Object Spaces

1
Object Spaces
2
History

• Object spaces is a computing paradigm originated
  by Dr. David Gelernter at Yale University. A
  language called Linda was developed to support
  the concept of global object coordination.
• A space is a shared, network-accessible
  repository for objects. Instead of communicating
  with each other, processes coordinate by
  exchanging objects through one or more spaces.

3
ObjectSpace - concept

• A space may be transient, or it may be
  persistent.
• An object may be identified using properties
  lookup if a desired object is not currently
  available, a process can wait until one arrives.
• Objects deposited in a space are passive. Unlike
  other models, processes dont modify objects in
  the space or invoke an objects methods directly.
• To modify an object, a process must explicitly
  remove it, update it, and reinsert it into the
  space.

4
Object spaces
5
Distributed applications using object spaces

• Applications based on object spaces--
• A distributed data structure is composed of
  multiple objects that are stored in one or more
  spaces.
• A distributed protocol defines the way
  participants share and modify these data
  structures in a coordinated way.

6
Distributed applications using object spaces - 2

• Example
• The participants are a set of controllers for
  printers.
• The data structure is a queue of printing tasks
  for multiple printers
• The protocol may be that each participant will
  try to coordinate with each other to avoid
  duplicating the same task and to handle errors.

7
Distributed applications using object spaces - 3

• Example applications
• - chatroom
• auction
• animation production house
• (see JavaSpaces TM Principles, Patterns, and
  PracticeChapter 1--Introduction)

8
JavaSpaces

• Is an implementation which supports the object
  space paradigm.
• Is based on Jini it runs on top of Jini, as a
  service.
• To install the package, you need
• jdk1.2.x
• Jini1_0_1 (jini toolkit)
• Jstk1_0_1 (java space toolkit)

9
JavaSpaces - 2

• To run JavaSpaces, you need an infrastructure in
  place
• An HTTP server, used to download code to
  JavaSpaces clients.
• An RMI Activation Daemon, for managing the states
  of Jini services.
• A Lookup service for Jini service location
  (reggie).
• A JavaSpaces service (outrigger) .
• Optionally, a transaction service (mahalo).

10
JavaSpaces API

• import net.jini.core.lease.Lease
• import net.jini.space.JavaSpace
• // to access a JavaSpace
• JavaSpace space SpaceAccessor.getSpace()
• // to deposit a net.jini.core.entry.Entry object
  into the JavaSpace
• SpaceObject msgnew SpaceObject(SWE622)
• space.write(msg, null, Lease.FOREVER)
• // to read an object from the JavaSpace
• SpaceObject templatenew SpaceObject()
• SpaceObject result (SpaceObject)space.read(
  template, null, Long.MAX_VALUE)
• ref textbook, p. 406

11
Samples

• From JAVASPACES PRINCIPLES, PATTERNS, AND
  PRACTICE
• http//java.sun.com/docs/books/jini/javaspaces/
• Chapter1/HelloWorld, HelloWorld2

12
Advantages of using JavaSpaces(Freeman, et al)

• Simplicity.
• Expressive Using a small set of operations, we
  can build a large class of distributed
  applications without writing a lot of code.
• It supports loosely coupled protocols.
• It eases the burden of writing client/server
  systems the functionalities of the server are
  provided for free, via the spaces in most cases
  only the client code needs to be written.

13
Sources of information

• The Nuts and Bolts of Compiling and Running
  JavaSpaces Programs have links for downloading
  packages and installation instructions.
  http//www.jiniworld.net/document/javaspace/The
  Nuts and Bolts of Compiling and Running
  JavaSpaces(TM).htm
• O'Reilly Network First Contact Is There Life in
  JavaSpace? Apr. 05, 2001