The Description Package

of the Transfer And Exchange Layer (txlayer)

Editor: T.Beiler (SRB), J.Brauckmann (SRB)
@version [CVS] @(#) $Id: index.html,v 1.3 1998/01/26 14:27:49 semper Exp $

Introduction

In order to perform an exchange, it is neccessary to have descriptions of the goods to be exchanged. An item for which a description shall be generated has to be 'describable'. In other words, the class has to implement the interface 'Describable'. and therefore to offer a method getDescription(), which returns an object of class Description.

There are two situations involving descriptions while exchanging goods. The first occurs, when a description of an expectation is compared against a description of a promise: The comparison of what one exchanging side expects to what it is offered. The other situation is given, when the received good is checked to suit the description expectation and/or the description of the promise.

There are two possibilities what a Description can look like:

• Possibility 1: A description has the same structure as the object it describes. For example, a description of a tree is again a tree.
  + Perhaps easier to compare
  - More difficult to handle: For almost each class of item there has to be an extra class of description
  - the structure may too strong influence the comparison.
  This kind of description is implemented with the interface Description.
• Possibility 2: A description is flat, regardless of whether the described Object might have a tree structure.
  + easy to handle: Just one class, no extensions
  - difficult to compare: no information about the structure from outside visible, for example a description of a container.
  This kind of description is implemented with the class WildCardDescription.

Problem essence: When are two items equal in the desired sense (bit by bit of course doesn't work solely)? And: Since a description has to be serializable, is it addionally necessary to rebuild an structured object from this, or does it suffice to compare the describing serialized streams/strings?

Classes and Interfaces

Interface Describable

Interface Description

Interface WildCardDescription

Examples