Representation

A representation is the concrete immutable form of a resource.

While a resource is an abstract notion about information, a representation is its concrete realization. Using the example of the resource "attendee-list", its representation contains the specific list of names of the people who have registered for a conference.

A representation is immutable - once created it will never change. NetKernel caches returned representations and tracks their validity. This means that re-requests of a resource when the representation has not change is a very inexpensive operation.

A representation is concrete - it contains specific information. However, the representation does not have a mandated data type. In fact, a representation may have multiple data types, and clients may request the type that is most efficient for them to use. For example, a resource representation may be in the form of a binary stream (read directly from disk), a Java String, or an object hierarchy. Each form contains the same information and any form could be substituted without a loss of information.

If a client requests a resource and does not specify a preference for the form of the representation, then NetKernel will supply the form it considers the most computationally valuable. Alternatively a client may request a resource representation in a particular form. If the code producing the resource can provide that representation, then that is immediately delivered. If not, then NetKernel seeks help from a transreptor - code that understands how to "transrepresent" or translate from one representational form to another.

In NetKernel 3.x, the specific forms of a representation are known as aspects.

In this diagram, a URI address identifies a representation which has three aspects: a Java String aspect, an XML aspect, and a binary aspect. All of these aspects have identical information represented in according to the dictates of their type.

For example, let's examine the resource identified as "ffcpl:/message". The representation is the following XML document fragment:

The BinaryAspect would contain the bits that are read from or written to a disk file. The StringAspect would be a Java String object that contains the following thirty-two characters: ("<","m","e","s","s","a","g","e",">","H","e", "l", "l", "o", ",", " ", "w", "o", "r", "l", "d", "!", "<","/","m","e","s","s","a","g","e", and ">"). The XMLAspect would refer to the internal DOM object hierarchy of the parsed XML document.

The notion of transreption is universal and broadly applicable in NetKernel. For example, the source code for a script program can be transrepted into Java virtual machine byte codes by a compiler.