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The Architecture of Aglets
The Inner Workings of an Agent Technology
by Bill Venners
First Published in JavaWorld, April 1997

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Interaction between aglet and host
An aglet interacts with its environment (its aglet host) through an AgletContext object. An aglet can obtain a handle to its context by invoking getAgletContext(), a method it inherits from base class Aglet. The aglet context has methods such as createAglet() and retractAglet(), which allow an aglet to add new aglets (or get an old aglet back) to its local host.

Interaction between aglets
To interact with each other, aglets do not normally invoke each other's methods directly. Instead they go through AgletProxy objects, which serve as aglet representatives. For example, if a BossAglet wishes to make a request of an EmployeeAglet, the BossAglet obtains a handle to a proxy object that "represents" the EmployeeAglet. The BossAglet then makes a request by invoking a method in the EmployeeAglet's proxy, which in turn forwards the request to the actual EmployeeAglet.

The AgletProxy class contains methods that allow aglets to request other aglets to take actions, such as dispatch(), clone(), deactivate(), and dispose(). The aglet that has been requested to take an action can comply, refuse to comply, or decide to comply later.

The proxy also allows an aglet to send a message, either synchronously or asynchronously, to another aglet. A Message object is supplied for this purpose; it carries a String to indicate the kind of message plus one other optional piece of data, either a String or one of Java's primitive types. To send a message you create a Message object and pass it as a parameter to the sendMessage() or sendAsynchMessage() method of the proxy object.

An aglet must go through a proxy object to interact with an aglet, even if both aglets are in the same aglet host. The reason aglets aren't allowed to directly interact with one another is that the aglet's callback and initialization methods are public. These methods should be invoked only by the aglet host, but if an aglet could get a handle to another aglet, it could invoke that aglet's callback or initialization methods. An aglet could become very confused if another aglet inadvertently or maliciously invoked these methods directly.

The aglet being represented by a proxy might be local or remote, but the proxy object is always local. For example, if a BossAglet in Silicon Valley wants to communicate with an EmployeeAglet on a South Pacific island, the BossAglet gets a local AgletProxy object, which represents the remote EmployeeAglet. The BossAglet merely invokes methods in the local proxy, which in turn communicates across the network to the EmployeeAglet. Only aglets, not proxies, migrate across the network. A proxy communicates with a remote aglet that it represents by sending data across the network.

You get a proxy to an aglet in one of three ways, each of which involves invoking a method in the context object:

  1. By creating the aglet in the first place with createAglet(). (This returns a proxy object.)

  2. By searching through an enumeration of local proxies returned by getAgletProxies().

  3. By supplying an "aglet identifier" and, if remote, an aglet location as parameters to getAgletProxy(). (Every aglet, upon creation or cloning, is assigned a globally unique aglet identifier.)

Security
Mobile-agent systems, such as aglets, require high levels of security, because they represent yet another way to transmit a malicious program. Before aglets can be used in practice, there must be an infrastructure of aglet hosts that prevent untrusted aglets from doing damage but provide trusted aglets with useful access to the host's resources. Security is amply provided for in Java's intrinsic architecture and in the extra security features of JDK 1.1, but as with applets, some attacks (such as denial of service by allocating memory until the host crashes) are still possible. Currently, the aglet hosts from IBM (named Tahiti and Fiji) place very severe security restrictions on the activities of any aglet that didn't originate locally.

Next month
Will aglets become as ubiquitous as their plastic cousins, which quietly perch on the ends of everyone's shoelaces? Aglets represent a good example of innovation on top of Java's network-oriented architecture, but what new benefits do they offer developers and end users that client/server, applets, and servlets don't already offer? In next month's Under The Hood, I will analyze the real-world utility of mobile agents in general and aglets in particular.

Resources

About the author
Bill Venners has been writing software professionally for 12 years. Based in Silicon Valley, he provides software consulting and training services under the name Artima Software Company. Over the years he has developed software for the consumer electronics, education, semiconductor, and life insurance industries. He has programmed in many languages on many platforms: assembly language on various microprocessors, C on Unix, C++ on Windows, Java on the Web. He is author of the book: Inside the Java Virtual Machine, published by McGraw-Hill. You can reach him at bv@artima.com.

This article was first published under the name Under the Hood: The architecture of aglets in JavaWorld, a division of Web Publishing, Inc., April 1997.

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