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JavaSpacesTMBy Stephan Roorda Source: JavaSpaces specification |
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Presentation OutlineReview of Linda Overview of JavaSpaces In depth description of JavaSpaces Why is JavaSpaces better? |
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Review of Linda |
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Linda BasicsTuple 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 |
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Tuple SpaceSender Sender Tuple Space Receiver Receiver |
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Linda OperationsThere 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. |
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Linda TemplatesSpecifies 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 |
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Matching TemplatesIn 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. |
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Linda Operationsrd 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 |
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Quote |
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Overview of JavaSpaces |
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Mapping Linda to JavaSpacesJavaSpace = Tuple Space entry = tuple write = out take = in read = rd |
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Differences between Linda and JavaSpacesEntries 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 |
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DifferencesSupport for multiple JavaSpaces transactions can span multiple threads and spaces Leasing frees system from garbage left behind from crashes JavaSpaces does not provide eval |
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JavaSpaces Design GoalsProvide a simple platform for designing and implementing distributed systems Thin clients simple quick to download run on limited local memory |
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JavaSpaces Design GoalsVariety of server implementations relational databases object oriented databases It should be possible to create a replicated JavaSpaces service |
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Requirements for Application ClientsMust be possible to write a 100% Pure Java client Clients implementation must be independent of the implementation details of the Server |
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Key features of JavaSpacesSpaces 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 |
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Key featuresSpaces 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 |
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EntryCollection of typed objects package net.jini.core.entry; public interface Entry extends java.io.Serializable { // this interface is empty } |
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Example Entryimport 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; } } |
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JavaSpace InterfaceAll 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 |
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JavaSpace Interfacepackage 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 ) } |
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Accessing a JavaSpaceSpace might be registered as a Jini lookup service Space might register with an RMI registry |
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OperationsWrite Read Take Notify |
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writeWrite the given entry into this JavaSpaces service public void writeShip( SpaceShip ship ) { try { space.write( ship, null, Lease.FOREVER ); } catch( Exception e ) { e.printStackTrace(); } } |
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read and readIfExistsRead 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 |
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read and readIfExistspublic 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; } } |
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Silly Samplepublic 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”) ); } |
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take and takeIfExistsSame as Read operations, except that the entry is removed from the space Will never return copies of the same Entry |
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take and ExceptionsRemoteException - 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 |
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notifyNotify 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 |
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Events in JavaEvent Source Event Object Event Listener |
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Distributed Events in JavaSpacesEvents 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 |
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Event notification in JavaSpaces |
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Notification Examplepublic 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 ); } |
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Notification Examplepublic 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 ); } } |
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notify and TransactionsTransactions can be: null non - null |
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notify and Transactionsentries 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 |
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Operation Orderingoperations 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 |
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Transactionsuses 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 |
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Benefits of JavaSpacesSimple Expressive Supports loosely coupled protocols Eases the implementation of client/server systems |
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Applications of JavaSpacesAny system/problem that needs a distributed solution Human Genome project Cryptography Rendering Chat program Auction server( such as e-bay, amazon, etc ) |
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Examples |
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ConclusionSimple to learn Easy to understand Map a lot of problems fairly new - not many “real-world” uses yet best implementation of Linda yet |
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References and SourcesJavaSpaces Principles, Patterns, and Practice by Freeman, Hupfer, Arnold Official JavaSpaces specification visit http://www.cs.rit.edu/~sjr1521 for full links and bibliography |
«JavaSpacesTM» |