Download Note - Academic Csuohio - Cleveland State University

EEC-681/781
Distributed Computing
Systems
Lecture 5
Wenbing Zhao
Department of Electrical and Computer Engineering
Cleveland State University
[email protected]
2
Outline
• Remote Procedure Call
• Remote Method Invocation
• Case study: Java RMI
– Material taken from
http://java.sun.com/developer/onlineTraining/rmi/RMI.html
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Conventional Procedure Call
count = read(fd, buf, bytes);
Parameter passing in a local
procedure call: the stack
before the call to read()
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The stack while the
called procedure is active
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Parameter Passing in
Procedure Call
• Passing by value
• Passing by reference
0x01400000
buffer
fd = 10;
bytes = 1024;
buf = 0x01400000;
count = read(fd, buf, bytes);
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Read()
fd = 10;
bytes = 1024;
buf = 0x01400000;
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Remote Procedure Call
• Observations:
– Application developers are familiar with simple
procedure model
– Well-engineered procedures operate in isolation
(black box)
– There is no fundamental reason not to execute
procedures on separate machine
• Conclusion: communication between caller &
callee can be hidden by using procedure-call
mechanism
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Client and Server Stubs
• Remote Procedure Call (RPC) achieves
distribution transparency by using a client stub
and a server stub
• The client stub provides the interface of the
procedure call => illusion of a local call interface
– It packs the parameters into a message and requests
that message be sent to the server
• The server stub is a piece of code that
transforms requests coming in over the network
into local procedure calls
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RPC between Client and Server
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Steps of Remote Procedure Call
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Parameter Passing for RPC
• Marshaling: must convert the parameters into a
message
• Marshaling is not trivial:
– Client and server machines may have different data
representations (think of byte ordering)
– Wrapping a parameter means transforming a value into
a sequence of bytes
– Client and server have to agree on the same encoding
– Client and server need to properly interpret messages,
transforming them into machine-dependent
representations
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Passing Value Parameters
Original message
on the Pentium
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The message after The message after being
receipt on the SPARC
inverted. The little
numbers in boxes
indicate the address of
each byte
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Parameter Passing
A procedure
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The corresponding message
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External Data Representation
• Data structures:
– “flattened” on transmission
– rebuilt upon reception
• Primitive data types:
– Byte order (big-endian: MSB comes first)
– ASCII vs UNICODE (2 bytes per character)
– Marshalling/unmarshalling
• To/from agreed external format
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External Data Representation
• XDR (RFC 1832), CDR (CORBA), Java:
– data -> byte stream
– object references
• HTTP/MIME:
– data -> ASCII text
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CORBA CDR Example
index in
sequence of bytes
0–3
4–7
8–11
12–15
16–19
20-23
24–27
4 bytes
notes
on representation
5
length of string
"Smit"
"h___"
‘Smith’
6
"Lond"
length of string
"on__"
1934
‘London’
unsigned long
The flattened form represents a Person struct with value: {‘Smith’, ‘London’, 1934}
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Interface Definition Language
• In order to allow servers to be accessed by
differing clients, Interface Definition Language
(IDL) is usually used to allow various platforms
to call the RPC
– The first popular implementation of RPC on Unix was
Sun's RPC, which was used as the basis for NFS
• From IDL, client and server stubs can be
generated automatically to facilitate
development
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Parameter Passing
• While passing value parameters is relatively
straightforward, passing reference parameters
is difficult
• If we introduce a remote reference mechanism,
access transparency can be enhanced:
– Remote reference offers unified access to remote
data
– Remote references can be passed as parameter in
RPCs
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Asynchronous RPC
The interconnection
between client and server
in a traditional RPC
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The interaction using
asynchronous RPC
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Asynchronous RPC
A client and server interacting through two asynchronous
RPCs. This scheme is also called deferred synchronous RPC
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Remote Distributed Objects
• Data and operations encapsulated in an object
• Operations are implemented as methods, and
are accessible through interfaces
• Object offers only its interface to clients
• Object server (or host server) is responsible for
a collection of objects
• Server skeleton handles (un)marshaling and
object invocation
• Client stub (proxy) implements interface
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Distributed Objects
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Client-to-Object Binding
• Object reference: denotes server, object, and
communication protocol. Having an object
reference allows a client to bind to an object:
– Client loads associated stub code
– Stub is instantiated and initialized for specific object
• Two ways of binding:
– Implicit: Invoke methods directly on the referenced
object
– Explicit: Client must first explicitly bind to object
before invoking it
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Remote Method Invocation Steps
•
•
•
•
Client invokes method at stub
Stub marshals request and sends it to server
Server ensures referenced object is active
Request is unmarshaled by object’s skeleton,
and referenced method is invoked
• Result is marshaled and passed back to client
• Client stub unmarshals reply and passes result
to client application
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RMI: Parameter Passing
• Passing objects by value: A client may
pass a complete object as parameter value:
– An object has to be marshaled:
• Marshall its state
• Marshall its methods, or give a reference to where an
implementation can be found
– Server unmarshals object. Note that we have
now created a copy of the original object
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RMI: Parameter Passing
• Passing objects by reference: Much
easier than in the case of RPC:
– One can simply bind to referenced object, and
invoke methods
– Unbind when referenced object is no longer
needed
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Parameter Passing
• Passing an object by reference or by value
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Java RMI Architecture
• Design goal: to create a Java distributed object
model that integrates naturally into the Java
programming language and the local object model
• The definition of a remote service is coded using a Java
interface
• The implementation of the remote service is coded in a
class
• Interfaces define behavior and classes define
implementation
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Java RMI Architecture Layers
• Stub and Skeleton layer: intercepts method calls made
by the client to the interface and redirects these calls to a
remote RMI service
• Remote reference layer: to interpret and manage
references made from clients to the remote service objects
• Transport layer: provides basic connectivity, as well as
some firewall penetration strategies
RMI
System
Client Program
Server Program
Stubs & Skeletons
Stubs & Skeletons
Remote Reference Layer
Remote Reference Layer
Transport Layer
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Stub and Skeleton Layer
• Java RMI follows the Proxy pattern
– The stub class plays the role of the proxy
– The remote service implementation class plays the role
of the RealSubject
<<Interface>>
Subject
Request()
RealSubject
Proxy
Request()
Request()
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Remote Reference Layer
• Defines and supports the invocation
semantics of the RMI connection
– This layer provides a RemoteRef object that
represents the link to the remote service
implementation object
– The stub objects use the invoke()method in
RemoteRef to forward the method call
– The RemoteRef object understands the
invocation semantics for remote services
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Transport Layer
• The Transport Layer makes network connection
between JVMs
• Java Remote Method Protocol (JRMP): Java
RMI’s wire protocol (proprietary) on top of TCP/IP
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RMI Registry
• RMI Registry (rmiregistry): a simple naming
service that comes with Java RMI
– It runs on each machine that hosts remote service
objects, by default on port 1099
– It accepts registration request only from the local RMI
servers
– It accepts queries for services anywhere
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RMI Registry
• To access the remote server from client side:
– Query a registry by invoking the lookup()method on
the static Naming class
– The method lookup()accepts a URL that specifies
the server host name and the name of the desired
service
• rmi://<host_name>[:<name_service_port>]/<service_name>
– The method returns a remote reference to the service
object
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Steps to Create a Remote Service
• First create a local object that implements that
service
• Next, export that object to Java RMI. When the
object is exported, Java RMI creates a listening
service that waits for clients to connect and
request the service
• After exporting, register the object in the Java
RMI Registry under a public name
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Using Java RMI
• A working Java RMI system is composed of
several parts:
–
–
–
–
–
Interface definitions for the remote services
Implementations of the remote services
Stub and Skeleton files
A server to host the remote services
An RMI Naming service that allows clients to find the
remote services
– A class file provider (an HTTP or FTP server)
– A client program that needs the remote services
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Steps to Build a System
• Design your system
• Write and compile Java code for interfaces
• Write and compile Java code for implementation
classes
• Generate Stub and Skeleton class files from the
implementation classes
• Write Java code for a remote service host program
• Develop Java code for Java RMI client program
• Install and run Java RMI system
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Service Interface
public interface Calculator extends java.rmi.Remote {
public long add(long a, long b) throws
java.rmi.RemoteException;
public long sub(long a, long b) throws
java.rmi.RemoteException;
public long mul(long a, long b) throws
java.rmi.RemoteException;
public long div(long a, long b) throws
java.rmi.RemoteException;
}
To compile it:
> javac Calculator.java
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Implementation of Remote Service
public class CalculatorImpl
extends java.rmi.server.UnicastRemoteObject
implements Calculator {
// Implementations must have an explicit constructor //
in order to declare the RemoteException exception
public CalculatorImpl() throws
java.rmi.RemoteException {
super();
}
public long add(long a, long b)
throws java.rmi.RemoteException {
return a + b;
}
…
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Stubs and Skeletons
• To generate the stub and skeleton files,
invoke the RMI compiler, rmic
• The compiler runs on the remote service
implementation class file
> rmic CalculatorImpl
• After you run rmic you should find the file
CalculatorImpl_Stub.class
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Host Server
• Remote RMI services must be hosted in a server
process
import java.rmi.Naming;
public class CalculatorServer {
public CalculatorServer() {
try {
Calculator c = new CalculatorImpl();
Naming.rebind("rmi://localhost:1099/CalculatorService", c);
} catch (Exception e) {
System.out.println("Trouble: " + e);
}
}
public static void main(String args[]) {
new CalculatorServer();
}
}
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Client
import java.rmi.Naming;
import java.rmi.RemoteException;
import java.net.MalformedURLException;
import java.rmi.NotBoundException;
public class CalculatorClient {
public static void main(String[] args) {
try {
Calculator c = (Calculator) Naming.lookup(
"rmi://localhost/CalculatorService");
System.out.println( c.sub(4, 3) );
System.out.println( c.add(4, 5) );
System.out.println( c.mul(3, 6) );
System.out.println( c.div(9, 3) );
}
…
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Running the RMI System
• Make sure you change to the directory that
contains the classes you have written
• Launch a terminal, start Java RMI registry
> rmiregistry
• Launch another terminal, start the server
> java CalculatorServer
• Launch the 3rd terminal, start the client
> java CalculatorClient
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Parameter Passing in Java RMI
• When Java RMI calls involve passing
parameters or accepting a return value
– How does RMI transfer these between JVMs?
– What semantics are used?
– Does RMI support pass-by-value or pass-byreference?
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Parameter Passing in Java RMI
• Primitive parameters: pass by value
– For both input parameter and return type,
Java RMI makes a copy of a primitive data
type and send it to the destination
• Object parameters: pass by value
– The object to be passed, together with all the
objects it references, are serialized and
copied over
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Parameter Passing in Java RMI
• Remote object parameters: pass by
reference
– A client can obtain a reference to a remote
object through the Java RMI Registry program
– A client can obtain a remote reference as a
result of making a remote method call
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Distributed Garbage Collection in
Java RMI
• Java takes care of memory management
• Distributed garbage collection is needed in Java
RMI
– Many challenges to do so. Most prominent problem is
that a client might quit without notice
– Solution: lease based. The resource is granted to a
client for certain period of time. It is the client’s
responsibility to renew the lease. The resource is
reclaimed if lease is not renewed
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