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Jim Waldo on Distributed Computing
A Conversation with Jini's Chief Architect, Jim Waldo
by Frank Sommers with Bill Venners
First Published in JavaWorld, November 2001

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Reliability in the Enterprise

Frank Sommers: For many MIS managers, change is not only a source of expense, but is also a source of fear, because change is also a chance for making errors. How does Jini address the issue of reliability in the presence of constant change?

Jim Waldo: Change is something that MIS managers rightfully fear. On the other hand, MIS managers also know that they must have change. Change was never really planned for in most distributed computing environments because we assumed networks were fairly small and static. But as networks became bigger, and as the rate of change became faster, the need to design a network system that allows for change has become more important.

Jini was designed with the idea that change occurs all the time, making change nearly automatic. MIS managers need, really, a way to change the broken things without causing other things to break. Jini gives you that isolation.

For instance, Jini lets you run your old and your new services simultaneously. You can plug in a service's new version, and your old clients can use it, even while the old version still exists, because those versions are just alternate implementations of the same interface. You can then instrument the new service, and if it has a problem, you can shut it down, and every service client can return to the old version immediately. That happens automatically. You just have to unplug the new service's machine, or unregister it, and then you can phase in those kinds of changes. You don't have to change the whole network; you can change one thing at a time.

To avoid human error, you avoid requiring human involvement. Many human errors occur in network configurations because of typing errors; for example, in assigning network addresses, where services are misdescribed or misapplied by the system administrator.

In the Jini world, when you plug in a service to the network, once it gets its IP address (automatically done via DHCP), a mechanism started programmatically registers the service with the Jini lookup service without any human intervention. The description is not human-entered, but is the Java type of the service being offered. The clients that need to use it already know what that type is in order to make the method calls to talk to the service. We avoid human error by automating the process that was error-prone.

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