JavaSpaces and Linda Language Ghaith Haddad EEL6897 Fall 2007

Walkthrough  Parallel Programming Models  What is Linda?  Linda Model RequirementsRequirements AdvantagesAdvantages Compare with MPICompare with MPI DisadvantagesDisadvantages  Linda Implementations in Java TSpacesTSpaces JavaspacesJavaspaces  Requirements  Complexity  Diagram  In Depth  Example  Transactions  Distributed Events GigaSpacesGigaSpaces ComparisonComparison Research ProjectsResearch Projects  References  Demo

Parallel Programming Models  Set of software technologies to express parallel algorithms and match applications with the underlying parallel systems [2]  Areas include: applications, programming languages, compilers, libraries, communications systems, and parallel I/O  “Roughly” categorized into two kinds of systems: Shared-MemoryShared-Memory Distributed-MemoryDistributed-Memory  Examples: POSIX Threads, MPI, Charm++, Linda, Oz (programming language)

What is Linda  “Linda is a coordination language for parallel and distributed processing, providing a communication mechanism based on a logically shared memory space called tuple space.” [1]  Developed by David Gelernter and Nicholas Carriero at Yale University.  Coordination Language: Model, not Grammar!  tuple space An implementation of the associative memory paradigm for parallel/distributed computingAn implementation of the associative memory paradigm for parallel/distributed computing Structured as a bag of tuplesStructured as a bag of tuples  Tuple Example: ("point", 12, 67), Three fields tupleThree fields tuple 12 and 67 are the x and y coordinates of the point represented by this tuple12 and 67 are the x and y coordinates of the point represented by this tuple Similar to struct or object without methodsSimilar to struct or object without methods

Tuple Space vs. Associative Memory [7]  Allow memory items to be selected by reference to their contents rather than by an address  Every word in the associative memory is searched in parallel against a key held in the comparand register which is the same width as memory

Linda Model Requirements  The original Linda model requires four operations that individual workers perform on the tuples and the tuplespace [3] in: atomically reads and removes -consumes- a tuple from tuplespacein: atomically reads and removes -consumes- a tuple from tuplespace rd: non-destructively reads a tuplespacerd: non-destructively reads a tuplespace out: produces a tuple, writing it into tuplespaceout: produces a tuple, writing it into tuplespace eval: creates new processes to evaluate tuples, writing the result into tuplespaceeval: creates new processes to evaluate tuples, writing the result into tuplespace

Linda Model Requirements  The specification of the tuple to be retrieved makes use of an associative matching technique A subset of the fields in the tuple have their values specifiedA subset of the fields in the tuple have their values specified Used to locate a matching tuple in the tuple spaceUsed to locate a matching tuple in the tuple space Example: in("point", ?x, ?y)is called an anti- tupleExample: in("point", ?x, ?y)is called an anti- tuple anti-tuple: Any tuple with the same number and type of fieldsanti-tuple: Any tuple with the same number and type of fields Anti-tuple is used toAnti-tuple is used to

Linda Model Advantages [3]  More orthogonal Treats process coordination as a separate activity from computation.Treats process coordination as a separate activity from computation. Allows processes computing in different languages and platforms to interoperate using the same primitivesAllows processes computing in different languages and platforms to interoperate using the same primitives  More general: Can subsume various levels of concurrency - uni-processor, multi-threaded multiprocessor, or networked- under a single model.Can subsume various levels of concurrency - uni-processor, multi-threaded multiprocessor, or networked- under a single model. Allows a multi-threaded Linda system to be distributed across multiple computers, or vice- versa, without changeAllows a multi-threaded Linda system to be distributed across multiple computers, or vice- versa, without change

Compare with MPI [3]  Message-passing models require tightly-coupled processes sending messages to each other in some sequence or protocol  Linda processes are decoupled from other processes, communicating only through the tuplespace a process need have no notion of other processes except for the kinds of tuples consumed or produced (data coupling).a process need have no notion of other processes except for the kinds of tuples consumed or produced (data coupling).

Linda Model Disadvantages  The tuple space is unprotected since any process can access any tuple in it [5]  No hierarchical organization of tuples, therefore no sense of name scoping [5]  Decreased speed of operations in Linda systems as compared to MPI systems [3]  At worst, inefficient, and, at best, subject to unpredictable performance [1]  More disadvantages in [6]

Linda Implementations in Java  TSpaces  JavaSpaces  GigaSpaces  Many Other Research Projects…

TSpaces  IBM’s implementation  "the common platform on which we build links to all system and application services“ [8]  Single server process running on the network which makes use of a textual configurations file [1]  Transports tuples across the network using the standard Java object serialization mechanisms and TCP/IP sockets [1]  New commands can be added relatively easily [1]

TSpaces

JavaSpaces  Forms part of the Jini system, and so makes extensive use of the Jini API [9]  Network support is provided by the Java RMI (Remote Method Invocation) protocol [10]  Distribution of classes to clients is handled by the standard Internet hypertext protocol (HTTP) [1]  Supports the basic Yale Linda operations (the names differ from the original names used by the Yale group, but essentially the same functionality is provided)

JavaSpaces

JavaSpaces – Requirements [1]  a web (HTTP) server (a minimal one is provided with the Jini/JavaSpaces release)  an RMI activation server (part of the standard RMI software bundled with Java)  a Jini lookup service (alternatively the RMI registry service can be used, but this is discouraged as support for this option may be discontinued by Sun in the future)  a Jini transaction manager  a JavaSpaces server  Applications are also required to run a security manager, whether security checking is required or not

JavaSpaces – Complexity [1]  A typical command line required to run a JavaSpaces application is as follows java -Djava.security.policy= D:\JavaProgs\policy.all -Doutrigger.spacename=JavaSpaces-Dcom.sun.jini.lookup.groups=public -Djava.rmi.server.codebase= examples-dl.jar -cp D:\JavaProgs\space- examples.jar;D:\JavaProgs\classes sun.applet.AppletViewer worker.html

JavaSpaces – Diagram [11]

JavaSpaces – In Depth [11]  All operations are invoked on an object that implements the net.jini.space.JavaSpace interface.  A space stores entries, each of which is a collection of typed objects that implements the Entry interface import net.jini.core.entry.*; public class MessageEntry implements Entry { public String content; public MessageEntry() { } public MessageEntry(String content) { this.content = content; } public String toString() { return "MessageContent: " + content; }}

Net.jini.space.JavaSpace Interface [12] EventRegistration notify(Entry tmpl, Transaction txn, RemoteEventListener listener, long lease, MarshalledObject handback) - When entries are written that match this template notify the given listener with a RemoteEvent that includes the handback object. Entry read(Entry tmpl, Transaction txn, long timeout) - Read any matching entry from the space, blocking until one exists. Entry readIfExists(Entry tmpl, Transaction txn, long timeout) - Read any matching entry from the space, returning null if there is currently is none. Entry snapshot(Entry e) - The process of serializing an entry for transmission to a JavaSpaces service will be identical if the same entry is used twice. Entry take(Entry tmpl, Transaction txn, long timeout) - Take a matching entry from the space, waiting until one exists. Entry takeIfExists(Entry tmpl, Transaction txn, long timeout) - Take a matching entry from the space, returning null if there is currently is none. Lease write(Entry entry, Transaction txn, long lease) - Write a new entry into the space. 

JavaSpaces – Example import net.jini.space.JavaSpace; public class SpaceClient { public static void main(String argv[]) { try { System.out.println("Searching for a JavaSpace..."); Lookup finder = new Lookup(JavaSpace.class); JavaSpace space = (JavaSpace) finder.getService(); System.out.println("A JavaSpace has been discovered."); System.out.println("Writing a message into the spce..."); MessageEntry msg = new MessageEntry(); msg.content = "Hello there"; space.write(msg, null, 60*60*1000); MessageEntry template = new MessageEntry(); System.out.println("Reading a message from the space..."); MessageEntry result = (MessageEntry) space.read(template, null, Long.MAX_VALUE); System.out.println(“Message read is: "+result.content); } catch(Exception e) {e.printStackTrace();} }}

JavaSpaces – Transactions [11]  Package: net.jini.core.transaction  Provides basic atomic transactions that group multiple operations across multiple JavaSpaces services into a bundle that acts as a single atomic operation  Either all modifications within the transactions will be applied or none will regardless of whether the transaction spans one or more operations or one or more JavaSpaces servicesregardless of whether the transaction spans one or more operations or one or more JavaSpaces services  Transactions can span multiple spaces and participants  A read(), write(), or take() operation that has a null transaction acts as if it were in a committed transaction that contained that operation.

JavaSpaces – Distributed Events [14]  Defined in the Jini Technology Core Platform Specification Defines interfaces and requirements placed on Jini which is capable of sending remote eventsDefines interfaces and requirements placed on Jini which is capable of sending remote events The JavaSapces Specification overrides Jini specification, it provides more details on how a JavaSpace implements remote eventsThe JavaSapces Specification overrides Jini specification, it provides more details on how a JavaSpace implements remote events  A remote event model allows an object in one JVM to register as a listener to an object on another JVM Example: a JavaSpace can send remote events when an entry matching a specified template is written into the spaceExample: a JavaSpace can send remote events when an entry matching a specified template is written into the space

GigaSpaces [13]   Developed as a commercial implementation of the JavaSpaces specification, with some features added: Operations on multiple tuples Updating, deleting and counting tuples Iterating over a set of tuples matching an anti-tuple Distributed implementations of the Java Collections List, Set and Map interfaces Message queuing mechanism   Efficient implementation of GigaSpaces Buffered writes Indexing of tuples (with or without intervention from the application developer) Support for non-Java clients to access GigaSpaces through the use of the SOAP protocol over HTTP Support for web servers to make use of GigaSpaces to share session information (potentially even between separate web servers)

Comparison [1]  TSpaces is unique in providing an extensible form of matching through the ability to add new commands

Research projects  XMLSpaces [15] Designed to support the use of XML data in tuplesDesigned to support the use of XML data in tuples Built over TSpacesBuilt over TSpaces Supports a distributed tuple space modelSupports a distributed tuple space model  CO 3 PS [16] "Computation, Coordination and Composition with Petri net Specifications“"Computation, Coordination and Composition with Petri net Specifications“  Java-Linda [17] a student project at Yale Universitya student project at Yale University  eLinda [18] A model close to the standard Linda modelA model close to the standard Linda model Three ImplementationsThree Implementations  eLinda1: Fully distributed tuple space  eLinda2: Centralised tuple space, like JavaSpaces, TSpaces  eLinda3: Like in eLinda2, but with “broadcast” tuples cached on each processing node

References  [1] Linda implementations in Java for concurrent systems- G. C. Wells1;x, A. G. Chalmers and P. G. Clayton – © 2003 John Wiley & Sons Ltd.  [2]  [3]  [4] G. Andrews, “ Concurrent Programming: Principles and Practice”, Benjamin/Cummings Publishing Company, Inc., Redwood City, CA  [5] Drucker, R.; Frank, A., "A C++/Linda model for distributed objects," Computer Systems and Software Engineering, 1996., Proceedings of the Seventh Israeli Conference on, vol., no., pp , Jun 1996  [6] Ericsson-Zenith (1992). Process Interaction Models. Paris University  [7]  [8] IBM. The TSpaces vision. URL:  [9] Sun Microsystems. Jini connection technology. URL:  [10] C. Austin and M. Pawlan. Advanced Programming for the Java 2 Platform. Addison-Wesley, September 2000  [11]  [12]  [13] GigaSpaces Technologies Ltd. Gigaspaces. URL:  [14] P. Bishop and N. Warren, JavaSpaces in Practice, Addison-Wesley, 2002  [15] R. Tolksdorf and D. Glaubitz. Coordinating web-based systems with documents in XMLSpaces. URL:  [16] T. Holvoet. An Approach for Open Concurrent Software Development. PhD thesis, Department of Computer Science, K.U.Leuven, December 1997  [17] A. Smith. Towards wide-area network Piranha: Implementing Java-Linda. URL:  [18] G.C. Wells. A Programmable Matching Engine for Application Development in Linda. PhD thesis, University of Bristol, U.K., 2001

Demo…