Spec Index JavaSpaces Service Specification

Version 2.2

JS - JavaSpacesTM Service Specification

JS.1 Introduction

Distributed systems are hard to build. They require careful thinking about problems that do not occur in local computation. The primary problems are those of partial failure, greatly increased latency, and language compatibility. The Java(TM) programming language has a remote method invocation system called RMI that lets you approach general distributed computation in the Java1 (JVM).

This specification describes the architecture of JavaSpaces technology, which is designed to help you solve two related problems: distributed persistence and the design of distributed algorithms. JavaSpaces services use RMI and the serialization feature of the Java programming language to accomplish these goals.

JS.1.1 The JavaSpaces Application Model and Terms

A JavaSpaces service holds entries. An entry is a typed group of objects, expressed in a class for the Java platform that implements the interface net.jini.core.entry.Entry. Entries are described in detail in the Jini Entry Specification.

An entry can be written into a JavaSpaces service, which creates a copy of that entry in the space2 that can be used in future lookup operations.

You can look up entries in a JavaSpaces service using templates, which are entry objects that have some or all of its fields set to specified values that must be matched exactly. Remaining fields are left as wildcards--these fields are not used in the lookup.

There are two kinds of lookup operations: read and take. A read request to a space returns either an entry that matches the template on which the read is done, or an indication that no match was found. A take request operates like a read, but if a match is found, the matching entry is removed from the space.

You can request a JavaSpaces service to notify you when an entry that matches a specified template is written. This is done using the distributed event model contained in the package net.jini.core.event and described in the Jini Distributed Events Specification.

All operations that modify a JavaSpaces service are performed in a transactionally secure manner with respect to that space. That is, if a write operation returns successfully, that entry was written into the space (although an intervening take may remove it from the space before a subsequent lookup of yours). And if a take operation returns an entry, that entry has been removed from the space, and no future operation will read or take the same entry. In other words, each entry in the space can be taken at most once. Note, however, that two or more entries in a space may have exactly the same value.

The architecture of JavaSpaces technology supports a simple transaction mechanism that allows multi-operation and/or multi-space updates to complete atomically. This is done using the two-phase commit model under the default transaction semantics, as defined in the package net.jini.core.transaction and described in the Jini Transaction Specification.

Entries written into a JavaSpaces service are governed by a lease, as defined in the package net.jini.core.lease and described in the Jini Distributed Leasing Specification.

JS.1.1.1 Distributed Persistence

Implementations of JavaSpaces technology provide a mechanism for storing a group of related objects and retrieving them based on a value-matching lookup for specified fields. This allows a JavaSpaces service to be used to store and retrieve objects on a remote system.

JS.1.1.2 Distributed Algorithms as Flows of Objects

Many distributed algorithms can be modeled as a flow of objects between participants. This is different from the traditional way of approaching distributed computing, which is to create method-invocation-style protocols between participants. In this architecture's "flow of objects" approach, protocols are based on the movement of objects into and out of implementations of JavaSpaces technology.

For example, a book-ordering system might look like this:

A method-invocation-style design would create particular remote interfaces for these interactions. With a "flow of objects" approach, only one interface is required: the net.jini.space.JavaSpace interface.

In general, the JavaSpaces application world looks like this:

explained in following paragraph

Clients perform operations that map entries or templates onto JavaSpaces services. These can be singleton operations (as with the upper client), or contained in transactions (as with the lower client) so that all or none of the operations take place. A single client can interact with as many spaces as it needs to. Identities are accessed from the security subsystem and passed as parameters to method invocations. Notifications go to event catchers, which may be clients themselves or proxies for a client (such as a store-and-forward mailbox).

JS.1.2 Benefits

JavaSpaces services are tools for building distributed protocols. They are designed to work with applications that can model themselves as flows of objects through one or more servers. If your application can be modeled this way, JavaSpaces technology will provide many benefits.

JavaSpaces services can provide a reliable distributed storage system for the objects. In the book-buying example, the designer of the system had to define the protocol for the participants and design the various kinds of entries that must be passed around. This effort is akin to designing the remote interfaces that an equivalent customized service would require. Both the JavaSpaces system solution and the customized solution would require someone to write the code that presented requests and bids to humans in a GUI. And in both systems, someone would have to write code to handle the seller's registrations of interest with the broker.

The server for the model that uses the JavaSpaces API would be implemented at that point.

The customized system would need to implement the servers. These servers would have to handle concurrent access from multiple clients. Someone would need to design and implement a reliable storage strategy that guaranteed the entries written to the server would not be lost in an unrecoverable or undetectable way. If multiple bids needed to be made atomically, a distributed transaction system would have to be implemented.

All these concerns are solved in JavaSpaces services. They handle concurrent access. They store and retrieve entries atomically. And they provide an implementation of the distributed transaction mechanism.

This is the power of the JavaSpaces technology architecture--many common needs are addressed in a simple platform that can be easily understood and used in powerful ways.

JavaSpaces services also help with data that would traditionally be stored in a file system, such as user preferences, e-mail messages, and images. Actually, this is not a different use of a JavaSpaces service. Such uses of a file system can equally be viewed as passing objects that contain state from one external object (the image editor) to another (the window system that uses the image as a screen background). And JavaSpaces services enhance this functionality because they store objects, not just data, so the image can have abstract behavior, not just information that must be interpreted by some external application(s).

JavaSpaces services can provide distributed object persistence with objects in the Java programming language. Because code written in the Java programming language is downloadable, entries can store objects whose behavior will be transmitted from the writer to the readers, just as in an RMI using Java technology. An entry in a space may, when fetched, cause some active behavior in the reading client. This is the benefit of storing objects, not just data, in an accessible repository for distributed cooperative computing.

JS.1.3 JavaSpaces Technology and Databases

A JavaSpaces service can store persistent data which is later searchable. But a JavaSpaces service is not a relational or object database. JavaSpaces services are designed to help solve problems in distributed computing, not to be used primarily as a data repository (although there are many data storage uses for JavaSpaces applications). Some important differences are:

These differences exist because JavaSpaces services are designed for a different purpose than either relational or object databases. A JavaSpaces service can be used for simple persistent storage, such as storing a user's preferences that can be looked up by the user's ID or name. JavaSpaces service functionality is somewhere between that of a filesystem and a database, but it is neither.

JS.1.4 JavaSpaces System Design and Linda3 Systems

The JavaSpaces system design is strongly influenced by Linda systems, which support a similar model of entry-based shared concurrent processing. In Section JS.4.1, "Linda Systems", you will find several references that describe Linda-style systems.

No knowledge of Linda systems is required to understand this specification. This section discusses the relationship of JavaSpaces systems with respect to Linda systems for the benefit of those already familiar with Linda programming. Other readers should feel free to skip ahead.

JavaSpaces systems are similar to Linda systems in that they store collections of information for future computation and are driven by value-based lookup. They differ in some important ways:

On the nomenclature side, the JavaSpaces technology API uses a more accessible set of terms than the traditional Linda terms. The term mappings are "entry" for "tuple," "value" for "actual," "wildcard" for "formal," "write" for "out," and "take" for "in." So the Linda sentence "When you `out' a tuple make sure that actuals and formals in `in' and `read' can do appropriate matching" would be translated to "When you write an entry make sure that values and wildcards in take and read can do appropriate matching."

JS.1.5 Goals and Requirements

The goals for the design of JavaSpaces technology are:

The requirements for JavaSpaces application clients are:

JS.1.6 Dependencies

This document relies upon the following other specifications:

JS.2 Operations

There are four primary kinds of operations that you can invoke on a JavaSpaces service. Each operation has parameters that are entries, including some that are templates, which are a kind of entry. This chapter describes entries, templates, and the details of the operations, which are:

As used in this document, the term "operation" refers to a single invocation of a method; for example, two different take operations may have different templates.

JS.2.1 Entries

The types Entry and UnusableEntryException that are used in this specification are from the package net.jini.core.entry and are described in detail in the Jini Entry Specification. In the terminology of that specification write is a store operation; read and take are combination search and fetch operations; and notify sets up repeated search operations as entries are written to the space.

JS.2.2 net.jini.space.JavaSpace

All operations are invoked on an object that implements the JavaSpace interface. For example, the following code fragment would write an entry of type AttrEntry into the JavaSpaces service referred to by the identifier space: 

JavaSpace space = getSpace();
AttrEntry e = new AttrEntry();
e.name = "Duke";
e.value = new GIFImage("dukeWave.gif");
space.write(e, null, 60 * 60 * 1000);	 // one hour
// lease is ignored -- one hour will be enough

The JavaSpace interface is: 

package net.jini.space;

import java.rmi.*;
import net.jini.core.event.*;
import net.jini.core.transaction.*;
import net.jini.core.lease.*;

public interface JavaSpace {
    Lease write(Entry e, Transaction txn, long lease)
        throws RemoteException, TransactionException;
    public final long NO_WAIT = 0; // don't wait at all
    Entry read(Entry tmpl, Transaction txn, long timeout)
        throws TransactionException, UnusableEntryException,
               RemoteException, InterruptedException;
    Entry readIfExists(Entry tmpl, Transaction txn, 
                       long timeout)
        throws TransactionException, UnusableEntryException,
               RemoteException, InterruptedException;
    Entry take(Entry tmpl, Transaction txn, long timeout)
        throws TransactionException, UnusableEntryException,
               RemoteException, InterruptedException;
    Entry takeIfExists(Entry tmpl, Transaction txn, 
                       long timeout)
        throws TransactionException, UnusableEntryException,
               RemoteException, InterruptedException;
    EventRegistration notify(Entry tmpl, Transaction txn,
              RemoteEventListener listener, long lease,
              MarshalledObject handback)
        throws RemoteException, TransactionException;
    Entry snapshot(Entry e) throws RemoteException;
}

The Transaction and TransactionException types in the above signatures are imported from net.jini.core.transaction. The Lease type is imported from net.jini.core.lease. The RemoteEventListener and EventRegistration types are imported from net.jini.core.event.

In all methods that have the parameter, txn may be null, which means that no Transaction object is managing the operation (see Section JS.3, "Transactions").

The JavaSpace interface is not a remote interface. Each implementation of a JavaSpaces service exports proxy objects that implement the JavaSpace interface locally on the client, talking to the actual JavaSpaces service through an implementation-specific interface. An implementation of any JavaSpace method may communicate with a remote JavaSpaces service to accomplish its goal; hence, each method throws RemoteException to allow for possible failures. Unless noted otherwise in this specification, when you invoke JavaSpace methods you should expect RemoteExceptions on method calls in the same cases in which you would expect them for methods invoked directly on an RMI remote reference. For example, invoking snapshot might require talking to the remote JavaSpaces server, and so might get a RemoteException if the server crashes during the operation.

The details of each JavaSpace method are given in the sections that follow.

JS.2.2.1 InternalSpaceException

The exception InternalSpaceException may be thrown by a JavaSpaces service that encounters an inconsistency in its own internal state or is unable to process a request because of internal limitations (such as storage space being exhausted). This exception is a subclass of RuntimeException. The exception has two constructors: one that takes a String description and another that takes a String and a nested exception; both constructors simply invoke the RuntimeException constructor that takes a String argument. 

package net.jini.space;

public class InternalSpaceException extends RuntimeException {
    public final Throwable nestedException;
    public InternalSpaceException(String msg) {...}
    public InternalSpaceException(String msg, Throwable e) {...}
    public printStackTrace() {...}
    public printStackTrace(PrintStream out) {...}
    public printStackTrace(PrintWriter out) {...}
}

The nestedException field is the one passed to the second constructor, or null if the first constructor was used. The overridden printStackTrace methods print out the stack trace of the exception and, if nestedException is not null, print out that stack trace as well.

JS.2.3 write

A write places a copy of an entry into the given JavaSpaces service. The Entry passed to the write is not affected by the operation. Each write operation places a new entry into the specified space, even if the same Entry object is used in more than one write.

Each write invocation returns a Lease object that is lease milliseconds long. If the requested time is longer than the space is willing to grant, you will get a lease with a reduced time. When the lease expires, the entry is removed from the space. An IllegalArgumentException will be thrown if the lease time requested is negative and not equal to Lease.ANY.

If a write returns without throwing an exception, that entry is committed to the space, possibly within a transaction (see Section JS.3, "Transactions"). If a RemoteException is thrown, the write may or may not have been successful. If any other exception is thrown, the entry was not written into the space.

Writing an entry into a space might generate notifications to registered objects (see Section JS.2.7, "notify").

JS.2.4 readIfExists and read

The two forms of the read request search the JavaSpaces service for an entry that matches the template provided as an Entry. If a match is found, a reference to a copy of the matching entry is returned. If no match is found, null is returned. Passing a null reference for the template will match any entry.

Any matching entry can be returned. Successive read requests with the same template in the same JavaSpaces service may or may not return equivalent objects, even if no intervening modifications have been made to the space. Each invocation of read may return a new object even if the same entry is matched in the JavaSpaces service.

A readIfExists request will return a matching entry, or null if there is currently no matching entry in the space. If the only possible matches for the template have conflicting locks from one or more other transactions, the timeout value specifies how long the client is willing to wait for interfering transactions to settle before returning a value. If at the end of that time no value can be returned that would not interfere with transactional state, null is returned. Note that, due to the remote nature of JavaSpaces services, read and readIfExists may throw a RemoteException if the network or server fails prior to the timeout expiration

A read request acts like a readIfExists except that it will wait until a matching entry is found or until transactions settle, whichever is longer, up to the timeout period.

In both read methods, a timeout of NO_WAIT means to return immediately, with no waiting, which is equivalent to using a zero timeout. An IllegalArgumentException will be thrown if a negative timeout value is used.

JS.2.5 takeIfExists and take

The take requests perform exactly like the corresponding read requests (see Section JS.2.4, "readIfExists and read"), except that the matching entry is removed from the space. Two take operations will never return copies of the same entry, although if two equivalent entries were in the JavaSpaces service the two take operations could return equivalent entries.

If a take returns a non-null value, the entry has been removed from the space, possibly within a transaction (see Section JS.3, "Transactions"). This modifies the claims to once-only retrieval: A take is considered to be successful only if all enclosing transactions commit successfully. If a RemoteException is thrown, the take may or may not have been successful. If an UnusableEntryException is thrown, the take removed the unusable entry from the space; the contents of the exception are as described in the Jini Entry Specification. If any other exception is thrown, the take did not occur, and no entry was removed from the space.

With a RemoteException, an entry can be removed from a space and yet never returned to the client that performed the take, thus losing the entry in between. In circumstances in which this is unacceptable, the take can be wrapped inside a transaction that is committed by the client when it has the requested entry in hand.

JS.2.6 snapshot

The process of serializing an entry for transmission to a JavaSpaces service will be identical if the same entry is used twice. This is most likely to be an issue with templates that are used repeatedly to search for entries with read or take. The client-side implementations of read and take cannot reasonably avoid this duplicated effort, since they have no efficient way of checking whether the same template is being used without intervening modification.

The snapshot method gives the JavaSpaces service implementor a way to reduce the impact of repeated use of the same entry. Invoking snapshot with an Entry will return another Entry object that contains a snapshot of the original entry. Using the returned snapshot entry is equivalent to using the unmodified original entry in all operations on the same JavaSpaces service. Modifications to the original entry will not affect the snapshot. You can snapshot a null template; snapshot may or may not return null given a null template.

The entry returned from snapshot will be guaranteed equivalent to the original unmodified object only when used with the space. Using the snapshot with any other JavaSpaces service will generate an IllegalArgumentException unless the other space can use it because of knowledge about the JavaSpaces service that generated the snapshot. The snapshot will be a different object from the original, may or may not have the same hash code, and equals may or may not return true when invoked with the original object, even if the original object is unmodified.

A snapshot is guaranteed to work only within the virtual machine in which it was generated. If a snapshot is passed to another virtual machine (for example, in a parameter of an RMI call), using it--even with the same JavaSpaces service--may generate an IllegalArgumentException.

We expect that an implementation of JavaSpaces technology will return a specialized Entry object that represents a pre-serialized version of the object, either in the object itself or as an identifier for the entry that has been cached on the server. Although the client may cache the snapshot on the server, it must guarantee that the snapshot returned to the client code is always valid. The implementation may not throw any exception that indicates that the snapshot has become invalid because it has been evicted from a cache. An implementation that uses a server-side cache must therefore guarantee that the snapshot is valid as long as it is reachable (not garbage) in the client, such as by storing enough information in the client to be able to re-insert the snapshot into the server-side cache.

No other method returns a snapshot. Specifically, the return values of the read and take methods are not snapshots and are usable with any implementation of JavaSpaces technology.

JS.2.7 notify

A notify request registers interest in future incoming entries to the JavaSpaces service that match the specified template. Matching is done as it is for read. The notify method is a particular registration method under the Jini Distributed Events Specification. When matching entries are written, the specified RemoteEventListener will eventually be notified. When you invoke notify you provide an upper bound on the lease time, which is how long you want the registration to be remembered by the JavaSpaces service. The service decides the actual time for the lease. You will get an IllegalArgumentException if the lease time requested is not Lease.ANY and is negative. The lease time is expressed in the standard millisecond units, although actual lease times will usually be of much larger granularity. A lease time of Lease.FOREVER is a request for an indefinite lease; if the service chooses not to grant an indefinite lease, it will return a bounded (non-zero) lease. The final argument to the notify request is a MarshaledObject called the handback.

Each notify returns a net.jini.core.event.EventRegistration object. When an object is written that matches the template supplied in the notify invocation, the listener's notify method is eventually invoked, with a RemoteEvent object. The RemoteEvent object's getID method will return the same value returned by the EventRegistration object's getID method. This value, the event ID, will be unique at least with respect to all other active event registrations on this JavaSpaces service with different templates or transactions. The RemoteEvent object's getSource method will return a reference to the JavaSpaces service. If the handback associated with the registration was non-null, the RemoteEvent object's getRegistrationObject method will return an object that is equivalent (in the sense of the equals method) to the handback, otherwise this method will return null. The RemoteEvent object's getSequenceNumber method will return the sequence number assigned to this event. Sequence numbers for a given event ID are strictly increasing. If there is no gap between two sequence numbers, no events have been missed; if there is a gap, events might (but might not) have been missed. For example, a gap might occur if the JavaSpaces service crashes, even if no events are lost due to the crash.

If the transaction parameter is null, the listener will be notified when matching entries are written either under a null transaction or when a transaction commits. If an entry is written under a transaction and then taken under that same transaction before the transaction is committed, listeners registered under a null transaction will not be notified of that entry.

If the transaction parameter is not null, the listener will be notified of matching entries written under that transaction in addition to the notifications it would receive under a null transaction. A notify made with a non-null transaction is implicitly dropped when the transaction completes.

The request specified by a successful notify is as persistent as the entries of the space. They will be remembered as long as an untaken entry would be, until the lease expires, or until any governing transaction completes, whichever is shorter.

The service will make a "best effort" attempt to deliver notifications. The service will retry at most until the notification request's lease expires. Notifications may be delivered in any order.

See the Jini Distributed Events Specification for details on the event types.

JS.2.8 Operation Ordering

Operations on a space are unordered. The only view of operation order can be a thread's view of the order of the operations it performs. A view of inter-thread order can be imposed only by cooperating threads that use an application-specific protocol to prevent two or more operations being in progress at a single time on a single JavaSpaces service. Such means are outside the purview of this specification.

For example, given two threads T and U, if T performs a write operation and U performs a read with a template that would match the written entry, the read may not find the written entry even if the write returns before the read. Only if T and U cooperate to ensure that the write returns before the read commences would the read be ensured the opportunity to find the entry written by T (although it still might not do so because of an intervening take from a third entity).

JS.2.9 Serialized Form

Class serialVersionUID Serialized Fields
InternalSpaceException -4167507833172939849L all public fields

JS.3 Transactions

The JavaSpaces API uses the package net.jini.core.transaction to provide basic atomic transactions that group multiple operations across multiple JavaSpaces services into a bundle that acts as a single atomic operation. JavaSpaces services are actors in these transactions; the client can be an actor as well, as can any remote object that implements the appropriate interfaces.

Transactions wrap together multiple operations. Either all modifications within the transactions will be applied or none will, whether the transaction spans one or more operations and/or one or more JavaSpaces services.

The transaction semantics described here conform to the default transaction semantics defined in the Jini Transaction Specification.

JS.3.1 Operations under Transactions

Any read, write, or take operations that have a null transaction act as if they were in a committed transaction that contained exactly that operation. For example, a take with a null transaction parameter performs as if a transaction was created, the take performed under that transaction, and then the transaction was committed. Any notify operations with a null transaction apply to write operations that are committed to the entire space.

Transactions affect operations in the following ways:

If a transaction aborts while an operation is in progress under that transaction, the operation will terminate with a TransactionException. Any statement made in this chapter about read or take apply equally to readIfExists or takeIfExists, respectively.

JS.3.2 Transactions and ACID Properties

The ACID properties traditionally offered by database transactions are preserved in transactions on JavaSpaces systems. The ACID properties are:

The timeout values in read and take allow a client to trade full isolation for liveness. For example, if a read request has only one matching entry and that entry is currently locked in a take from another transaction, read would block indefinitely if the client wanted to preserve isolation. Since completing the transaction could take an indefinite amount of time, a client may choose instead to put an upper bound on how long it is willing to wait for such isolation guarantees, and instead proceed to either abort its own transaction or ask the user whether to continue or whatever else is appropriate for the client.

Persistence is not a required property of JavaSpaces technology implementations. A transient implementation that does not preserve its contents between system crashes is a proper implementation of the JavaSpace interface's contract, and may be quite useful. If you choose to perform operations on such a space, your transactions will guarantee as much durability as the JavaSpaces service allows for all its data, which is all that any transaction system can guarantee.

JS.4 Further Reading

JS.4.1 Linda Systems

  1. "How to Write Parallel Programs: A Guide to the Perplexed," Nicholas Carriero and David Gelernter, ACM Computing Surveys, Sept., 1989.

  2. "Generative Communication in Linda," David Gelernter, ACM Transactions on Programming Languages and Systems, Vol. 7, No. 1, pp. 80-112 (January 1985).

  3. "Persistent Linda: Linda + Transactions + Query Processing," Brian G. Anderson and Dennis Shasha, Proceedings of the 13th Symposium on Fault-Tolerant Distributed Systems, 1994.

  4. "Adding Fault-tolerant Transaction Processing to LINDA," Scott R. Cannon and David Dunn, Software--Practice and Experience, Vol. 24(5), pp. 449-446 (May 1994).

  5. ActorSpaces: An Open Distributed Programming Paradigm, Gul Agha, Christian J. Callsen, University of Illinois at Urbana-Champaign, UILU-ENG-92-1846.

JS.4.2 The Java Platform

  1. The Java Programming Language, Third Edition, Ken Arnold, James Gosling, and David Holmes, Addison Wesley, 2000.

  2. The Java Language Specification, James Gosling, Bill Joy, and Guy Steele, Addison Wesley, 1996.

  3. The Java Virtual Machine Specification, Second Edition, Tim Lindholm and Frank Yellin, Addison Wesley, 1999.

  4. The Java Class Libraries, Second Edition, Patrick Chan, Rosanna Lee, and Doug Kramer, Addison Wesley, 1998.

JS.4.3 Distributed Computing

  1. Distributed Systems, Sape Mullender, Addison Wesley, 1993.

  2. Distributed Systems: Concepts and Design, George Coulouris, Jean Dollimore, and Tim Kindberg, Addison Wesley, 1998.

  3. Distributed Algorithms, Nancy A. Lynch, Morgan Kaufmann Publishers, 1997.

JS.5 History

Version Description
v1.0 Initial release of this specification.
v2.0 (formerly called v1.2) Section JS.2.7 was changed to no longer require that event sequence numbers be "fully ordered."4
v2.1 (formerly called v1.2.1) A paragraph in Section JS.2.7 was inadvertently left in the document during the v1.2 revision. That paragraph has been deleted in this release.
v2.2 Removed dependency on Jini Entry Utilities Specification.

1 As used in this document, the terms "Java virtual machine" or "JVM" mean a virtual machine for the Java platform.

2 The term "space" is used to refer to a JavaSpaces service implementation.

3 "Linda" is the name of a public domain technology originally propounded by Dr. David Gelernter of Yale University. "Linda" is also claimed as a trademark for certain goods by Scientific Computing Associates, Inc. This discussion refers to the public domain "Linda" technology.

4 "Fully ordered" in defined in the Jini Technology Core Platform Specification, "Distributed Events".

License

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Spec Index JavaSpaces Service Specifications
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