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JavaSpaces: Data, Decoupling, and Iteration
A Conversation with Ken Arnold, Part V
by Bill Venners
October 7, 2002

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Summary
Ken Arnold, the original lead architect of JavaSpaces, talks with Bill Venners the data-driven nature of JavaSpaces, how JavaSpaces facilitates decoupling, and why iteration isn't supported in the JavaSpace interface.

Ken Arnold has done a lot of design in his day. While at Sun Microsystems, Arnold was one of the original architects of Jini technology and was the original lead architect of JavaSpaces. Prior to joining Sun, Arnold participated in the original Hewlett-Packard architectural team that designed CORBA. While at UC Berkeley, he created the Curses library for terminal-independent screen-oriented programs. In Part I of this interview, which is being published in six weekly installments, Arnold explains why there's no such thing as a perfect design, suggests questions you should ask yourself when you design, and proposes the radical notion that programmers are people. In Part II, Arnold discusses the role of taste and arrogance in design, the value of other people's problems, and the virtue of simplicity. In Part III, Arnold discusses the concerns of distributed systems design, including the need to expect failure, avoid state, and plan for recovery. In Part IV, Arnold describes the basic idea of a JavaSpace, explains why fields in entries are public, why entries are passive, and how decoupling leads to reliability. In this fifth installment, Arnold talks about the data-driven nature of JavaSpaces, how JavaSpaces lets you "throw in a grain and watch it grow," and why iteration isn't supported in the JavaSpace interface.

Type versus State

Bill Venners: In both the Jini lookup service and in JavaSpaces, you can look up objects by both type and state. For Jini lookups, you can specify the types of service you want plus the types and values of attribute entries. In JavaSpace reads and takes, you specify an entry type and field values. How do the kinds of questions you ask in your query differ when you look up objects by type versus by state?

Ken Arnold: The difference is function. If you are writing code that talks to some data source, you can ask it questions. Method calls let you ask the data source for the information you want, and get the answer back. You can view a particular set of method calls as a particular socket shape. So you've written code that translates when you compile it into these method invocations. If those method invocations are all resolved locally, you would have another class to plug into that socket. With a Jini lookup service, you actually go on a network and ask: Is there anything on the network that plugs into this socket? Data is not the point there. It is only methods that matter in that part of the query.

JavaSpaces tries to accomplish something rather different. Yes, JavaSpaces is data-driven. It is object-oriented in the sense that the entries have type and you can match subtypes, and that entry fields can be object types. But at some point, you get past objects in any system. At some point, you call a method with an integer value. In some languages that integer is still logically an object, but it doesn't contain other objects. At some point, you hit the bottom. JavaSpaces is a bottom point in the sense that it is a way to make an asynchronous method call. You can consider that the entry fields are like the parameters to the method call. The fact that those are data shouldn't bother you, because that is when you hit bottom.

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