Distributed Information Systems
Distributed Objects and Remote Invocation
• Conventional models
– Procedure call.
• Objectives
– RPC and RMI.
– Remote invocation semantics.
– Implementation of RMI.
• References
– DSCD: Chapter 5
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– Method invocation in
OO programming.
• Extension to DS
– Remote procedure
call (RPC).
– Remote method
invocation (RMI).
– Event-based
programming.
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Middleware Layer
• Provide location transparency.
• Provide independence from details of communication
protocols, OS, and hardware.
• Support components in different programming
languages.
Applications
RMI, RPC and events
Request reply protocol
Programming Models For Distributed Applications
Middleware
layers
External data representation
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– Distributed eventbased programs.
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Interface b/w Program Modules In DS
• Parameter-passing mechanisms for local procedure
calls do not work.
– Ex: Pointer argument in C function
void insert(float *farray, float num);
– Implication to parameter-passing in DS?
• Service interface in RPC: a server provides a set of
procedures available to clients.
• Remote interface in RMI: specifies methods of an
object available for remote invocation.
– A subset of methods available to local objects.
Operating System
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Distributed Information Systems
Remote Invocation Semantics
Remote Object References
• When a client invokes a method in a remote object, a
message is directed to the object.
– Object at different servers and providing different
services may be named identically.
– A remote object may be deleted and re-created at
a different state.
– How to direct message to the right object?
• A remote object reference must be unique
throughout DS and over time.
– Host IP addr + port number + creation time
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Remote Invocation Semantics
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Remote Invocation Semantics
• At-least-once semantics
– Request is retransmitted on timeout and each
received request is executed.
– Invoker receives either a result or an exception.
– What does invoker know upon receiving result?
– What does it know upon receiving exception?
– Useful with idempotent operations.
– Ex: Adopted by Sun RPC.
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• As middleware, the semantics of a RMI service
should be known to the applications that use it.
• Maybe semantics
– No request retransmission.
– Invoker receives a result or a timeout exception.
– What does invoker know upon receiving result?
– What does invoker know upon receiving timeout?
– Useful when occasional failure is acceptable.
– Ex: A update request in a series.
– Ex: Allowed by CORBA for some methods.
• At-most-once semantics
– Request retransmission, duplicate request deletion,
and history are applied.
– Invoker receives either a result or an exception.
– What does invoker know upon receiving result?
– What does invoker know after receiving exception?
– Ex: Adopted by Java RMI and CORBA.
• Summery of alternative remote invocation semantics.
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Distributed Information Systems
RMI Transparency
• Transparent RMI
– Same syntax for local & remote procedure calls.
– Locating and contacting remote objects, marshalling
and message passing are hidden.
• Issues
– RMI is more vulnerable than local invocation.
– Latency difference b/w RMI and local invocation
• Current consensus: keep invocation syntax the same
but invocation interface different.
– Ex: Java RMI.
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Develop Java RMI Server And Client
1.
2.
3.
4.
Remote Invocation In Java RMI
• Ex: Computing a product through a remote
calculator service.
– Client side:
Calc c = (Calc) Naming.lookup(“//bayes.cis/CalcServ”);
result = c.multiply(numb1, numb2);
– Any difference from local invocation?
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Implementation Of RMI
Define a remote interface (Calculator.java).
Implement the remote service (CalculatorImpl.java).
Host the service (CalculatorServer.java).
Use the service in a client (CalculatorClient.java).
• Several objects and modules are involved in a RMI.
server
client
object A proxy for B
skeleton
Request
remote
object B
for B
Reply
Communication
Remote
reference module
module
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Communication
module
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Remote reference
module
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Distributed Information Systems
Implementation Of RMI
Implementation Of RMI
• Proxy (stub): behaves like a local ‘server’ to the client
object.
– One for each remote object.
– Request marshalling, request passing, and reply
unmarshalling.
• Skeleton: behaves like a local ‘client’ to the server
object.
– Receiving request from communication module,
request unmarshalling, remote (local) method
invocation, reply marshalling, and reply passing.
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Implementation Of RMI
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Remote Object Registration
• Communication module
– Two cooperative modules transmit request/reply.
– Use sender id, request id, and remote object
reference info to provide a specified remote
invocation semantics.
– Interact with proxy object at client, and skeleton
object at server.
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• Generation of proxy and skeleton
– Their classes are generated automatically by an
interface compiler (e.g., java rmic).
– What information is the generation based on?
• Remote reference module: maintains remote object
table for translation b/w local and remote object
reference (ROR).
– At client: RORs ↔ proxies
– At server: ROR ↔ remote (local) object
– Used by proxy and skeleton.
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• Where does a client find the remote object reference
for a remote service?
• Binder: A service that maintains mapping b/w remote
object names (a string) and their remote object
reference.
• Upon creation, a remote object is registered at the
binder.
• A client can look up a service from binder using
remote object name to obtain the remote object
reference.
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Distributed Information Systems
Java Binder
RMI Compilation And Execution
• The binder program rmiregistry is supplied with jdk.
• Each host has at least one rmiregistry running.
• Server uses methods in class Naming to register a
remote object.
• Ex: Registering a calculator service.
Calc c = new CalcImpl();
Naming.rebind(“//bayes.cis:1099/CalcServ”, c);
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RMI Compilation And Execution
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Obtain Stub for Client
• Server side
– Compile 1,2 & 3 with javac.
– Create stub/skeleton: rmic CalculatorImpl
– Start RMI registry: rmiregistry &
– Start server: java CalculatorServer &
• Client side
– Compile 1,4 with javac.
– Obtain CalculatorImpl_Stub.class.
– Run client: java CalculatorClient
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• Components:
1. Remote interface: Calculator.java
2. Service implementation: CalculatorImpl.java
3. Server: CalculatorServer.java
4. Client: CalculatorClient.java
• How does a client developer obtain stub?
• Option 1: run rmic from service implementation.
– A client developer may not have the service
implementation code.
• Option 2: download from server’s public site.
– Downloading adds an extra step in client
development.
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Distributed Information Systems
Dynamic Stub Loading
Dynamic Stub Loading
Idea: Client obtains stub at runtime from a URL.
java -Djava.security.policy=policy_file CalculatorClient
1. Place the stub class file at a world-readable,
published location called the codebase.
2. Start server with codebase info:
java -Djava.security.policy=policy_file
-Djava.rmi.server.codebase=codebase_url
CalculatorServer &
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3. When server registers, registry saves codebase_url.
4. Start client as follows:
5. If client cannot find stub in its classpath, it uses
codebase_url from registry and downloads stub
from there.
6. From then on, client interacts with server through
the stub.
• Levels of sophistication in RMI deployment.
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On Demand Remote Object Creation
On Demand Remote Object Activation
• Idea: To reduce resource consumption, an object is
not created until needed.
• A server initializes and registers one remote (factory)
object T.
• A client invokes a remote method on T in order to
obtain a remote reference to another object Q.
• If Q does not exist yet, T creates it on demand.
• T returns to client the remote reference for Q.
• The client invokes a remote method on Q.
• Advantages?
• Idea: To reduce resource consumption, an object is
not activated until needed.
• Active: a remote object is instantiated and available
for invocation in a running process.
• Passive: a remote object is not instantiated but can be
made active.
• A passive remote object consists of two parts:
– the implementation of its methods, and
– its state in marshaled form.
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Distributed Information Systems
On Demand Remote Object Activation
• Activator: a process that starts server processes to
host remote objects.
– Map activation id to info on passive object.
– Start a server process which creates a new instance
of the passive object class and initializes instance
variables to the stored state.
– Java RMI usually uses one activator per host.
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