_3 = strconc_1(&strconc_1_arg, clnt);
if (result_3 == (char **) NULL) {
clnt_perror (clnt, "call failed");
}
printf( "strconc( \"xyz\", \"abc\" ) = %s\n", *result_3 );
clnt_destroy (clnt);
CSS434 RMI
16
Sun RPC (Server)
#include "example.h"
#include <math.h>
#include <string.h>
int *
fact_1_svc(int *argp, struct svc_req *rqstp)
{
static int result;
int i;
result = 1;
for ( i = *argp; i > 0; i-- )
result *= i;
}
return &result;
double *
power_1_svc(doubles *argp, struct svc_req *rqstp)
{
static double result;
result = pow( argp->a, argp->b );
}
return &result;
char **
strconc_1_svc(strings *argp, struct svc_req *rqstp)
{
static char * result;
result = strcat( argp->dst, argp->src );
}
return &result;
CSS434 RMI
17
SunRPC v.s. Java RMI
Advantages
SunRPC
Java RMI
Template generation
Dynamic port assignment
High performance
Automatic serialization
Manual IP port handling
Good security
Disadvantages Manual serialization
Weak to hackers
CSS434 RMI
Low performance
18
RMI Mechanism
Convert object/method/arguments in a TCP
message.
Find the corresponding remote object,
unmarshal arguments, and invoke the object.
server
client
object A proxy for B
Request
skeleton
& dispatcher
for B’s class
remote
object B
Reply
Communication
Remote
reference module
module
Communication Remote reference
module
module
Exchange a request and a replay in TCP.
Maintain a table of local objects and their
remote object references (i.e., object proxies).
CSS434 RMI
19
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.
CSS434 RMI
20
RMI
Programming Procedure
(4) Program a Client.java class
(5) javac Client.java
(8) Run Client with java Client
Application Layer:
Client .java
(2) Program a Server.java class
(7) Run Server with java Server
Server.java
(implements remote interface)
(1) Define a remote interface
Stub/Skeleton:
Stub
(3) javac Server.java
rmic Server
Skeleton
Server_Stub.class Server_Skel.class
(6) Invoke a rmiregistry
Remote Reference:
rmiregistry [port#]
(object manager/name service)
request and result
Transport Layer:
TCP/IP
CSS434 RMI
21
RMI
Remote Interface & Return Object
// Remote interface: A server must implements this interface in its class define
import java.rmi.*
public interface ServerInterface extends Remote {
public ReturnObj get( ) throws RemoteException; // A server returns a ReturnObj to a client
}
// Define the class of a object returned from a server to a client (ReturnObj)
import java.io.*
import java.util.*
public class ReturnObj implements Serializable { // A return object must be serializable.
private id;
SubObject subObj;
public ReturnObj( int i, SubObject s ) {
id = i;
subObj = s;
}
public void print( ) throws IOException { System.out.println( “ReturnObj: id=“ + id ); }
}
CSS434 RMI
22
RMI
Server Implementation
import java.io.*;
import java.util.*;
import java.rmi.*;
import java.rmi.server.*;
(1) Implement a remote interface
public class Server extends UnicastRemoteObject implements ServerInterface {
static private int i = 0; (2) Inherit a RemoteObject
public Server( ) throws RemoteException{
(3) Define a constructor
}
public static void main( String args[] ) {
try {
Server server = new Server( ); (4) Instantiate a RemoteObject
Naming.rebind( “server”, server );
} catch ( Exception e ) {
System.err.println( “Exception: “ + e );
System.exit( 1 );
}
(5) Implement all methods declared in a remote interface
}
public ReturnObject get( ) throws RemoteException {
ReturnObject f = new ReturnObject( i++, new SubObject( i ) );
return f;
}
}
CSS434 RMI
23
RMI
Client Implementation
Import java.rmi.*;
Import java.io.*;
public class Client {
public static void main( String[] args ) {
try {
(1) RMI registry returns a reference to RemoteObject
ServerInterface si = (ServerInterface)Naming.lookup( “rmi://adonis1/server” );
ReturnObject f = si.get( ); (2) Call a method of this RemoteObject
f.print( );
f.subObj.print( );
} catch ( RemoteException re ) { (3) Exception handling for RMI
System.out.println( “Exception : “ + re );
System.exit( 1 );
} catch ( IOException ie ) { (4) Exception handling for I/O
System.out.println( “Exception : “ + ie );
System.exit( 1 );
} catch ( NotBoundException nbe ) { (5) Exception handling for Naming
System.out.println( “Exception : “ + nbe );
System.exit( 1 );
}
}
}
CSS434 RMI
24
RMI
Compilation and Execution
% javac ReturnObject.java
// Compile a return class
% javac Server.java
// Compile a server class
% rmic Server
// Create a stub and a skelton
% javac Client.java
// Compile a client class
% ls
ReturnObject.java ReturnObject.class Server.java
Server.class Server_Stub.class Server_Skel.class
% rmiregistry&
// Invoke RMI registory
% java Server&
// Invoke a server
% java Client
// Invoke a client
ReturnObject id = 0
SubObject id = 1
% java Client
// Invoke a client again
ReturnObject id = 2
SubObject id = 3
%
CSS434 RMI
25
Exercises (No turn-in)
1.
2.
3.
4.
5.
6.
The caller process of an RPC/RMI must wait for a reply from the callee process after
making a call. Explain how this can actually be done.
Which types of server did you implement for the programming assignments 3 and 4, a
stateful or a stateless server? Then, why did you implement such a type of server?
Discuss the similarities and differences among the following parameter passing:
1.
Call-by-object-reference
2.
Call-by-move
3.
Call-by-visit
Discuss about the pros and the cons of dynamic binding in RPC.
Textbook p227, Q5.12: Discuss the invocation semantics that can be achieved when the
request-reply protocol is implemented over a TCP connection, which guarantees that data
is delivered in the order sent, without loss or duplication. Take into account all of the
conditions causing a connection to be broken.
Textbook p227, Q5.14: The Election service must ensure that a vote is recorded whenever
any user thinks they have cast a vote. Discuss the effect of maybe call semantics on the
Election service. Would at-least-once call semantics be acceptable for the Election service
or would you recommend at-most-once call semantics?
CSS434 RMI
26
Exercises (No turn-in)
7.
8.
Textbook p227, Q5.15: A request-reply protocol is implemented over a communication
service with omission failures to provide at-least-once RMI invocation semantics. In the
first case the implementor assumes an asynchronous distributed system. In the second
case the implementor assumes that the maximum time for the communication and the
execution of a remote method is T. In what way does the latter assumption simplify the
implementation?
Textbook p228, Q5.22: A client makes remote procedure calls to a server. The client takes
5 milliseconds to compute the arguments for each request, and the server takes 10
milliseconds to process each request. The local operating system processing time for each
send or receive operation is 0.5 milliseconds, and the network time to transmit each
request or reply message is 3 milliseconds. Marshalling or unmarshalling takes 0.5
milliseconds per message.
Calculate the time take by the client to generate and return from two requests:
(i) if it is single-threaded, and
(ii) if it has two threads that can make requests concurrently on a single processor.
You can ignore context-switching times.
CSS434 RMI
27