In NetKernel modules are functional building blocks and the unit of deployment. Modules can hold components: static resources, java classes, accessors, aspects, transreptors, transports, fragmentors and caches. Let's first examine the types of resources that a module may contain:

Universal Resource Accessors

A Universal Resource Accessors is an endpoint that accepts and processes requests for resources. Accessor development is discussed in detail in the Extension Guide. Accessors can be written in Java for ultimate control and flexibility, or in one of the scripting languages.

Transports

A transport is an external-to-internal request translator. An external request could be for example an HTTP request, a JMS message, an email, a SOAP message or even a GUI event. A transport's role is to take an external request, interpret it and issue a kernel request for a resource. A detailed guide to developing transports is available in the Extensions Guide.

Resources

We've intimated already that a module can contain pretty much any type of resource: Java classes, images, static XML, PDF documents ... anything you like. We'll show below how general resources are exported so that other modules or transports can access them.

Transreptors

A transreptor changes the interface on a representation of a resource. The best way to describe transreptors is by example. Suppose a request is made to NetKernel for an XML DOM version of a file-based xml resource. NetKernel will issue the request to the File URA, however the File URA is general purpose and can only return a binary stream. NetKernel is aware that the request was made for a DOM version and so it looks in the Module Manager to see if a BinaryStream to DOM transreptor is registered. The NetKernel finds just such a transreptor, the binary stream is parsed to DOM and the result is returned to the requestor. A detailed guide to developing transreptors is available here.

Transreptors can be created for mapping from almost anything to anything. Typical examples are the DOM parser and serializers in the ext_xml module. It would be straightforward to provide a Java image to PNG binary transreptor. One important rule to stick to though, is that transreptors do not fundamentally process or transform a resource's content, they simply make the same abstract entity available in another form. Therefore generative transformations or processing should always occur in accessors.

Fragmentors

A fragmentor provides a transformation on a resource defined by a fragment identifier on a URI. A fragmentor in many ways operates like a transforming accessor; the key differences are in the way that they are invoked and their flexibility. A fragmentor can operate on only a single resource and it should use the fragment identifier as a kind of operator to determine what transformation to apply.

Cachelets

Each module can host a single cachelet. A cachelet is a cache component that is responsible for providing caching of any module resource. Cachelets are resolved hierarchically and a single module can provide a cachelet for a whole set of modules. Alternatively every module can provide it's own module specific cachelet. In general a single cachelet for any given application is sufficient, this is usually hosted in the module which is providing a transport. A detailed discussion of the NetKernel dependency-based caching architecture is provided here.

Java code libraries

An important location (not shown in the diagram) is the /lib directory of a module. Jar libraries placed in the /lib directory will automatically be picked up by the Java ClassLoader for the module. A module is fully encapsulated and has it's own classloader therefore any Java libraries required by classes in the module must be provided here.