JavaSpacesTM

By Stephan Roorda

Source: JavaSpaces specification

Presentation Outline

• Review of Linda

• Overview of JavaSpaces

• In depth description of JavaSpaces

• Why is JavaSpaces better?

Review of Linda

Linda Basics

• Tuple space is Linda's name for its shared data space– A Tuple is simply a list of fields, separated by

commas and enclosed in parentheses

• A tuple is accessed by specifying its contents– Associative memory model– There is no address associated with a tuple

Tuple Space

Tuple Space

Sender

Sender

Receiver Receiver

Linda Operations

• There are four basic operations:– out

• Generates a data (passive) tuple.

• Each field is evaluated and put into tuple space.

– in • Uses a template to retrieve tuple from tuple space

• Once retrieved, the tuple is taken out of tuple space and is no longer

• If no matching tuple is found process will block. Provides for synchronization between processes.

Linda Templates

• Specifies tuple to retrieve – Consists of sequence of typed fields

• Two kinds of fields – Actuals

• Variables, constants or expression that resolve to constant

– Formals • Holders for data to retrieve

• Preceded by a question mark

• Assigned values of corresponding fields in matched tuple

Matching Templates

• In order for a template to match a tuple: – Have to have the same number of fields– Actuals must have same type, length and values as

those in corresponding tuple fields– Formals in template must match type and length of

corresponding fields in tuple

• If several tuples match the template, impossible to predict which will be selected

• The order of evaluation of fields within a tuple or template is undefined.

Linda Operations

– rd • Uses a template to copy data without removing it from tuple

space.

• Once read, the tuple is still available for others.

• If no matching tuple is found process will block.

– eval • Generates process (active) tuple

• Control is immediately returned to invoking program – Logically, each field is evaluated concurrently, by a separate

process and then placed into tuple space

Quote

Overview of JavaSpaces

Mapping Linda to JavaSpaces

• JavaSpace = Tuple Space

• entry = tuple

• write = out

• take = in

• read = rd

Differences between Linda and JavaSpaces

• Entries in Java are typed as objects – associates behavior with entries

• JavaSpaces allows matching of subtypes– result of having typed entries

• Fields in an entry are objects in Java– systems built with this are object-oriented

Differences

• Support for multiple JavaSpaces– transactions can span multiple threads and

spaces

• Leasing– frees system from garbage left behind from

crashes

• JavaSpaces does not provide eval

JavaSpaces Design Goals

• Provide a simple platform for designing and implementing distributed systems

• Thin clients – simple – quick to download– run on limited local memory

JavaSpaces Design Goals

• Variety of server implementations– relational databases– object oriented databases

• It should be possible to create a replicated JavaSpaces service

Requirements for Application Clients

• Must be possible to write a 100% Pure Java client

• Clients implementation must be independent of the implementation details of the Server

Key features of JavaSpaces

• Spaces are shared– handles the details of concurrent access

• Spaces are persistent– objects can outlive the processes that created them

• Spaces are associative– associative lookup is used to locate objects– this is based on content and not memory location

Key features

• Spaces are transactionally secure– transaction model ensures that an operation on a

space is atomic– supported for one or more spaces

• Spaces allow us to exchange executable content– objects are passive in the space( immutable )– when removed we can change their attributes and

invoke methods on them

Entry

• Collection of typed objects

package net.jini.core.entry;

public interface Entry extends java.io.Serializable {

// this interface is empty

}

Example Entry

import net.jini.core.entry.*;

public class SpaceShip implements Entry {

public Integer score;

public String name;

public MessageEntry() {

}

public SpaceShip( String n, int s ) {

score = s;

name = n;

}

}

JavaSpace Interface

• All of the operations have to be invoked on an object that implements the JavaSpace interface

• Not a remote interface

• Exports objects that implement the JavaSpace interface locally on the client

JavaSpace Interface

package net.jini.space;

<import statements>

public interface JavaSpace {

public final long NO_WAIT = 0;

Lease write( Entry e, Transaction txn, long lease )

Entry read( Entry tmpl, Transaction txn, long timeout )

Entry take( Entry tmpl, Transaction txn, long timeout )

EventRegistration notify( Entry tmpl, Transaction txn,

RemoteEventListener listener, long lease,

MarshalledObject handback )

}

Accessing a JavaSpace

• Space might be registered as a Jini lookup service

• Space might register with an RMI registry

Operations

• Write

• Read

• Take

• Notify

write

• Write the given entry into this JavaSpaces service

public void writeShip( SpaceShip ship ) {

try {

space.write( ship, null, Lease.FOREVER );

}

catch( Exception e ) {

e.printStackTrace();

}

}

read and readIfExists

• Read an entry from the JavaSpaces service that matches the given template

• Passing a null reference for the template will match any Entry

• Multiple read requests may return different Entry objects even if no changes are made to the space in between each

read and readIfExists

public int getScore( String name ) {

SpaceShip template = new SpaceShip();

template.name = name;

try {

SpaceShip ship =

(SpaceShip)space.read( template, null, Long.Max_VALUE );

return ship.score.intValue();

}

catch( Exception e ) {

e.printStackTrace();

return -1;

}

}

Silly Sample

public static void main( String args[] ) {

JavaSpace space = SpaceAccessor.getSpace();

SpaceGame game = new SpaceGame( space );

// create an entry

SpaceShip enterprise = new SpaceShip( “enterprise”, 10 );

// demonstrate read and write

game.writeShip( enterprise );

System.out.println(enterprise.name + “ written into space”);

System.out.println(“The “ + enterprise.name + “’s score is “

+ game.getScore(“enterprise”) );

}

take and takeIfExists

• Same as Read operations, except that the entry is removed from the space

• Will never return copies of the same Entry

take and Exceptions

• RemoteException - may or may not have been successful

• UnusableEntryException - removes the unusable entry from the space

• Any other exception - take did not occur and no entry was removed from the space

notify

• Notify a specific object when entries that match the given template are written into this JavaSpaces service

• A lease time is given which is how long you want the registration to be remembered by the server

Events in Java

• Event Source

• Event Object

• Event Listener

Distributed Events in JavaSpaces

• Events might have to travel from one JVM to another over a network

• Events may arrive:– multiple times– out of order– not at all

• Programmer’s responsibility to ensure correctness

Event notification in JavaSpaces

Notification Example

public static void main( String args[] ) {

JavaSpace space = SpaceAccessor.getSpace();

Listener listener = new Listener( space );

Message template = new Message();

space.notify(template, null, listener, Lease.FOREVER, null);

Message msg = new Message();

msg.content = “Hello World”;

space.write( msg, null, Lease.FOREVER );

}

Notification Example

public class Listener implement RemoteEventListener {

private JavaSpace space;

public Listener( JavaSpace space ) throws RemoteException {

this.space = space;

UnicastRemoteObject.exportObject( this );

}

public void notify( RemoteEvent ev ) {

Message result =

(Message)space.read( template, null, Long.MAX_VALUE );

System.out.println( result.content );

}

}

notify and Transactions

• Transactions can be:– null– non - null

notify and Transactions

• entries that are written and taken in the same transaction - before a commit - listeners will not be notified that are registered under a null transaction

• server retries until the notification request’s lease expires

• notifications may be delivered in any order

Operation Ordering

• operations on a space are unordered• Example: if T and U are 2 threads. T performs a

write and U performs a read with a template that matches the written entry, the read may not find the written entry even if the write returns before the read.

• The only way to guarantee this is if the threads work together and that is independent of JavaSpaces

Transactions

• uses net.jini.core.transaction to group operations into a bundle that act as a single transaction

• either all operations within the transaction complete or none do

• null - performs as if a transaction was created just for that operation

Benefits of JavaSpaces

• Simple

• Expressive

• Supports loosely coupled protocols

• Eases the implementation of client/server systems

Applications of JavaSpaces

• Any system/problem that needs a distributed solution– Human Genome project– Cryptography– Rendering– Chat program– Auction server( such as e-bay, amazon, etc )

Examples

Conclusion

• Simple to learn

• Easy to understand

• Map a lot of problems

• fairly new - not many “real-world” uses yet

• best implementation of Linda yet

References and Sources

• JavaSpaces Principles, Patterns, and Practice by Freeman, Hupfer, Arnold

• Official JavaSpaces specification

• visit http://www.cs.rit.edu/~sjr1521 for full links and bibliography