Mobile code

In distributed computing, code mobility is the ability for running programs, code or objects to be migrated (or moved) from one machine or application to another. This is the process of moving mobile code across the nodes of a network as opposed to distributed computation where the data is moved.

It is common practice in distributed systems to require the movement of code or processes between parts of the system, instead of data.

Examples of code mobility include scripts downloaded over a network (for example JavaScript, VBScript), Java applets, ActiveX controls, Flash animations, Shockwave movies (and Xtras), and macros embedded within Microsoft Office documents.

The purpose of code mobility is to support sophisticated operations. For example, an application can send an object to another machine, and the object can resume executing inside the application on the remote machine with the same state as it had in the originating application.

According to a classification proposed by Fuggetta, Picco and Vigna, code mobility can be either strong or weak: strong code mobility involves moving both the code, data and the execution state from one host to another, notably via a process image (this is important in cases where the running application needs to maintain its state as it migrates from host to host), while weak code mobility involves moving the code and the data only. Therefore, it may be necessary to restart the execution of the program at the destination host.

Several paradigms, or architectural styles, exist within code mobility:

Within code mobility, the Mobile Agent paradigm has conventionally attracted the most interest and research, however some recent work has produced general purpose implementations.

Mobile code can also be encapsulated or embedded in other file formats not traditionally associated with executable code. An example of this form of encapsulation is the presence of JavaScript in a PDF.

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