Distributed Objects and Remote Invocation

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Distributed Objects and Remote Invocation

• Communication between distributed objects
• Remote Procedure Call (RPC)
• Events and notifications
• Java RMI case study

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Introduction

• Middleware
• "Software that provides a programming model above
  the basic building blocks of processes and
  message passing..." Coulouris2001, p166
• Interface
• "...specifies the procedures and the variables
  that can be accessed..." Coulouris2001, p167

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• cannot specify direct access to variables
  (different processes can not access each other's
  data)
• attributes implemented as a pair of
  get-set-operations
• procedure parameters are input, output or both
• in difference to local procedure calls
• no call by value
• no call by reference
• no passing of pointers
• state of the parameter is passed, not the
  parameter or a reference to it

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Communication between distributed objects

• remote method call
• synchronous
• event-based communication
• asynchronous
• transparency
• calling can be transparent, but interfaces should
  express remoteness
• Java RMI

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• distributed garbage collecting
• is hard
• leasing as in Jini
• client ? stub ? channel ? skeleton ? server

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Invocation semanticsFailure models

• Maybe, At-least-once and At-most-once can suffer
  from crash failures when the server containing
  the remote object fails.
• Maybe - if no reply, the client does not know if
  method was executed or not
• omission failures if the invocation or result
  message is lost
• At-least-once - the client gets a result (and the
  method was executed at least once) or an
  exception (no result)
• arbitrary failures. If the invocation message is
  retransmitted, the remote object may execute the
  method more than once, possibly causing wrong
  values to be stored or returned.
• if idempotent operations are used, arbitrary
  failures will not occur
• At-most-once - the client gets a result (and the
  method was executed exactly once) or an exception
  (instead of a result, in which case, the method
  was executed once or not at all)

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Remote Procedure Call (RPC)

• client program
• ? client stub procedure
• marshall request / unmarshall reply
• ? client communication module
• carry out request-reply protocol
• ? communication channel

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• ? server communication module
• carry out request-reply protocol
• ? dispatcher
• choose the server the request belongs to
• choose the method of a server stub (skeleton) the
  request is passed to
• ? server stub procedure (skeleton)
• unmarshall request / call the service procedure /
  marshall reply
• ? service procedure

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Events and Notifications
Event service
Picture like in Coulouris2001
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Events and notifications

• publish-subscribe
• event publisher ? event channel ? event
  subscriber
• pull
• push
• systems are heterogeneous and asynchronous

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• object of interest
• event source, publisher, generator
• event has type and attributes
• objects representing events are called
  notifications
• subscriber
• event sink, listener, subscriber

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• observer
• third-party agent, maybe "channel"
• roles for observers
• filter
• forwarding
• notification mailboxes
• events hold until receiver can receive them
• patterns of events
• a pattern specifies a relationship between events

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• publisher
• either the object of interest or the observer
• quality of service

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Java RMI case study

• just to show how transparent it is
• examples do not compile

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Java RMIas in Coulouris2001
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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• import java.rmi.public interface AcmeService
  extends Remote int foo() throws
  RemoteException

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• import java.rmi.public class
  AcmeServiceServer public static void
  main(String args) System.setSecurityManager(
  new RMISecurityManager()) try
  AcmeService as new AcmeServiceServant()
  Naming.rebind("Acme Service", as) catch
  (Exception e) // Kids, do not do this at
  home! // Always catch as exact //
  exceptions as possible!

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• import java.rmi.import java.rmi.server.Unicast
  RemoteObject
• public class AcmeServiceServant extends
  UnicastRemoteObjectimplements AcmeService
  public AcmeServiceServant() throws
  RemoteException ... public void foo() throws
  RemoteException ...

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• import java.rmi.
• import java.rmi.server.
• public class AcmeServiceClient public static
  void main(String args) System.setSecurityMana
  ger( new RMISecurityManager()) try
  AcmeService as (AcmeService)Naming.lookup(
  "//acme.AcmeService") as.foo() catch
  (RemoteException re)
• catch (Exception e)

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Summaryas in Coulouris2001

• Heterogeneity is an important challenge to
  designers
• Distributed systems must be constructed from a
  variety of different networks, operating systems,
  computer hardware and programming languages.
• The Internet communication protocols mask the
  difference in networksand middleware can deal
  with the other differences.
• External data representation and marshalling
• CORBA marshals data for use by recipients that
  have prior knowledge of the types of its
  components. It uses an IDL specification of the
  data types
• Java serializes data to include information about
  the types of its contents, allowing the recipient
  to reconstruct it. It uses reflection to do
  this.
• RMI
• each object has a (global) remote object
  reference and a remote interface that specifies
  which of its operations can be invoked remotely.
• local method invocations provide exactly-once
  semantics the best RMI can guarantee is
  at-most-once
• Middleware components (proxies, skeletons and
  dispatchers) hide details of marshalling, message
  passing and object location from programmers.

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References

• Coulouris2001
• Distributed Systems - Concepts and Design, 3rd
  edition
• Coulouris, Dollimore, Kindberg
• Addison Wesley 2001